論文

公開件数: 264 件
No. 掲載種別 単著・共著区分 タイトル 著者 誌名 出版者 巻号頁 出版日 ISSN DOI URL 概要
1 研究論文(学術雑誌)
共著
Spontaneous structural distortion of the metallic Shastry-Sutherland system DyB4 by quadrupole-spin-lattice coupling
Sim, H, Lee, S, Hong, KP, Jeong, J., Zhang, J. R., Kamiyama, T., Adroja, D. T., Murray, C. A., Thompson, S. P., Iga, F., Ji, S, Khomskii, D., Park, Je-Geun
Physical Review B
American Physics Society (APS)
94/ 19, 195128(1-?)
2016/11/15

10.1103/PhysRevB.94.195128

DyB4 has a two-dimensional Shastry-Sutherland (Sh-S) lattice with strong Ising character of the Dy ions. Despite the intrinsic frustrations, it undergoes two successive transitions: a magnetic ordering at T-N = 20K and a quadrupole ordering at T-Q = 12.5K. From high-resolution neutron and synchrotron x-ray powder diffraction studies, we have obtained full structural information on this material in all phases and demonstrate that structural modifications occurring at quadrupolar transition lead to the lifting of frustrations inherent in the Sh-S model. Our paper thus provides a complete experimental picture of how the intrinsic frustration of the Sh-S lattice can be lifted by the coupling to quadrupole moments. We show that two other factors, i.e., strong spin-orbit coupling and long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in metallic DyB4, play an important role in this behavior.
2 研究論文(学術雑誌)
共著
Surface Kondo Effect and Non-Trivial Metallic State of the Kondo Insulator YbB12
K. Hagiwara, Y. Ohtsubo, M. Matsunami, S. Ideta, K. Tanaka, H. Miyazaki, J. Rault, P. Le Fèvre, F. Bertran, A.T.-Ibrahimi, R. Yukawa, M. Kobayashi , K. Horiba, H. Kumigashira, K. Sumida, T. Okuda , F. Iga, S. Kimura
Nature Communications
Nature
7, 12690(1-?)
2016/08/31

10.1038/ncomms12690

A synergistic effect between strong electron correlation and spin–orbit interaction has been theoretically predicted to realize new topological states of quantum matter on Kondoinsulators (KIs), so-called topological Kondo insulators (TKIs). One TKI candidate has been experimentally observed on the KI SmB6(001), and the origin of the surface states (SS) and the topological order of SmB6 has been actively discussed. Here, we show a metallic SS on the clean surface of another TKI candidate YbB12(001) using angle-resolved photoelectron spectroscopy. The SS shows temperature-dependent reconstruction corresponding to the Kondo effect observed for bulk states. Despite the low-temperature insulating bulk, the reconstructed SS with c–f hybridization is metallic, forming a closed Fermi contour surrounding G on the surface Brillouin zone and agreeing with the theoretically expected behaviour for SS on TKIs. These results demonstrate the temperature-dependent holistic reconstruction of two-dimensional states localized on KIs surface driven by the Kondo effect.
3 研究論文(学術雑誌)
共著
Synchrotron Radiation Mossbauer Spectroscopy Using Sm-149 Nuclei
S. Tsutsui, R. Masuda, Y. Kobayashi, Y. Yoda, K. Mizuuchi, Y. Shimizu, H. Hidaka, T. Yanagisawa, T H. Amitsuka, F. Iga, M. Seto
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN
日本物理学会
85/ 8, 83704(1-?)
2016/08/15

10.7566/JPSJ.85.083704

Synchrotron radiation Sm-149 Mossbauer spectroscopy in the energy domain has successfully been carried out on Sm2O3, SmBe13, and SmB6. The isomer shift values were determined in these compounds to discuss their Sm valence state. We also investigated the time window effect on spectra, which is useful for measuring spectra with "high-energy-resolution". This effect makes it possible to discuss the frequency of valence fluctuation in Sm compounds as well as determine Sm valence state precisely using the Sm-149 Mossbauer effect. We discussed the fluctuation of the Sm valence in SmB6.
4 研究論文(学術雑誌)
共著
Momentum microscopy of SmB6 single crystals with detailed surface characterization
M. Ellguth, C. Tusche, F. Iga, S. Suga
Philosophical Magazine

96/ 31(SI), 3284-3306
2016/04/29

10.1080/14786435.2016.1185185


5 研究論文(学術雑誌)
共著
Hybridization gap formation in the Kondo insulator YbB12 observed using time-resolved photoemission spectroscopy
M. Okawa, Y. Ishida, M. Takahashi, T. Shimada, F. Iga, T. Takabatake, T. Saitoh, and S. Shin
Physical Review B

92, 161108(R)
2015/10/12

http://dx.doi.org/10.1103/PhysRevB.92.161108

A detailed low-energy electronic structure of a Kondo insulator YbB12 was revealed by a synergetic combination of ultra-high-resolution laser photoemission spectroscopy (PES) and time-resolved PES. The former confirmed a 25-meV pseudogap corresponding to the Kondo temperature of this material, and more importantly, it revealed that a 15-meV gap and a Kondo-peak feature developed below a crossover temperature of T∗∼110K. In harmony with this, the latter discovered a very long recombination time exceeding 100 ps below ∼T∗. This is a clear manifestation of photoexcited carriers due to the bottleneck in the recovery dynamics, which is interpreted as a developing hybridization gap of a hard gap.
6 研究論文(学術雑誌)
共著
Evidence for Gamma(8) Ground-State Symmetry of Cubic YbB12 Probed by Linear Dichroism in Core-Level Photoemission
Y. Kanai, T. Mori, S. Naimen, K. Yamagami, H. Fujiwara, A. Higashiya, T. Kadono, S. Imada, T. Kiss, A. Tanaka, K. Tamasaku, M. Yabashi, T. Ishikawa, F. Iga, and A. Sekiyama
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN

84/ 7, 73705(1-4)
2015/07/15

10.7566/JPSJ.84.073705

We have successfully observed linear dichroism in angle-resolved Yb3+ 3d(5/2) core-level photoemission spectra for YbB12 in cubic symmetry. Its anisotropic 4f charge distribution due to the crystal-field splitting is responsible for the linear dichroism, which has been verified by spectral simulations using ionic calculations with the full multiplet theory for a single-site Yb3+ ion in cubic symmetry. The observed linear dichroism as well as the polarization-dependent spectra in two different photoelectron directions for YbB12 are quantitatively reproduced by theoretical analysis for the Gamma(8) ground state, indicating the Gamma(8) ground-state symmetry for the Yb3+ ions mixed with the Yb2+ state.
7 研究論文(学術雑誌)
共著
Emergent photovoltage on SmB6 surface upon bulk-gap evolution revealed by pump-and-probe photoemission spectroscopy
Y. Ishida, T. Otsu, T. Shimada, M. Okawa, Y. Kobayashi, F. Iga, T. Takabatake & S. Shin
SCIENTIFIC REPORTS

5, 8955
2015/05/26

10.1038/srep08955

Recent studies suggest that an exemplary Kondo insulator SmB6 belongs to a new class of topological insulators (TIs), in which non-trivial spin-polarized metallic states emerge on surface upon the formation of Kondo hybridization gap in the bulk. Remarkably, the bulk resistivity reaches more than 20 Omega cm at 4 K, making SmB6 a candidate for a so-called bulk-insulating TI. We here investigate optical-pulse responses of SmB6 by pump-and-probe photoemission spectroscopy. Surface photovoltage effect is observed below similar to 90 K. This indicates that an optically-active band bending region develops beneath the novel metallic surface upon the bulk-gap evolution. The photovoltaic effect persists for >200 mu s, which is long enough to be detected by electronics devices, and could be utilized for optical gating of the novel metallic surface.
8 研究論文(国際会議プロシーディングス)
共著
Multipole Order and Fluctuation in Ce0.7La0.3B6 Studied by Resonant X-ray Diffraction
Takeshi Matsumura, Shinji Michimura, Toshiya Inami, Toru Otsubo, Hiroshi Tanida, Fumitoshi Iga, and Masafumi Sera
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN Conferene Proceedings

3, 011050
2014/09/15

10.7566/JPSCP.3.012024

Recent studies suggest that an exemplary Kondo insulator SmB6 belongs to a new class of topological insulators (TIs), in which non-trivial spin-polarized metallic states emerge on surface upon the formation of Kondo hybridization gap in the bulk. Remarkably, the bulk resistivity reaches more than 20 Omega cm at 4 K, making SmB6 a candidate for a so-called bulk-insulating TI. We here investigate optical-pulse responses of SmB6 by pump-and-probe photoemission spectroscopy. Surface photovoltage effect is observed below similar to 90 K. This indicates that an optically-active band bending region develops beneath the novel metallic surface upon the bulk-gap evolution. The photovoltaic effect persists for >200 mu s, which is long enough to be detected by electronics devices, and could be utilized for optical gating of the novel metallic surface.
9 研究論文(国際会議プロシーディングス)
共著
Pressure induced insulator-to-metal transition at 170 GPa of Kondo semiconductor YbB12
Shuei KAYAMA, Shigeki TANAKA, Atsushi MIYAKE, Tomoko KAGAYAMA,
Katsuya SHIMIZU, Fumitoshi IGA
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN Conference Proceedings

3, 012024
2014/09/15

10.7566/JPSCP.3.012024

YbB12 is the only one Yb-based Kondo semiconductor. We have succeeded in electrical resistance measurements on single crystal of YbB12 at ultra high pressure of up to 195 GPa. The electronic state in YbB12 is discussed from analysis of an activation energy in electrical resistance measurements under high pressure.
10 研究論文(国際会議プロシーディングス)
共著
Substitution Effect of Non-magnetic Rare-earth ion R (R = Lu, Sc, Y and Zr) of Kondo Semiconductor YbB12
Kento HAYASHI, Katsuya ISHII, Fumitoshi IGA, Tomoaki NOGUCHI, Toshiro TAKABATAKE, Akihiro KONDO, Kouichi KINDO
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN Conference Proceedings

3, 011050
2014/09/15

10.7566/JPSCP.3.011050

YbB12 is well known as a Kondo semiconductor in which its energy gap formation is proposed by c-f hybridization model or Kondo singlet model. In order to obtain the information on energy gap formation mechanism, we have investigated details of substitution effect on the energy gap by using non-magnetic ion R = Lu3+, Sc3+, Y3+ and Zr4+. From the magnetic susceptibility, specific heat and electrical resistivity measurements of Yb1-xRxB12, only Zr-substitution case has caused enhancement with synchronization between the magnitudes of the energy gap and Kondo temperature. By increase of number of conduction electrons the enhancement of the c-f hybridization seems to cause increase of the energy gap in Kondo semiconductor.
11 研究論文(国際会議プロシーディングス)
共著
Synthesis and characterization of rare-earth boride thin
films of CeB6 fabricated by MBE method
Daichi Hatanaka, Eisuke Asanuma, Kouki Takeda, Takeshi Ikeda, Mitsuteru
Nakamura, Yoshiki Nakanishi, Fumitoshi Iga, Yoshitomo Harada, Hirotaka
Yamaguchi, Masahito Yoshizawa
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, Conference Proceedings

3, 011049
2014/09/15

10.7566/JPSCP.3.011049

We synthesized CeB6 thin films by using a molecular beam epitaxy (MBE) method. Optimal growth conditions for this material were investigated. Structure, constituent element and electronic state of the film were evaluated. The grown films show a Kondo behavior as a function of temperature, which is consistent with previous works. These results show that CeB6 was successfully grown by MBE method. However, different physical properties from bulk have appeared in detail. It may be ascribed to a possible change in the crystal structure due to either intrinsic effect arising from the interface between substrates and grown films or extrinsic effect such as impurities.
12 研究論文(学術雑誌)
共著
Appearance of Antiferromagnetic Dipole Order in Ce0.5La0.5B6 with Pr Ion Doping
Takeshi Matsumura, Keisuke Kunimori, Akihiro Kondo, Kei Soejima, Masafumi Sera, and Fumitoshi Iga
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN

83/ 9, 094724
2014/09/15

10.7566/JPSJ.83.094724

We have performed a neutron diffraction experiment on Pr-doped Ce0.5Pr0.1La0.4B6, in which an antiferromagnetic octupole order with q = (1/2; 1/2; 1/2) could be anticipated by analogy with Ce0.7La0.3B6. Contrary to this natural expectation, we detected an unambiguous magnetic peak at q = (1/4; 1/4; 1/2), which is the same q-vector frequently realized in the magnetic ordered phases of RB6 ( R = rare earth) compounds. No significant signal was observed at q = (1/2; 1/2; 1/2) at zero magnetic field. This result shows that the normal antiferromagnetic dipole moment is also one of the competing multipole order parameters in the CexLa1-xB6 system. The relevant order parameters are close in energy and can be tuned by a weak perturbation.
13 研究論文(学術雑誌)
共著
Universality of Anharmonic Motion of Heavy Rare-Earth Atoms in Hexaborides
Kazuaki Iwasa, Fumitoshi Iga, Ari Yonemoto, Yuka Otomo, Satoshi Tsutsui, and Alfred Q. R. Baron
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN

83/ 9, 094604
2014/09/15

10.7566/JPSJ.83.094604

Phonons of TbB6 are studied by inelastic X-ray scattering and compared with those of GdB6 and DyB6. Measured low-lying phonon modes located near 6 meV soften distinctly with decreasing temperature, particularly the longitudinal phonon that is dominated by the motion of Tb ions at q(1) = (1/2, 0, 0). The magnitude and temperature dependence of phonon energy are between those of GdB6 and DyB6. We derive a Landau-type free-energy expansion with respect to the atomic displacement in TbB6, and discuss a general feature among these three compounds. The systematic variation in the phonon characteristics of these hexaborides and the first-order-like instability indicate the universality of the anharmonic motion of heavy rare-earth atoms in RB6 (R = Gd, Tb, and Dy).
14 研究論文(学術雑誌)
共著
Large ferroquadrupole moment induced in the octupole-ordered Ce0.7La0.3B6 revealed by high-resolution x-ray diffraction
T. Inami, S. Michimura, Y. Hayashi, T. Matsumura, M. Sera, F. Iga
PHYSICAL REVIEW B

90/ 4, 041108
2014/07/25

10.1103/PhysRevB.90.041108

We carried out a high-resolution x-ray diffraction experiment on Ce0.7La0.3B6 that exhibits staggered order of 4f octupole moments below To = 1.4 K. Theoretically, ferroquadrupole moments that accompany the antiferro-octupole order and a resultant rhombohedral deformation of the cubic lattice were predicted. In contrast, experimentally, no direct evidence of the rhombohedral lattice has been obtained. We observe the splitting of Bragg peaks below To and find that the unit cell is a rhombohedron being elongated along the [111] axis. The response of rhombohedral domains to magnetic fields also well agrees with theoretical calculations. A particular outcome of this experiment is that the magnitude of the induced quadrupole moments is precisely evaluated from the obtained shear strain. Using this result, the magnitude of the hidden octupole moments is also discussed.
15 研究論文(学術雑誌)
共著
Synchrotron radiation-based Mössbauer spectra of 174Yb measured with internal conversion electrons
R. Masuda, Y. Kobayashi, S. Kitao, M. Kurokuzu, M. Saito, Y. Yoda, T. Mitsui, F. Iga, M. Seto
Applied Physics Letters

104/ 8, 082411(1-6)
2014/02/14

10.1063/1.4866280

A detection system for synchrotron-radiation (SR)-based Mössbauer spectroscopy was developed to enhance the nuclear resonant scattering counting rate and thus increase the available nuclides. In the system, a windowless avalanche photodiode (APD) detector was combined with a vacuum cryostat to detect the internal conversion (IC) electrons and fluorescent X-rays accompanied by nuclear de-excitation. As a feasibility study, the SR-based Mössbauer spectrum using the 76.5 keV level of 174Yb was observed without 174Yb enrichment of the samples. The counting rate was five times higher than that of our previous system, and the spectrum was obtained within 10 h. This result shows that nuclear resonance events can be more efficiently detected by counting IC electrons for nuclides with high IC coefficients. Furthermore, the windowless detection system enables us to place the sample closer to the APD elements and is advantageous for nuclear resonant inelastic scattering measurements. Therefore, this detection system can not only increase the number of nuclides accessible in SR-based Mössbauer spectroscopy but also allows the nuclear resonant inelastic scattering measurements of small single crystals or enzymes with dilute probe nuclides that are difficult to measure with the previous detection system.
16 研究論文(学術雑誌)
共著
Evidence for hidden quadrupolar fluctuations behind the octupole order in Ce0.7La0.3B6 from resonant x-ray diffraction in magnetic fields
T. Matsumura, S. Michimura, T. Inami, T. Otsubo, H. Tanida, F. Iga, M. Sera
Physical Review B

89/ 1, 014422(1-13)
2014/01/24

10.1103/PhysRevB.89.014422

The multipole ordered phase in Ce0.7La0.3B6, emerging below 1.5 K and named phase IV, has been studied by resonant x-ray diffraction in magnetic fields. By utilizing diamond x-ray phase plates to rotate the incident linear polarization and a conventional crystal analyzer system, full linear polarization analysis has been performed to identify the order parameters. The analysis shows that the Γ5g (Oyz, Ozx, Oxy) quadrupoles are more induced by the field than the Γ3g (O20 and O22) quadrupoles on the Γ5u (Tβx+y+z) antiferro-octupole order in phase IV. The problem is that this result is contradictory to a mean-field calculation, which inevitably gives the Γ3g quadrupole as the main induced moment. This result indicates that the Γ5g quadrupole order is close in energy. We consider that a large fluctuation of the Γ5g quadrupole is hidden behind the primary ordering of the Γ5u octupole and that the multipolar fluctuation significantly affects the ordering phenomenon.
17 研究論文(学術雑誌)
共著
Spin-Polarized Angle-Resolved Photoelectron Spectroscopy of the So-Predicted Kondo Topological Insulator SmB6
S. Suga, K. Sakamoto, T. Okuda, K. Miyamoto, K. Kuroda, A. Sekiyama, J. Yamaguchi, H. Fujiwara, A. Irizawa, T. Ito, S. Kimura, T. Balashov, W. Wulfhekel, S. Yeo, F. Iga, S. Imada
Journal of the Physical Society of Japan

83/ 1, 014705(1-6)
2014/01/01

10.7566/JPSJ.83.014705

Undoped and slightly Eu-doped SmB6 samples show the opening of a gap with decreasing temperature below ~150 K. The spectral shapes near the Fermi level (EF) at 15 K have shown a marked increase in intensity of the peak at a binding energy (EB) of approximately 12 meV with decreasing photon energy (hν) from 17 to 7 eV. Angle-resolved spectra of SmB6 measured at hν = 35 eV just after the in situ cleavage showed clear dispersions of several bands in the EB region from EF to 4 eV. Spin-polarized photoelectron spectra were then measured at 12 K and a light incidence angle of ~50°. In contrast to the lack of spin polarization for the linearly polarized light excitation, clear spin polarization was observed in the case of circularly polarized light excitation. The two prominent peaks at EB ~ 12 and ~150 meV have shown opposite signs of spin polarization which are reversed when the helicity of the light is reversed. The sign and magnitude of spin polarization are consistent with a theoretical prediction for the 6H5/2 and 6H7/2 states.
18 研究論文(学術雑誌)
共著
Microstructure and hydrogen desorption characteristics of hydrogenated ScH2-MBn (M = Mg and Ca) systems synthesized by mechanical milling
T. Nakagawa,T. Ichikawa, K. Shimoda, M. Tsubota, H. Miyaoka, S. Isobe, T. Honma, S. Michimura, F. Iga, S. Ohnuki, Y. Kojima
International Journal of Hydrogen Energy

38/ 16, 6744-6749
2013/05/30

10.1016/j.ijhydene.2013.03.102

The mixtures of scandium hydride ScH2 and metal boride MBn, which is MgB2 or CaB6, were hydrogenated by mechanical milling under hydrogen pressure at room temperature. ScH2–MgB2 and ScH2–CaB6 desorbed 3.4 and 2.3 mass% of H2, respectively, with peaks below 300 °C. The results of synchrotron radiation X-ray powder diffraction and X-ray absorption spectroscopy at the Sc K-edge indicated that ScB2 was produced by milling. Fourier-transform infrared spectroscopy suggested that hydrogen was stored as B–H bonds in the as-milled samples. Nuclear magnetic resonance spectroscopy clarified the presence of metal borohydrides M(BH4)2 (M = Mg and Ca) in the as-milled ScH2–MBn mixtures. These results indicate that M(BH4)2 is synthesized by milling the ScH2–MBn mixtures under hydrogen pressure at room temperature, and hydrogen was desorbed from M(BH4)2. The by-products of M(BH4)2 are MgH2 in the M = Mg case, which was observed by transmission electron microscopy, and ScB2 in both cases.
19 研究論文(学術雑誌)
共著
A new method for determining the valence of lanthanide compounds: Lγ4 emission spectroscopy
H. Hayashi, N. Kanai, N. Kawamura, M. Mizumaki, K. Imura, N. K. Sato, H. S. Suzuki, F. Iga
Journal of Analytical Atomic Spectrometry

28/ 3, 373-378
2013/05/13

10.1039/c0xx00000x

We have examined the possibility of Lγ4 emission spectroscopy as a new method for valence determination. Sm Lγ4 emission spectra of a mixed valence compound, SmB6, as well as Sm standard references were measured using a multicrystal, multidetector spectrometer at 298 K. The Lγ4 spectra exhibited a large chemical shift (~4.5 eV) and substantial profile changes depending on the Sm oxidation state. Splitting of the Sm Lγ4 band was attributed to spin-dependent atomic multiplets, and valence-dependent edge-shifts of the Sm LI XANES spectra strongly suggest that the chemical shift of the Lγ4 emission is mainly due to the screening effects of the Sm LI core. The Lγ4 spectrum of SmB6 was well reproduced by the averaged spectra of SmS and SmF3, indicating a SmB6 valence of 2.65± 0.03. The advantages of the Lγ4 method, including simple data treatment and no need for an excitation energy scan, will encourage its use in several areas of materials science.
20 研究論文(学術雑誌)
共著
Different evolution of the intrinsic gap in strongly correlated SmB6 in contrast to YbB12
J. Yamaguchi, A. Sekiyama, M. Y. Kimura, H. Sugiyama, Y. Tomida, G. Funabashi, S. Komori, T. Balashov, W. Wulfhekel, T. Ito, S. Kimura, A. Higashiya, K. Tamasaku, M. Yabashi, T. Ishikawa, S. Yeo, S-I. Lee, F. Iga, T. Takabatake, S. Suga
New Journal of Physics

15, 043042(1-13)
2013/04/23

10.1088/1367-2630/15/4/043042

Dependence of the spectral functions near the Fermi level on temperature and rare-earth atom doping was studied in detail for strongly correlated alloys Sm1−xEuxB6 and Yb1−xLuxB12 by photoelectron spectroscopy at ~8000 eV as well as at 7 and 8.4 eV. It was found that the 4f lattice coherence and intrinsic gap are robust for Sm1−xEuxB6 at least up to the Eu substitution of x = 0.15 while both collapse by Lu substitution already at x = 0.125 for Yb1−xLuxB12. As for the temperature dependence of the spectral shapes near the Fermi level at low temperatures, rather contrasting results were observed between YbB12 and SmB6. Although the gap shape does not change below 15 K for YbB12 with the characteristic temperature T* of 80 K, the spectral shape of SmB6 with a T* of 140 K shows that the peak beyond the gap is further increased below 15 K. The temperature dependence of the spectra near the intrinsic gap is clearly different between SmB6 and YbB12, although both materials have so far been categorized in the same kind of strongly correlated semiconductor. The possibility of the surface contribution is discussed for SmB6.
21 研究論文(学術雑誌)
共著
New Type of the Domain-Redistribution at Low Magnetic Fields in Phase II of CeB6
K. Kunimori1, M. Sera1, H. Tanida1, T. Matsumura1, F. Iga2
(1 Hiroshima Univ., 2 Ibaraki Univ.)
Journal of the Physical Society of Japan

81/ 10, 104706(1-6)
2012/10

10.1143/JPSJ.81.104706

We performed the detailed investigation of the magnetization (M) of CeB6 at low magnetic fields in the Oxy-type antiferro-quadrupole (AFQ) ordered phase II to clarify the mechanism of the domain redistribution by magnetic field. Although M in phase II is isotropic above ∼1 kOe, it is anisotropic at lower magnetic fields and the magnetic field of the domain redistribution (Hc^IIA–IIB) is highly anisotropic. M[001]<M[110]<M[111] and Hc^IIA–IIB[001]>Hc^IIA–IIB[110]>Hc^IIA–IIB[111]. Here, M[001] and Hc^IIA–IIB[001] are M and Hc^IIA–IIB for H∥[001], etc. respectively. Although there exist three domains of Oxy-, Oyz-, and Ozx-AFQ ordered states at H=0 as in a usual AF magnet, these domains are coupled in magnetic fields and the single domain state is formed by the linear combination of Oxy, Oyz, and Ozx quadrupole moments above Hc^IIA–IIB through the magnetic field induced higher order Txyz-antiferro-octupole (AFO) interaction. The domain redistribution takes place due to the competition between the small tetragonal lattice distortion stabilized by the Oxy-type AFQ order at H=0 and the Zeeman interaction. For H∥[001], M is smallest and Hc^IIA–IIB is highest because only the Oxy-domain couples with the Txyz-AFO moment inducing the effective ferromagnetic interaction but the two domains of Oyz and Ozx are not. For H∥[111], M is largest below ∼1 kOe and Hc^IIA–IIB is lowest because the Txyz-AFO moment is induced in all the three domains and each of Oxy, Oyz, and Ozx domain changes continuously to the (Oyz+Ozx+Oxy)/√3 single domain with increasing magnetic field. The coefficients of the linear combination of three domains gradually changes with magnetic field through the magnetic field induced Txyz-AFO moment. This type of the continuous domain redistribution is different from that in the usual antiferro-magnet, where there does not exist the coupling among the different domains.
22 研究論文(学術雑誌)
共著
Interplay of low-energy phonons and magnetic excitations in the Kondo insulator YbB12
P. A. Alekseev, J-M. Mignot, K. S. Nemkovski, A. V. Rybina, V. N. Lazukov, A. S. Ivanov, F. Iga, T. Takabatake
JOURNAL OF PHYSICS-CONDENSED MATTER

24/ 20, 205601
2012/05/23

10.1088/0953-8984/24/20/205601

Peculiarities in the lattice dynamics of the Kondo insulator YbB12 have been studied by inelastic neutron scattering. Selected phonon modes were traced above and below the temperature region (T ~ 50 K) where the gap opens in the electron density of states. The intensities of some low-energy modes exhibit an anomalous temperature dependence for q vectors close to the Brillouin zone boundary, suggesting a renormalization of the phonon eigenvectors. This effect is thought to arise from a coupling with magnetic excitations of the same symmetry, which exist at nearby energies. It is argued that this magnetovibrational coupling may in turn play a role in the steep temperature crossover existing in YbB12 between the low-temperature (Kondo insulator) and high-temperature (incoherent spin-fluctuation) regimes, which is rapidly suppressed by lighter Zr substitution.
23 研究論文(学術雑誌)
共著
Antiferroquadrupole order and magnetic field induced octupole in CeB6
T. Matsumura, T. Yonemura, K. Kunimori, M. Sera, F. Iga, T. Nagao, J. Igarashi
PHYSICAL REVIEW B

85/ 17, 174417(1-10)
2012/05/14

10.1103/PhysRevB.85.174417

We have studied the antiferroquadrupole ordered phase of CeB6 in magnetic fields by resonant x-ray diffraction. By analyzing the significant change in the energy spectrum on reversing the field direction along [1̅ 1 0], we have deduced field dependencies of the antiferro components of magnetic dipole, electric quadrupole, and magnetic octupole moments which are simultaneously induced in the Ce 4f orbital with a propagation vector (1/2,1/2,1/2). The data treatments are based on theoretically calculated spectral functions. The existence of the field-induced octupole is also concluded for other field directions. We also show direct evidence for the formation of a linear-combination-type antiferroquadrupole order parameter in magnetic fields, which is expressed as 〈αOyz+βOzx+γOxy〉 and changes continuously with the field direction (α,β,γ). A possibility of observing the quadrupolar fluctuation is also pointed out.
24 研究論文(学術雑誌)
共著
Motion of the guest ion as precursor to the first-order phase transition in the cage system GdB6
K. Iwasa, R. Igarashi, K. Saito, C. Laulhé, T. Orihara, S. Kunii, K. Kuwahara, H. Nakao, Youichi Murakami, F. Iga, M. Sera, S. Tsutsui, H. Uchiyama, A. Q. R. Baron
PHYSICAL REVIEW B

84/ 21, 214308
2011/12/28

10.1103/PhysRevB.84.214308

The motion of guest Gd ions in oversized boron cages in GdB6 was investigated from phonon spectra measurements obtained by inelastic x-ray scattering. The measured phonon modes soften by about 10% from 300 K down to TN=16 K, in particular, the longitudinal phonon for the propagation vector q1=(1/2,0,0) that characterizes the distorted structure below TN. Besides, the dispersion relation curves show kinklike anomalies at qk=(0.38,0.38,0). The observed results imply that the motion of the guest Gd ion interplays with the f electrons magnetoelastically and with carriers via Fermi surface nesting. The anomalous properties previously reported for this material far above TN originate from the strong electron-phonon coupling, which causes the motion of guest ions as precursors to the first-order phase transition.
25 研究論文(国際会議プロシーディングス)
共著
Raman Scattering of YB6
H. Bando, T. Hasegawa, N. Ogita, M. Udagawa, and F. Iga
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN

80/ Supplement A, SA053
2011/07

10.1143/JPSJS.80SA.SA053

Temperature and polarization dependencies of Raman scattering spectra of YB6 have been measured in the temperature region from room temperature to 2.4K. All Raman active modes have been observed and well assigned by the polarization dependence. The observation of the doublet structure of T-2g and E-g peaks suggests the crystallographic symmetry of P4/mmm for YB6, not Pn (3) over barm from group theoretical consideration. In the lower energy region than 200cm(-1), the peaks due to the Y vibration have been observed. Within the present experimental accuracy, any anomalies of phonon spectra have not been observed at the superconducting transition temperature.
26 研究論文(国際会議プロシーディングス)
共著
Existence Region of Phase III' in CeB6
Keisuke Kunimori, Masahiro Kotani, Hiroshi Funaki, Hiroshi Tanida, Masafumi Sera, Takeshi Matsumura, and Fumitoshi Iga
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN

80/ Supplement A, SA056
2011/07

10.1143/JPSJS.80SA.SA056

We measured the magnetization in the antiferro-magnetic phase of CeB6 in magnetic field whose direction is rotated in the {110} plane. It was found that phase III' exists between phases III and II in magnetic field independent of the applied field direction, although phase III' with 1-k-k' collinear magnetic structure and only chi(perpendicular to) component was believed to exist only for H parallel to[111] in order to release the Zeeman energy loss from the chi(parallel to) component which exists inevitably in the thrre domain state with K-xy, K-yz, and K-zx in phase III for H parallel to[111]. The most strange result is that phase III' exists even for H parallel to[001] where the K-xy single domain without the Zeeman energy loss due to the chi(perpendicular to) configuration is realized in phase III. This indicates the existence of the unknown multipole interaction which acts as a buffer between phases III and II in magnetic field.
27 研究論文(国際会議プロシーディングス)
共著
Pressure Effect of Ce0.75La0.25B6
Keisuke Kunimori, Hiroshi Tanida, Takeshi Matsumura, Masafumi Sera, and Fumitoshi Iga
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN

80/ Supplement A, SA055
2011/07

10.1143/JPSJS.80SA.SA055

We performed the magnetization measurement of Ce0.75La0.25B6 under hydrostatic pressure. It was found that there exists the direct phase transition from phase II to phase IV at the ambient pressure in this compound. Together with the results of Ce0.8La0.2B6, it is confirmed that there exists the II-IV direct phase transition at the ambient pressure independent of x in CexLa1-xB6 system. By applying the pressure, the temperature region of the II-IV direct phase transition is expanded largely. The II-IV direct phase transition seems to be that of the second order from the magnetization curve under pressure.
28 研究論文(国際会議プロシーディングス)
共著
Behavior of the Antiferroquadrupolar Moments in the Antiferromagnetic Ordered Phase of CeB6
Takeshi Matsumura, Takumi Yonemura, Keisuke Kunimori, Masafumi Sera, and Fumitoshi Iga
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN

80/ Supplement A, SA054
2011/07

10.1143/JPSJS.80SA.SA054


29 研究論文(国際会議プロシーディングス)
共著
Raman Scattering Study of Lattice Dynamics in Caged Compounds
N. Ogita, T. Hasegawa, M. Udagawa, T. Onimaru, F. Iga, T. Takabatake, H. Sugawra, and H. Sato
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN

80/ Supplement A, SA037
2011/07

10.1143/JPSJS.80SA.SA037

Raman scattering spectra of caged compounds PrB 6 , PrT 4 Sb 12 , and PrRu 2 Zn 20 have been measured at room temperature. In there spectra, cage modes due to B, Sb, and Zn vibrations and Pr guest modes are observed. In the excitation energy dependence, the guest modes and some of cage modes show the resonance behavior at low energy excitation.
30 研究論文(国際会議プロシーディングス)
共著
Elastic anomalies of TbB4 in pulsed high magnetic fields
T. K. Fujita, M. Yoshizawa, R. Kamiya, H. Mitamura, T. Sakakibara, K. Kindo, F. Iga, I. Ishii and T. Suzuki
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN

80/ Supplement A, SA084
2011/07

10.1143/JPSJS.80SA.SA084

In this paper we first introduce a measurement method for the elastic moduli in a pulsed magnetic field. By using this method, we measured the elastic moduli in a pulsed magnetic field for the rare earth tetraborides TbB4. We observed elastic anomalies at most of the magnetization jumps.
31 研究論文(国際会議プロシーディングス)
共著
Polarization-dependent Ti K X-ray absorption and emission studies of Ti2O3 single crystal
H. Sato, H. Maso, Y. Utsumi, H. Kurihara, Y. Mukaegawa, Y. Tezuka, T. Iwazumi, F. Iga, M. Tsubota, H. Namatame, M. Taniguchi
JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA

184/ 3-6, 184-187
2011/04

10.1016/j.elspec.2011.02.003

Polarization-dependent Ti K X-ray absorption and Ti Kα X-ray emission spectroscopy (XAS and XES) experiments have been carried out for single-crystalline Ti2O3 with metal-insulator transition (MIT) around 450 K. Both the XAS and XES spectra exhibit remarkable polarization dependences between E//c and E⊥c conditions, where the c-axis is parallel to the Ti ion pair direction and E represents a polarization vector of incident photons. The main peak of the E//c-XAS spectrum is 3 eV lower in energy than the E⊥c-spectrum at 300 K, reflecting the highly anisotropic unoccupied Ti 4p states, and the energy separation of the main peaks decreases to 2 eV at 520 K, in consistent with the reduced anisotropy of electronic structure followed by MIT. The broad features of the Ti Kα XES spectra taken far below the Ti K absorption edge are similar to the XAS features including the polarization dependence, suggesting that the 2p4p monopole excitation causes the broad XES features, where 2p denotes a 2p core hole.
32 研究論文(学術雑誌)
共著
Softening of the Longitudinal Phonon Mode along the [100] Direction in GdB6
K. Iwasa, R. Igarashi,K. Saito, C. Laulhe,T. Orihara, S. Kunii, K. Kuwahara, H. Nakao, Y. Murakami, F. Iga, M. Sera, S. Tsutsui, H. Uchiyama, AQR. Baron
CHINESE JOURNAL OF PHYSICS

49/ 1, 231-238
2011/02



We have investigated the phonon of GdB6, which exhibits antiferromagnetic and structural first-order phase transition by using the inelastic x-ray scattering technique. The dispersion relation curve of the longitudinal acoustic mode propagating along the simple cubic [100] axis shows the maximum energy around the wave vector q = xi, 0, 0), xi = 0.2 - 0.3, and it bends clown with approaching the Brillouin zone boundary. The lower-energy zone-boundary mode at q(1) = (1/2, 0, 0) corresponds to the structural modulation in the ordered phase. The energy of this mode is 75% of the maximum value on the branch at 300 K and further decreases by 10% with decreasing temperature down to the transition temperature T-N = 16 K so that this phonon mode softens considerably far above the transition temperature. On the other hand, the reference material YbB6 that is considered to have no localized magnetic moments does not show such softening in the inelastic neutron scattering measurement. The observation indicates a strong electron-phonon coupling in GdB6, which is expected to be magnetoelastic-type interaction between 4f states and displacement of Gd ions.
33 研究論文(学術雑誌)
共著
Stable Existence of Phase IV inside Phase II under Pressure in Ce0.8La0.2B6
K. Kunimori, H. Tanida, T. Matsumura, M. Sera, F. Iga
Journal of the Physical Society of Japan

79/ 7, 73703(1-4)
2010/07

10.1143/JPSJ.79.073703

We investigate the pressure effect of the electrical resistivity and magnetization of Ce0.8La0.2B6. The situation in which phase IV stably exists inside phase II at H=0 T could be realized by applying a pressure above P∼1.1 GPa. This originates from the fact that the stability of phase II under pressure is larger than those of phases IV and III. The results seem to be difficult to reproduce by taking the four interactions of Γ5u-type AFO, Oxy-type AFQ, Txyz-type AFO, and AF exchange into account within a mean-field calculation framework.
34 研究論文(学術雑誌)
共著
X-ray atomic orbital analysis of 4f and 5d electron cofiguration of SmB6 at 100,165, 230 and 298 K
S. Funahashi, K. Tanaka, F. Iga
Acta Crystallography

B66, 292-306
2010/06

10.1107/S0108768110009250

Accurate electron-density measurement of SmB6 at 100, 165, 230 and 298 K, and X-ray atomic orbital (XAO) analysis were carried out. The 4f-electron density around Sm and 5d electron density at similar to 1 angstrom from Sm were analysed by XAO analysis. The 5d electron density is due to the electrons of the 5d(J = 5/2)Gamma(8) orbitals which stem from the e(g) orbitals in the strong field approximation. The change in electron populations of the 5d(5/2)Gamma(8) orbitals with temperature is similar to that of the resistivity. Since the conduction band consists of 5d(5/2)Gamma(8) and B-2p orbitals according to band theory, this indicates that the larger populations of the 5d(5/2)Gamma(8) orbitals correspond to the larger number of localized electrons and are correlated to the resistivity of SmB6. The occupation of the bulky 5d(5/2)Gamma(8) orbitals may be the reason for the elongation of the lattice parameter below 150 K. The 4f(7/2)Gamma(6) orbitals are obviously occupied except at 100 K, which seems to be caused by the energy gap between 4f(5/2) and 4f(7/2) states, which begins to exist between 100 and 150 K, and may represent one of the properties of a Kondo insulator.
35 研究論文(国際会議プロシーディングス)
共著
Strongly correlated electronic states of Yb1−xLuxB12 and Sm1−yEuyB6 studied by highly bulk-sensitive photoelectron spectroscopy
J. Yamaguchi, A. Sekiyama, S. Imada, A. Higashiya, K. Tamasaku, M. Yabashi, T. Ishikawa, T. Ito, S. Kimura, F. Iga, T. Takabatake, S. Yeo, S. -I. Lee, H. -D. Kim, S. Suga
Journal of Physics: Coference series

200, 012230(1-4)
2010/02

10.1088/1742-6596/200/1/012230

Impurity-substituted Kondo semiconductors Yb1¡xLuxB12 and Sm1-yEuyB6 have been studied by the highly bulk-sensitive hard x-ray (hν~ 8 keV) photoelectron spectroscopy. The valence-band spectra for Yb1-xLuxB12 have been analyzed by the single-impurity Anderson model (SIAM). The temperature dependence of the bulk Yb 4f peak positions for x = 0 cannot
be interpreted within the SIAM, whereas that for x = 1/8 could be understood by the SIAM. This suggests that the Kondo lattice coherence, which is essential for pure YbB12, collapses due to the substitution of x = 1/8. For Sm1-yEuyB6, the y dependence of the Sm and Eu 3d core level spectra has been systematically examined. We have found that the Sm valence increases with y due to the reduction of the hybridization between the Sm 4f and valence-bands.
36 研究論文(国際会議プロシーディングス)
共著
Neutron diffraction study of the high magnetic field phase diagram of La-doped PrB6
J. Robert, J.-M. Mignot, M. Sera, F. Iga
Journal of Physics: Coference series

200, 012166(1-4)
2010/02

10.1088/1742-6596/200/1/012166

The high magnetic field phase diagram of lanthanum-doped PrB6 was investigated by single-crystal neutron diffraction. In this compound, a peculiar incommensurate phase denoted IC2 was observed in magnetic fields applied along directions close to [111]. Unlike the double-k IC1 phase [kIC1 = (1/4 − δ,±1/4, 1/2)] formed in zero field below TN = 7 K, IC2 is characterized by a propagation vector with two incommensurate components, kIC2 = (1/4−δ , 1/4−δ , 1/2) (δ  0.03). The field and temperature evolutions of domain populations further indicate that, contrary to the low-field phases C and IC1, the magnetic structure in phase IC2 is single-k.
37 研究論文(国際会議プロシーディングス)
共著
Anisotropic magnetoresistance and collapse of the energy gap in Yb1-xLuxB12
F. Iga, K. Suga, K. Takeda, S. Michimura, K. Murakami, T. Takabatake, K. Kindo
Journal of Physics: Coference series

200, 012064(1-4)
2010/02

10.1088/1742-6596/200/1/012064

強磁場での半導体ギャップ消失を合金で調べた。
38 研究論文(国際会議プロシーディングス)
共著
Large Softening of Longitudinal Elastic Modulus in TbB4
T. Suzuki, T. Fujita, I. Ishii, S. Michimura, F. Iga,
T. Takabatake
Journal of Physics: Coference series

150, 042194(1-4)
2009

10.1088/1742-6596/150/4/042194

Rare earth tetraborides have a tetragonal crystalline structure with the space group P4/mbm. The rare earth ions form the Shastry-Sutherland lattice in the c plane in these compounds, which evokes us an interesting electronic property due to the geometric frustration of spins and orbitals. TbB4 undergoes two phase transitions at TN1 = 42.1 K and TN2 = 21.7 K, respectively. Because of the clear cusp-type anomaly in the temperature dependence of magnetic susceptibility, the transition at TN1 should be an antiferromagnetic one. The origin of the transition at TN2 is unclear at present. To investigate the possibility of multipolar ordering, we measured temperature dependencies of elastic moduli using the ultrasonic phase comparison method. The longitudinal elastic modulus C11 shows large softening with more than 17% reduction of the stiffness at TN2 in contrast to no anomaly at TN1. This result suggests ferro-type ordering of the quadrupole OΓ3 at TN2. The macroscopic spontaneous strain in the c plane emerging due to the ferro-quadrupolar ordering may lift the geometric frustration.
39 研究論文(国際会議プロシーディングス)
共著
Raman scattering of rare earth hexaborides
N. Ogita, T. Hasegawa, M. Udagawa, F. Iga, S. Kunii
Journal of Physics: Coference series

176, 012032(1-4)
2009

10.1088/1742-6596/176/1/012032

Raman scattering spectra were measured for the rare-earth hexaborides RB6 (R = Ce, Gd, or Dy). All Raman-active phonons due to B6 vibrations were observed in the range 600 - 1400 cm¡1. Anomalous peaks were detected below 200 cm¡1, which correspond to vibrations of rare-earth ion excited by second-order Raman scattering process. The intensity and energy of the rare-earth mode decrease with decreasing temperature. This suggests that the rare-earth ion vibrates in a shallow and anharmonic potential due to the boron cage. Using the reported values of mean square displacement of rare-earth ion, we estimated the anharmonic contribution for the rare-earth vibrations.
40 研究論文(国際会議プロシーディングス)
共著
149Sm nuclear resonant inelastic scattering of SmB6
S. Tsutsui, T. Hasegawa, Y. Takasu, N. Ogita,
M. Udagawa, Y. Yoda, F. Iga
Journal of Physics: Coference series

176, 012033(1-5)
2009

10.1088/1742-6596/176/1/012033

149Sm nuclear resonant inelastic scattering was carried out in SmB6 in order to investigate the lattice dynamics due to Sm atoms. A sharp phonon excitation associated with Sm atoms is observed around 10 meV, suggesting the presence of a Sm dispersionless mode. The Sm phonon energy shows significant temperature dependence, correlated with temperature dependence of Sm valence state. The nuclear resonant inelastic scattering spectra also indicate that the electronic configuration at the Sm site is 4f5 hybridized with the conduction electrons.
41 研究論文(国際会議プロシーディングス)
共著
Temperature dependence of Sm valence in SmB6 studied by
X-ray absorption spectroscopy
M. Mizumaki, S. Tsutsui, F. Iga
Journal of Physics: Coference series

176, 012034(1-4)
2009

10.1088/1742-6596/176/1/012034

X-ray absorption spectra of a valence-fluctuated Kondo insulator SmB6 have been studied near the Sm L3-edge at temperatures below 300 K. The average valence of Sm ions in SmB6 was deduced as 2.59 at 300 K; it shows a characteristic temperature behaviour with three anomalies at T = 15, 50 and 150 K. These temperatures correspond to the characteristic energies of SmB6 in macroscopic measurements. The present results imply that the Sm-4f electrons strongly hybridize with the conduction electrons and play an important role in the gap formation.
42 研究論文(学術雑誌)
共著
Competition between the Quadrupole Interaction and Crystalline Electric Field Effect in the Antiferromagnetic Ordered Phase of NdB6
T. Yonemura, H. Tanida, M. Sera, F. Iga
Journal of the Physical Society of Japan

78/ 11, 114705(1-9)
2009/11

10.1143/JPSJ.78.114705

We have studied the thermal and transport properties of NdB6 focused on the anisotropic behaviors depending on the applied magnetic field direction. The magnetic field dependence of the physical properties in the type-I antiferromagnetic phase is small for H parallel to [001] or [110] but is large for H parallel to [111]. These anisotropic behaviors are associated with the different domain distribution depending on the applied magnetic field direction. For H parallel to [001] or [110], the domain with only the chi(perpendicular to) component is realized, and for H parallel to [111], that with both the chi(perpendicular to) and chi(parallel to) components is realized, where the competition among the crystalline electric field effect leading to the three-fold easy axis and the O-2(0)-type ferro-quadrupole interaction leading to the four-fold easy axis and also the antiferromagnetic interaction plays an important role. This competition might be important for the appearance of the successive metamagnetic phase transitions for H parallel to [111]. The anisotropic behaviors of the magnetization, the specific heat and the electrical resistivity depending on the applied magnetic field direction in the type 1-AFMI phase were analized by the mean field calculation by considering the different domain distribution depending on the magnetic field direction and could explain the characteristics of these anisotropic behaviors in low magnetic field region in the AFMI phase rather well.
43 研究論文(学術雑誌)
共著
Stabilization of Phase IV in CexLa1-xB6 (x=0.4, 0.5) by Pr and Nd Ion Dopings
A. Kondo, T. Taniguchi, H. Tanida, T. Matsumura, M. Sera, F. Iga, H. Tou, T. Sakakibara, S. Kunii
Journal of the Physical Society of Japan

78/ 9, 093708(1-4)
2009/09

10.1143/JPSJ.78.093708

We have Studied the effect of magnetic rare-earth ion (Pr, Nd) doping on phase IV in CexLa1-xB6 (x = 0.4, 0.5) systems. An unexpected large increase in the IV-1 transition temperature TIV-1 by Pr and Nd dopings was observed, while no such increase was observed for x >= 0.6. Although we do not know the reason why the doping effect markedly differs between x <= 0.5 and x >= 0.6 at present. the order parameter in phase IV for x <= 0.5 is coupled with the magnetic dipole moment of Pr and Nd ions and phase IV is stabilized.
44 研究論文(学術雑誌)
共著
Magnetic octupole order in Ce0.7La0.3B6: A polarized neutron diffraction study
K. Kuwahara, K. Iwasa, M. Kohgi, N. Aso, M. Sera, F. Iga, M. Matsuura, K. Hirota
Physica B

404/ 17, 2527-2528
2009/09

10.1016/j.physb.2009.06.016

Recently, in phase IV of CexLa1-xB6, weak but distinct superlattice reflections from the order parameter of phase IV have been detected by our unpolarized neutron scattering experiment [K. Kuwahara, K. Iwasa, M. Kohgi, N. Aso, M. Sera, F. Iga, J. Phys. Soc. Japan 76 (2007) 093702]. The scattering vector dependence of the intensity of superlattice reflections is quite unusual; the intensity is stronger for high scattering vectors. This result strongly indicates that the order parameter of phase IV is the magnetic octupole. However, the possibility that the observed superlattice reflections are due to lattice distortions could not be completely ruled out only on the basis of the unpolarized neutron scattering experiment. To confirm that the superlattice reflections are magnetic, therefore, we have performed a single crystal polarized neutron diffraction experiment on Ce0.7La0.7B6. The obtained result has clearly shown that the time reversal symmetry is broken by the order parameter of phase IV. This is further evidence for the magnetic octupole order in CexLa1-xB6.
45 研究論文(学術雑誌)
共著
Neutron Diffraction Study on the Multiple Magnetization Plateaus in TbB4 under Pulsed High Magnetic Field
S. Yoshii, K. Ohoyama, K. Kurosawa, H. Nojiri, M. Matsuda, P. Frings, F. Duc, B. Vignolle, G. L. Rikken, L.-P. Regnault, S. Michimura, F. Iga
Physical Review Letters

103/ 7, 077203(1-4)
2009/08/14

10.1103/PhysRevLett.103.077203

We present the first application of pulsed high magnetic fields up to 30 T for neutron diffraction experiments. As the first study, field variations of a couple of magnetic Bragg reflections have successfully been measured in the frustrated antiferromagnet TbB4. The results show that the conventional models fail, and a model, which is a mixture of the XY- and the Ising-type moments, matches for the half-magnetization state. We deduce an interaction that stabilizes an orthogonal moment arrangement as an origin of the unusual magnetization plateaus. Our results demonstrate the powerfulness of the present pulsed magnetic fields neutron diffraction system.
46 研究論文(学術雑誌)
共著
Magnetic Field Induced 4f Octupole in CeB6 Probed by Resonant X-Ray Diffraction
T. Matsumura, T. Yonemura, K. Kunimori, M. Sera, F. Iga
Physical Review Letters

103/ 1, 017203(1-4)
2009/07/03

10.1103/PhysRevLett.103.017203

A mysterious antiferroquadrupolar ordered phase of CeB6 is considered as originating from the T-xyz-type magnetic octupole moment in magnetic fields. By resonant x-ray diffraction, we have verified that the T-xyz-type octupole is indeed induced in the 4f orbital of Ce with a propagation vector (1/2, 1/2, 1/2), thereby supporting the theory. We observed an asymmetric field dependence of the intensity for an electric quadrupole (E2) resonance when the field was reversed and extracted a field dependence of the octupole by utilizing the interference with an electric dipole (E1) resonance. The result is in good agreement with that of the NMR-line splitting, which reflects the transferred hyperfine field at the boron nucleus from the anisotropic spin distribution of Ce with an O-xy-type quadrupole.
47 研究論文(学術雑誌)
共著
Effect of Nd substitution on the magnetic order in CexNd1−xB6 solid solutions
J.-M. Mignot, J. Robert, G. André, M. Sera, F. Iga
Physical Review B

79/ 22, 224426(1-7)
2009/06/24

10.1103/PhysRevB.79.224426

Neutron powder-diffraction measurements have been performed on CexNd1-xB6 (x=0.5, 0.6, 0.7, and 0.8) solid solutions to determine the type of magnetic order occurring in these compounds as a result of the interplay between magnetic dipole exchange and antiferroquadrupolar interactions. In the Ce-rich range, the sequence of two magnetic phases, with an incommensurate order [k=(1/4-delta,1/4-delta,1/2)] forming below T-N followed by a lock-in-type transition at lower temperature, is quite similar to that reported earlier for CexPr1-xB6. For x=0.5, on the other hand, the same antiferromagnetic order as in pure NdB6 first occurs at T-N, then coexists with an incommensurate component below the lower transition temperature. These results are in good agreement with previous resistivity measurements and support the idea that Ce and Nd magnetic moments in this system can be relatively decoupled.
48 研究論文(学術雑誌)
共著
Kondo lattice effects and the collapse of lattice coherence in Yb1-xLuxB12 studied by hard x-ray photoelectron spectroscopy
J. Yamaguchi, A. Sekiyama, S. Imada, H. Fujiwara, M. Yano, T. Miyamachi, G. Funabashi, M. Obara, A. Higashiya, K. Tamasaku, M. Yabashi, T. Ishikawa, F. Iga, T. Takabatake, S. Suga
Physical Review B

79/ 12, 125121(1-8)
2009/03/30

10.1103/PhysRevB.79.125121

Kondo semiconductor YbB12 and the Lu-substituted Yb1-xLuxB12 (x=1/8) alloy have been studied by the bulk-sensitive hard x-ray (h nu similar to 8 keV) photoelectron spectroscopy at temperatures from 200 down to 20 K. To check the Kondo lattice effects for the bulk electronic states, the temperature dependence of the bulk Yb valence and 4f peak positions in both systems is analyzed with use of the single-impurity Anderson model (SIAM) by considering the crystalline electric field effects. For x=0, the temperature dependence of the bulk electronic states cannot be interpreted within the SIAM, whereas those for x=1/8 could be understood by the SIAM predictions due to the collapse of the lattice coherence by the Lu substitution. These results indicate that the Kondo lattice effects are important for pure YbB12 and play essential roles for forming a narrow gap at low temperatures.
49 研究論文(学術雑誌)
共著
X-ray resonant scattering study of the structural and magnetic transitions in PrB6
H. C. Walker, K. A. McEwen, D. F. McMorrow, M. Bleckmann, J.-G. Park, S. Lee, F. Iga, D. Mannix
Physical Review B

79/ 5, 054402(1-4)
2009/02/02

10.1103/PhysRevB.79.054402

We have made an extensive study of PrB6 using x-ray resonant scattering to investigate both the lattice and magnetic properties. We have identified a structural distortion associated with the incommensurate to commensurate magnetic phase transition at T=4.5 K. Magnetic satellite reflections have been observed in the incommensurate and commensurate phases. The azimuthal dependence of the scattered intensity from the commensurate magnetic satellite reflection at (1/2, 5/4, 5/4) is consistent with the model for the magnetic structure deduced from earlier neutron-diffraction results. Evidence for possible quadrupolar ordering is discussed.
50 研究論文(学術雑誌)
共著
Complex Magnetic Structures of a Shastry-Sutherland Lattice TmB4 Studied by Powder Neutron Diffraction Analysis
S. Michimura, A. Shigekawa, F. Iga, T. Takabatake, K. Ohoyama
Journal of the Physical Society of Japan

78/ 2, 024707(1-5)
2009/01/26

10.1143/JPSJ.78.024707

Powder neutron diffraction analysis was performed on (TmB1)-B-11, which undergoes successive transitions at T-N1 = 11.7K, T* = 11 K, aid T-N2 = 10K. An incommensurate structure with two propagation vectors, k(2) [1 +/- k' +/- k" +/- k", 0] and k(2)' = [1 +/- k' +/- 3k", +/- k", 0] (k' = 0.1226 and k" similar or equal to 0.012). is suggested for phase II (T-3 <= T <= T-N1). Phase III (T-N2 <= T <= T-(sic)) is described by one propagation vector, k(3) = [1 +/- k',0,0]. The propagation vector for phase IV (T <= T-N2) further changes to a commensurate one characterized by k(4) = [100]. Four Tm moments in the c-plane order in a (+-+-) type, where + and - mean the directions of the moments parallel and antiparallel to the c-direction, respectively. The diffuse scattering intensity around the (100) magnetic reflection remains above T-N2 up to 20 K.
51 研究論文(国際会議プロシーディングス)
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Pressure effect on the multipole interactions in CexPr1−xB6
S. Nagai, S. Ikeda, H. Iwakubo, H. Tou, M. Sera, F. Iga
Journal of the Physical Society of Japan

77/ suppl.A, 288-290
2008



We have studied the electrical resistivity of Ce0.8Pr0.2B6 for H //<110> both at the ambient pressure and under pressure of P=1.36GPa. At the ambient pressure, Pr-doping acts to enhance TQ different from the large suppression of TQ in Ce0.7Pr0.3B6. This difference is ascribed to the different magnitude of the magnetic interaction between these two compounds. TN shows the unusual increase with increasing magnetic field, which is explained by considering the effect of the magnetic field induced Txy z-AFO order. By applying the pressure, the low temperature region of the magnetic phase diagram is largely modified. This originates from the existence of the effective ferromagnetic moment induced by the Oxy-AFQ and Txy z-AFO order by the magnetic field.
52 研究論文(国際会議プロシーディングス)
共著
Neutron Diffraction Study of multipole order in light rare-earthhexaborides
J. M. Mignot, J. Robert, M. Sera, F. Iga
Pramana-Journal of Physics

71/ 4 Special Issue, 837-845
2008/10/01

10.1007/s12043-008-0274-y

Multipole interactions are known to play a central role in the unconventional properties of light rare-earth hexaborides and especially of CeB(6). Substituting Pr at the Ce sites has the effect of enhancing exchange interactions and changing the symmetry of the local 4f charge distribution, while suppressing the octupole moment. The (T, H) magnetic phase diagrams of the Ce(x)Pr(1-x)B(6) compounds display a large variety of ordered phases involving magnetic and/or charge degrees of freedom. Here we focus on the compound Ce(0.7)Pr(0.3)B(6), which is located slightly beyond the Pr concentration where the antiferroquadrupolar phase of pure CeB(6) is suppressed in zero field. The different magnetic structures have been characterized by neutron diffraction and their origin is discussed in connection with recent non-resonant X-ray results by Tanaka et al.
53 研究論文(学術雑誌)
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Multistep magnetization plateaus in the Shastry-Sutherland System TbB4
S. Yoshii, T. Yamamoto, M. Hagiwara, S. Michimura, A. Shigekawa, F. Iga, T. Yakabatake, K. Kindo
Physical Review Letters

101/ 8, 087202(1-4)
2008/08/22

10.1103/PhysRevLett.101.087202

We report high-field magnetization and magnetostriction measurements on the rare-earth-metal tetraboride TbB(4), in which the Tb moments form a Shastry-Sutherland lattice in the tetragonal basal plane. A number of magnetization plateaus appear when the magnetic field is perpendicular to the magnetic easy plane. We propose that the magnetization plateaus arise from the combined effect of magnetic frustration and quadrupole interaction in the unique two-dimensional network.
54 研究論文(学術雑誌)
共著
Magnetic order and multipole interactions in CexPr1−xB6 solid solutions
J.-M. Mignot, G. André, J. Robert, M. Sera, F. Iga
Physical Review B

78/ 1, 014415(1-10)
2008/07

10.1103/PhysRevB.78.014415

Magnetic ordering phenomena in Ce(x)Pr(1-x)B(6) solid solutions have been studied using both powder and single-crystal neutron diffraction. A variety of magnetic structures are observed depending on temperature and Ce concentration. Over a broad composition range (x <= 0.7), Pr-Pr interactions play a dominant role, giving rise to incommensurate structures with wave vectors of the form k(IC1)=(1/4-delta,1/4,1/2) or k(IC2)=(1/4-delta,1/4-delta,1/2). The crossover to a CeB(6)-like regime takes place near x=0.7-0.8. For the latter composition, the antiferroquadrupolar phase transition observed in transport measurements precedes the onset, at lower temperature, of a commensurate magnetic order similar to that existing in CeB(6). However, unlike in the pure compound, an incommensurate magnetic order is formed at even higher temperature and persists in the antiferroquadrupolar phase down to the lock-in transition. These results are shown to reflect the interplay between various type of dipole exchange and higher multipole interactions in this series of compounds.
55 研究論文(学術雑誌)
共著
Pressure-induced phase transitions in PrB6 : Electrical resistivity measurements
H. Iwakubo, S. Ikeda, Y. Kishino, H. Tanida, M. Sera, F. Iga
Physical Review B

78/ 1, 012409(1-4)
2008/07

10.1103/PhysRevB.78.012409

We have studied the electrical resistivity of PrB(6) under pressure up to P=0.8 GPa, with magnetic fields applied along the [001] and [111] directions. The commensurate (C) magnetic ordered phase is significantly stabilized by pressure, and another extra phase (denoted phase A) is observed under a low pressure of similar to 0.3 GPa. This phase appears above the transition temperature T(IC1)=7 K at which the incommensurate magnetic order sets in at the ambient pressure. The magnetic phase diagram for H parallel to[111] changes drastically under pressure. Short-range interactions, which are strongly dependent on the Pr-Pr distances, are likely to play a dominant role in the relative stability of the different phases.
56 研究論文(学術雑誌)
共著
Magnetic excitations in the ordered phases of praseodymium hexaboride
M.D. Le, K. A. McEwen, J.G. Park, S. Lee, F. Iga, K.C. Rule
Journal of Physics:Condensed Matter

20/ 10, 10423(1-6)
2008/03/12

10.1088/0953-8984/20/10/104231

We have made the first inelastic neutron scattering measurements on single crystal PrB(6), studying the magnetic excitations in the ordered phases. The lowest energy mode has a minimum energy of approximate to 1.0 meV at the magnetic zone centre, with a significant dispersion along the [001] direction, but very little dispersion along [110]. Interactions out to fourth nearest neighbours are required to explain the [001] dispersion. Measurements of the temperature dependence of the excitations show a strong softening and broadening of the lowest energy mode at the magnetic zone centre between 2 K (commensurate phase) and 5 K (incommensurate phase).
57 研究論文(国際会議プロシーディングス)
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Lattice dynamics and magneto-elastic coupling in Kondoinsulator YbB12
A. V. Rybina, P. A. Alekseev, J. M. Mignot, E. V. Nefeodova, K. S. Nemkovski, R. I. Bewley, N. Yu. Shitsevalova, Yu. B. Paderno, F. Iga, T. Takabatake
Journal of Physics: Conference Series

92, 012074(1-4)
2007

10.1088/1742-6596/92/1/012074

Lattice dynamics and magneto-elastic coupling effects have been studied in the Kondo insulator YbB12 by means of inelastic neutron scattering. The analysis of the phonon density of states, dispersion, and symmetry properties is presented in connection with a possible magneto-elastic coupling. Manifestation of such effects was found for the phonons corresponding to the vibrations of Yb atoms.
58 研究論文(学術雑誌)
共著
Rapid Suppression of Phase IV by Nd Doping in Ce0.7La0.3B6
A. Kondo, H. Tou, M. Sera, F. Iga, T. Sakakibara
Journal of the Physical Society of Japan

76/ 10, 103708(1-4)
2007/10

10.1088/0953-8984/19/40/406224

We have studied the effect of Nd doping on phase IV of Ce0.7NdyLa0.3-yB6 (y = 0.005, 0.01, 0.03, and 0.05) to obtain information on the order parameter in phase IV. Phase IV is rapidly suppressed and phase III is stabilized by the Nd doping. Phase IV disappears at the critical concentration y(c) similar to 0.015 above which only phase III exists below 1.4 K. The macroscopic properties in phase IV are not affected by the Nd doping. For y = 0.01, there exists a region where phases III and IV coexist at a low magnetic field. This mixed state is unstable and is easily changed to phase III by applying the magnetic field. This indicates that phases III and IV are competitive and phase IV is not the magnetic dipole ordered phase. The long-range ordering temperatures from phase IV or III to the paramagnetic region are constant at least up to y = 0.05. This suggests the existence of some relationship between phases III and IV.
59 研究論文(学術雑誌)
共著
Terahertz conductivity of localized photoinduced carriers in a Mott insulator YTiO3 at low excitation density, contrasted with the metallic nature in a band semiconductor Si
J. Kitagawa, Y. Kadoya, M. Tsubota, F. Iga, T. Takabatake
Journal of Physics:Condensed Matter

19/ 40, 406224(1-12)
2007/10

10.1143/JPSJ.76.103708

We performed optical-pump terahertz-probe measurements of a Mott insulator YTiO3 and a band semiconductor Si using a laser diode (1.47 eV) and a femtosecond-pulse laser (1.55 eV). Both samples possess long energy-relaxation times (1.5 ms for YTiO3 and 15 mu s for Si); therefore, it is possible to extract terahertz complex conductivities of photoinduced carriers under equilibrium. We observed highly contrasting behaviour-Drude conductivity in Si and localized conductivity possibly obeying the Jonscher law in YTiO3. The carrier number at the highest carrier-concentration layer in YTiO3 is estimated to be 0.015 per Ti site. Anisotropic conductivity of YTiO3 is determined. Our study indicates that localized carriers might play an important role in the incipient formation of photoinduced metallic phases in Mott insulators. In addition, this study shows that the transfer-matrix method is effective for extracting an optical constant of a sample with a spatially inhomogeneous carrier distribution.
60 研究論文(学術雑誌)
共著
Polarized-Neutron Study of Spin Dynamics in the Kondo Insulator YbB12
K. S. Nemkovski, J.-M. Mignot, P. A. Alekseev, A. S. Ivanov, E. V. Nefeodova, A. V. Rybina, L.-P. Regnault, F. Iga, T. Takabatake
Physical Review Letters

99, 137204(1-4)
2007/09/28

10.1103/PhysRevLett.99.137204

Inelastic neutron scattering experiments have been performed on the archetype compound YbB12, using neutron polarization analysis to separate the magnetic signal from the phonon background. With decreasing temperature, components characteristic for a single-site spin-fluctuation dynamics are suppressed, giving place to specific, strongly Q-dependent, low-energy excitations near the spin-gap edge. This crossover is discussed in terms of a simple crystal-field description of the incoherent high-temperature state and a predominantly local mechanism for the formation of the low-temperature singlet ground state.
61 研究論文(学術雑誌)
共著
Detection of Neutron Scattering from Phase IV of Ce0.7La0.3B6: A Confirmation of the Octupole Order
K. Kuwahara, K. Iwasa, M. Kohgi, N. Aso, M. Sera, F. Iga
Journal of the Physical Society of Japan

76/ 9, 093702(1-4)
2007/09/01

10.1143/JPSJ.76.093702

We have performed a single crystal neutron scattering experiment on Ce0.7La0.3B6 to microscopically investigate the order parameter of phase IV. Below the phase transition temperature 1.5 K of phase IV, weak but distinct superlattice reflections at the scattering vector K = (h/2, h/2, l/2) (h, l = odd number) have been observed for the first time by neutron scattering. The intensity of the superlattice reflections is stronger for high scattering vectors, which is quite different from the usual magnetic form factor of magnetic dipoles. This result directly evidences that the order parameter of phase IV has a complex magnetization density, consistent with the recent experimental and theoretical prediction in which the order parameter is the magnetic octupoles T-beta with Gamma(5) symmetry of the point group Oh. Neutron scattering experiments using short wavelength neutrons, as done in this study, could become a general method to study the high-rank multipoles in f electron systems.
62 研究論文(学術雑誌)
共著
Resonant X-Ray Scattering Study of Quadrupole-Strain Coupling in DyB4
S. Ji, C. Song, J. Koo, J. Park, Y. J. Park, K.-B. Lee, Seongsu Lee, J.-G. Park, J. Y. Kim, B. K. Cho, K.-P. Hong, C.-H. Lee, F. Iga
Physical Review Letters

99/ 7, 076401(1-4)
2007/08/17

10.1103/PhysRevLett.99.076401

X-ray and neutron scattering studies were performed on DyB4 which exhibits both a quadrupolar ordering and a macroscopic lattice distortion. A forbidden reflection at 7.792 keV near the Dy L-3 absorption edge is identified as a quadrupolar ordering peak, and the quadrupolar order and a monoclinic structural distortion develop concomitantly below 12.3 K as second-order-type phase transitions. Coupling between the quadrupolar order and the strain in DyB4 is directly demonstrated by observing that both order parameters are proportional to each other.
63 研究論文(学術雑誌)
共著
X-ray Absorption and Inverse Photoemission Study of Y1-xCaxTiO3
M. Arita, H. Sato, M.i Higashi, K. Yoshikawa, K. Shimada, M. Sawada, Y. Ueda, H. Namatame, M. Taniguchi, S. Fujiomori, Y. Saitoh, M. Tsubota, F. Iga, T. Takabatake
Journal of the Physical Society of Japan

76/ 7, 074720(1-3)
2007/07/01

10.1143/JPSJ.074720

We have performed the O 1s x-ray absorption (XAS) and resonant inverse photoemission spectroscopy (RIPES) in Mott insulator YTiO3, band insulator CaTiO3 and Y0.61Ca0.39TiO3 with a metal-insulator transition. In CaTiO3, the spectrum has strong pi* and sigma* peaks derived from the Ti 3d-O 2p hybridization bands near the Fermi level. On the other hand the peaks almost disappear in the XAS spectrum of YTiO3. In Y0.61Ca0.39TiO3, the intensities of the pi* and sigma* structures increase with decreasing temperature, showing that degree of the p-d hybridization increases from the insulator to metal phases.
64 研究論文(学術雑誌)
共著
Dynamical magnetic correlations in the YbB12 Kondo insulator: Neutron investigations with a polarization analysis
P. A. Alekseev, K. S. Nemkovski, J. M. Mignot, A. S. Ivanov, E. V. Nefeodva, I. P. Sadikov, F. Iga, T. Takabatake
Crystallography Reports

52/ 3, 387-392
2007/05/06

10.1134/S1063774507030066

The results of investigations into the spin dynamics in the YbB12 Kondo insulator with the use of inelastic neutron scattering, including experiments with a polarization analysis, are discussed. It is shown that, at low temperatures, the dynamic magnetic response in the structure of the YbB12 Kondo insulator is characterized by three dispersive excitations with a nontrivial q-dependence of the intensity. An increase in the temperature results in a crossover to the single-site spin fluctuation regime accompanied by suppression of collective excitations.
65 研究論文(国際会議プロシーディングス)
共著
Observation of bulk electronic states of Kondo semiconductor YbB12 by high-resolution soft X-ray photoemission spectroscopy
A. Shigemoto, J. Yamaguchi, S. Imada, A. Yamasaki, A. Irizawa, T. Ukawa, T. Muro, F. Iga, T. Takabatake, S. Suga
Journal of Electron Spectroscopy and Related Phenomena

156, 472-475
2007/05/01

10.1016/j.elspec.2006.12.052

Bulk valence band photoemission spectra of the Kondo semiconductor YbB12 were measured with use of soft X-ray (hv = 700 eV) at 20 and 200 K and the Kondo, resonance behavior was confirmed. Sharp bulk Yb2+ 4f peaks are observed in the vicinity of Fermi level on fractured single crystals. From the spectra is estimated the Yb mean valence as 2.86 +/- 0.01 at 20 K and 2.94 +/- 0.01 at 200 K. The whole peaks of the Yb3+ 4f multiplets are found to shift by similar to 30 meV toward higher binding energies with decreasing the temperature from 200 to 20 K. On the other hand, the shift of the peak energy position of the Yb2+ 4f spectra toward lower binding energies with decreasing the temperature is found to be less than 10 meV. The temperature dependence of the Yb 4f photoemission spectra is well explained by the non-crossing approximation (NCA) calculation based on the single impurity Anderson model (SIAM).
66 研究論文(国際会議プロシーディングス)
共著
High resolution soft X-ray photoemission of kondo insulator YbB12
A. Shigemoto, J. Yamaguchi, S. Imada, A. Sekiyama, A. Yamasaki, A. Irizawa, T. Muro, F. Iga, T. Takabatake, S. Suga
Journal of Electron Spectroscopy and Related Phenomena

156, CIV-CIV
2007/05/01

10.1016/j.elspec.2006.12.052


67 研究論文(国際会議プロシーディングス)
共著
Bulk valence band of Ce(Pd(1-x)Cux)(3) studied by 3d -> 4f resonant photoemission
S. Kasai, S. Imada, A. Yamasaki, A. Sekiyama, F. Iga, M. Kasaya, S. Suga
Journal of Electron Spectroscopy and Related Phenomena

156, 441-444
2007/05/01

10.1016/j.elspec.2006.12.071

CePd(3)is a typical heavy fermion system with Kondo temperature (T-K) of about 250K. By partially substituting Pd with Cu, T-K decreases drastically. In order to clarify how the bulk Ce 4f state depends upon x (x = 0-0.133) in Ce(Pd1-xCux)(3) bulk sensitive soft X-ray photoemission (PES) has been performed around the Ce 3d -> 4f photoabsorption (XAS) edge with a state-of-the-art resolution. The PES spectra have been found to depend drastically on x. A peak just below the Fermi energy (E-F), interpreted as the tail of the Kondo peak just above E-F, dominates the PES spectrum in CePd3. With increasing x, a structure around 0.3eV, interpreted as the spin-orbit partner of the Kondo tail, grows and becomes stronger than the Kondo tail in x = 0.133. The Ce 3d -> 4f XAS spectrum also depends upon x, which is characterized by the decrease of the 3d(9)4f(1) peak reflecting the 4f(0) initial state with increasing x. These results are compared with the calculation based on the single impurity Anderson model (SIAM).
68 研究論文(国際会議プロシーディングス)
共著
Ti 2p soft X-ray emission spectroscopy of Ti2O3
H. Sato, K. Tsuji, K. Yoshikawa, Y. Tezuka, S. Nishimoto, M. Higashi, F. Iga, M. Tsubota, M. Takemura, T. Takabatake, M. Taniguchi
Journal of Electron Spectroscopy and Related Phenomena

156, 365-368
2007/05/01

10.1016/j.elspec.2006.12.050

Ti 3d electronic structure of Ti2O3 has been investigated by means of soft X-ray photoemission spectroscopy (SXES) in the Ti 2p-3d absorption region. In the SXES spectra, the Raman peaks due to the a(1g)-e(g)(pi) and a(1g)-e(g)(sigma) excitations are observed at 1.3 and 3.3 eV below the incident photon energy, respectively. The intensity of the a(1g)-e(g)(pi) peak in the SXES spectrum measured between the 2P(3/2)-3d and 2p(1/2)-3d absorption peaks depends on the polarization of the incident photon E; the peak observed clearly in case of E parallel to the c-axis, almost disappears in case of E perpendicular to the c-axis.
69 研究論文(学術雑誌)
共著
Unoccupied electronic structure of Y1−xCaxTiO3 investigated by inverse photoemission spectroscopy
M. Arita, H. Sato, M. Higashi, K. Yoshikawa, K. Shimada, M. Nakatake, Y. Ueda, H. Namatame, M. Taniguchi, M. Tsubota, F. Iga, and T. Takabatake
Physical Review B

75, 205124(1-7)
2007/05/01

10.1103/PhysRevB.75.205124

We have studied the unoccupied electronic structure of the band insulator CaTiO3, the Mott insulator YTiO3, and Y0.61Ca0.39TiO3 with a metal-insulator transition (MIT) around 150 K by means of Ti 3p-3d resonant inverse photoemission spectroscopy (RIPES). The Ti 3d partial density of states of CaTiO3 deduced from the on- and off-resonance RIPES spectra is in good agreement with results of a band-structure calculation, while that of YTiO3 is explained based on a calculation with the dynamical mean-field theory taking into account an electron correlation. In the case of Y0.61Ca0.39TiO3, we have successfully observed the temperature dependence of RIPES spectra across MIT; the intensity of the incoherent part (a remnant of the upper Hubbard bands) relative to the coherent part (quasiparticle bands) is reduced with decreasing temperature from the insulator to metal phases.
70 研究論文(国際会議プロシーディングス)
共著
High-field Magnetization of TbB4
S. Yoshii, T. Yamamoto, M. Hagiwara, T. Takeuchi, A. Shigekawa, S. Michimura, F. Iga, T. Takabatake and K. Kindo
Journal of Magnetism and Magnetic Materials

310/ 2, 1282-1284
2007/03/01

10.1016/j.jmmm.2006.10.440

The magnetization of single crystalline TbB4 has been investigated in the antiferromagnetically ordered state up to 54 T. At 4.2 K, for B vertical bar vertical bar[0 0 1] the magnetization shows field-induced multi-step jumps and several plateau regions above 16T, and finally reaches saturation at about 28 T. The magnetization in the (0 0 1) plane, on the other hand, shows only one large jump at B-c= 16 and 12 T for B vertical bar vertical bar [1 0 0] and B vertical bar vertical bar[1 1 0], respectively, and gradually increases toward saturation above B-c. The magnetization curves have also been measured at higher temperatures to establish the magnetic phase diagram.
71 研究論文(国際会議プロシーディングス)
共著
Field-induced lattice distortion in single crystal ErB4 observed by X-ray diffraction
S. Michimura, A. Shigekawa, F. Iga, M. Sera, T. Takabatake, A. Kikkawa, Y. Tanaka and K. Katsumata
Journal of Magnetism and Magnetic Materials

310/ 2, e446-e447
2007/03/01

10.1016/j.jmmm.2006.10.843

We have found a field-induced first-order transition in ErB4 for B perpendicular to c in the paramagnetic state by specific heat and X-ray diffraction measurements. The spontaneous distortion observed by the X-ray diffraction indicates that the transition originates from strong multipole interactions.
72 研究論文(国際会議プロシーディングス)
共著
Highly anisotropic magnetic phase diagram of a 2-dimensional orthogonal dimer system TmB4
F. Iga, A. Shigekawa, Y. Hasegawa, S. Michimura, T. Takabatake, S. Yoshii, T. Yamamoto, M. Hagiwara and K. Kindo
Journal of Magnetism and Magnetic Materials

310/ 2, e443-e445
2007/03/01

10.1016/j.jmmm.2006.10.476

We have measured magnetization processes of a Shastry-Sutherland lattice (SSL) TmB4. For B parallel to c, plateaus appear at (1)/(8) and (1)/(2) of the full moment 7 mu B/Tm at 1.7-1.4 and 2-3 T, respectively. For B parallel to a, a metamagnetic transition occurs at similar to 30 T which is followed by a plateau at similar to 32 T. These plateaus are originated from geometric frustration effect in SSL. Magnetic field and temperature dependences of magnetoresistance and specific heat were also measured in detail below T-N = 11.7 K. The magnetic phase diagram possesses complex features which have an antiferromagnetic phase, spin flop saturated one, and a few additional phases by long periodic modulations originated from frustration effect in SSL.
73 研究論文(国際会議プロシーディングス)
共著
Unusual pressure dependence of the multipolar interactions in CexLa1-xB6
S. Ikeda, K. Umeo, H. Tou, M. Sera, F. Iga and S. Kunii
Journal of Magnetism and Magnetic Materials

310/ 2, e163-e165
2007/03/01

10.1016/j.mmm.2006.10.120

We performed the mean field calculation of the magnetization under pressure for the four sublattice model to understand the unusual pressure effect of CeB6. The calculated results are in good agreement with the experimental results and the canted ferromagnetic ground state is predicted to appear at higher pressure. We studied the electrical resistivity of Ce0.75La0.25B6 under pressure. We found that the phase III is rapidly suppressed by pressure and TIV-I increases with pressure. At P = 0.6 GPa, the direct phase transition from IV to II is found, which will be the clue to understanding the phase IV.
74 研究論文(国際会議プロシーディングス)
共著
Magnetic rare-earth ion doping effect on phase IV of Ce0.7La0.3B6
A. Kondo, H. Tou, M. Sera, F. Iga, T. Morie, T. Sakakibara
Journal of Magnetism and Magnetic Materials

310/ 2, e160-e162
2007/03/01

10.1016/j.jmmm.2006.10.119

We have investigated the Pr-doping effect on phase IV of CexLa1-xB6. For Ce0.7Pr0.03La0.27B6, the region of phase III is expanded and the phase IV disappears regardless of only 3% Pr doping. By Pr 8% doping, the phase II which exists above H similar to 0.8 T in Ce0.7Pr0.03La0.27B6 seems to separate the phase III and I at low fields. In Ce0.7Pr0.1La0.2B6, we observed the new phase defined as phase IV ' whose characteristics are very similar to that of phase IV. This phase IV ' exists inside phase II. These indicate that this system is formed under the subttle balance of several kinds of multipole interactions.
75 研究論文(国際会議プロシーディングス)
共著
Ce site substitution effect on the multipolar ordering in CeB6
M. Sera, M. Kawaguchi, S. Kishimoto, A. Kondo, H. Iwakubo, H. Tou, K. Umeo, F. Iga
Journal of Magnetism and Magnetic Materials

310/ 2, e158-e159
2007/03/01

10.1016/j.jmmm.2006.10.118

We have studied the Ce-cite substitution effect by R(= Pr, Nd) ion on the multipole interactions in CeB6. We found that T-Q of Ce0.9Nd0.1B6 in magnetic fields is higher than that of Ce0.9Pr0.1B6 and the magnetization is larger in the former than in the latter. The larger suppression of T-Q in CexPr(1-x)B(6) suggests the larger reduction of the T-xyz antiferro-octupolar (AFO) interaction in this system considering that T-Q at H = 0 is nearly the same in both systems. The smaller magnetization in CexPr1-xB6 than in CexNd1-xB6 originates from the larger suppression of T-xyz AFO interaction and also the larger Ce-Pr AF exchange interaction than in CexNd1-xB6.
76 研究論文(国際会議プロシーディングス)
共著
Terahertz time-domain spectroscopy of photoinduced carriers in YTiO3 at low excitation density, contrasted with the metallic nature in a band semiconductor Si
J. Kitagawa, Y. Kadoya, M. Tsubota, F. Iga, T. Takabatake
Journal of Magnetism and Magnetic Materials

310/ 2, 913-915
2007/03/01

10.1016/j.jmmm.2006.10.139

We have performed an optical characterization of photoinduced carriers in the Mott insulator YTiO3 by using the optical pump-terahertz probe spectroscopy. The complex conductivity, sigma(omega), of photoinduced carriers is deduced to be similar to that of bad metals. The energy relaxation time of the photoinduced carriers is longer than 13 ns. The anisotropic terahertz response of photoinduced state is confirmed.
77 研究論文(国際会議プロシーディングス)
共著
Metal-insulator transition in Y1-xCaxTiO3 studied by X-ray scattering
H. Nakao, S. Kodama, K. Kiyoto, D. Bizen, Y. Murakami, M. Tsubota, F. Iga, T. Takabatake
Journal of Magnetism and Magnetic Materials

310/ 2, 895-897
2007/03/01

10.1016/j.jmmm.2006.10.134

The precise crystal structure of Y1-xCaxTiO3 (x = 0.38) has been investigated by the single crystal X-ray scattering. We have demonstrated the sequence of structural phase transitions, i.e. the high-temperature orthorhombic structure to monoclinic one (Mono) at T-mono and the Mono to the low-temperature orthorhombic structure (LTO) at the lower temperature. It provides a strong evidence that the structural phase transition, LTO-Mono, is responsible for the metal-insulator (MI) transition, though the phase separation of LTO and Mono was reported to be the origin of the MI transition. The pressure-temperature phase diagram of the crystal structure was also determined to elucidate the relation between the MI transition and the LTO-Mono phase transition.
78 研究論文(国際会議プロシーディングス)
共著
Magnetic phase diagram of CexNd1-xB6 solid solutions
J.-M. Mignot, G. Andre, M. Sera, F. Iga
Journal of Magnetism and Magnetic Materials

310/ 2, 738-740
2007/03/01

10.1016/j.jmmm.2006.10.523

We have measured magnetization processes of a Shastry-Sutherland lattice (SSL) TmB4. For B parallel to c, plateaus appear at (1)/(8) and (1)/(2) of the full moment 7 mu B/Tm at 1.7-1.4 and 2-3 T, respectively. For B parallel to a, a metamagnetic transition occurs at similar to 30 T which is followed by a plateau at similar to 32 T. These plateaus are originated from geometric frustration effect in SSL. Magnetic field and temperature dependences of magnetoresistance and specific heat were also measured in detail below T-N = 11.7 K. The magnetic phase diagram possesses complex features which have an antiferromagnetic phase, spin flop saturated one, and a few additional phases by long periodic modulations originated from frustration effect in SSL.
79 研究論文(学術雑誌)
共著
Magnetization Process in GdB4
A. Kikkawa, K. Katsumata, Y. Narumi, K. Suga, T. Fukui, T. Sugaya, K. Kindo, F. Iga, S. Michimura
Journal of the Physical Society of Japan

76/ 2, 024711(1-4)
2007/02/01

10.1143/JPSJ.76.024711

Magnetization measurements are made on single crystal samples of GdB4 under static fields up to 14 T and pulsed fields up to similar to 60 T. When the external magnetic field, H, is applied along the [ 110] spin easy axis, the magnetization increases non-linearly at around 12T, indicating a spin reorientation from the easy axis to the direction perpendicular to H. The values of saturation field, H-s, are, H-s([110]) similar or equal to 52T, H-s([100]) similar or equal to 52T and H-s([001]) similar or equal to 54T. We analyze the results based on the two-dimensional model proposed by Shastry and Sutherland [B. S. Shastry and B. Sutherland: Physica B 108 (1981) 1069]. We obtain J(1)/k(B) = -8.9 K and J(2)/k(B) = -0.68 K, where, J(1) and J(2) are the diagonal exchange interaction and the exchange interaction along the edges of a square, respectively.
80 研究論文(学術雑誌)
共著
Comments on the Hidden Order in Phase IV of CexLa1-xB6
A. Kondo, H. Tou, M. Sera, F. Iga
Journal of the Physical Society of Japan

76/ 1, 013701(1-4)
2007/01/01

10.1143/JPSJ.76.013701

We have studied the effect of the Q(xy)-antiferro-quadrupole (AFQ), T-xyz-antiferro-octupole (AFO), and AF exchange interactions on the Gamma(5u)-AFO order, which is a strong candidate of the order in phase IV of CexLa1-xB6, by mean field calculation for the two-sublattice model. The magnetic phase diagram obtained by the calculation seems to reproduce those of CexLa1-xB6, which seems to provide some clues to understanding phase IV. However, a peak of the magnetic susceptibility disappears easily after introducing the Q(xy)-AFQ interaction, which is because the Q(xy)-FQ order that accompanies the Gamma(5u)-AFO order is easily suppressed by the O-xy-AFQ interaction. The present work indicates that in order to understand the physics of CexLa1-xB6, further studies are necessary both experimentally and theoretically.
81 研究論文(学術雑誌)
共著
Macroscopic Properties in incommensurate magnetic ordered phases of PrB6 in magnetic field around the <111> direction
M. Sera, S. Goto, T. Koshikawa, M.-S. Kim, H. Tou, F. Iga
Journal of the Physical Society of Japan

75, 014706(1-7)
2006/12/26

10.1143/JPSJ.75.014706

We measured the thermal expansion and magnetostriction of PrB6 under the longitudinal magnetic fields up to 15 T for three magnetic field directions. We found the following results in the IC1 and IC2 phases for H parallel to (111). The magnetostriction shows only a small magnetic field dependence in the IC1 phase but exhibits a large shrinkage with increasing magnetic field roughly proportional to -H-2 in the IC2 phase. The thermal expansion exhibits a small change in the IC1 phase but a large increase in the IC2 phase with decreasing temperature. These different behaviors between the IC1 and IC2 phases originate from the existence of both chi(parallel to) and chi perpendicular to components exist in the IC1 phase and only the chi perpendicular to component in the IC2 phase. The characteristic magnetic and magnetoelastic properties of the experimental results are reproduced by the simple calculation for the two sublattice model, although this model does not represent the incommensurate magnetic strucutres of PrB6. The important point is the weight of chi(parallel to) and chi perpendicular to components which determines the macroscopic properties such as the magnetization, magnetostriction and thermal expansion.
82 研究論文(学術雑誌)
共著
Infrared spectroscopy of the intermediate-valence semiconductor YbB12
B. P. Gorshunov, A. S. Prokhorov, I. E. Spektor, A. A. Volkov, M. Dressel, F. Iga
Journal Experimental and Theoretical Physics

103/ 6, 897-903
2006/12/01

10.1134/S1063776106120077

The dynamic conductivity and permittivity spectra of the intermediate-valence compound YbB12 are measured in the frequency range (6-10(4)) cm(-1) (quantum energy 0.75 meV-1.24 eV) at temperatures of 5-300 K. Analysis of the spectral singularities associated with the response of free charge carriers has made it possible for the first time to determine the temperature dependences of their microscopic parameters, viz., concentration, effective mass, relaxation frequency and time, mobility, and plasma frequency. It is shown that the relaxation frequency decreases upon cooling from 300 K to the coherence temperature T* = 70 K for YbB12, which is mainly associated with the phonon mechanism of scattering of charge carriers. For cooling below the coherence temperature T* = 70 K, the temperature dependence of the relaxation frequency for charge carriers of the Fermi-liquid type is found to be gamma similar to gamma(0) + T-2, while their effective mass and relaxation time increase, respectively, to m*(20 K) = 34m(0) (m(0) is the free electron mass) and tau(20 K) = 4 x 10(-13) s, indicating the establishment of coherent scattering of carriers from localized magnetic moments of the f centers. At a temperature of T = 5 K, the conductivity spectrum contains an absorption line at a frequency of 22 cm(-1) (2.7 meV); the origin of this line can be associated with the exciton-polaron bound state. Since such a state was observed earlier in other intermediate-valence semiconductors (such as SmB6, TmSe1-x Te, and (Sm, Y)S), it is probably typical of this class of compounds.
83 研究論文(学術雑誌)
共著
Dynamics of the coherent ground state in intermediate-valent YbB12
B. Gorshunov, P. Haas, O. Ushakov, M. Dressel, F. Iga
Physical Review B

74, 174303(1-5)
2006/11/22

10.1103/PhysRevB.73.045207

The optical properties of the intermediate valence semiconductor YbB12 are studied at frequencies 8 cm(-1)-10 000 cm(-1) (quantum energies 1 meV to 1.2 eV) and temperatures 5 K-300 K. Microscopic parameters of charge carriers are determined: effective mass, mobility, relaxation rate, relaxation time, plasma frequency. A strong decrease of the relaxation rate is observed when cooled down to 70 K, indicating that scattering is mediated mainly by phonons and not by the Kondo effect. Below the coherence temperature of T*=70 K a Drude-like response of renormalized quasiparticles is observed with a scattering rate gamma following the Fermi liquid behavior gamma proportional to T-2, an effective mass m(*)approximate to 34m(0), and an enhanced relaxation time tau=4x10(-13) s, implying that coherent effects play a crucial role in formation of the ground state in YbB12. At T=5 K an absorption peak is found at 2.7 meV whose origin can be ascribed to an exciton-polaronic bound state.
84 研究論文(学術雑誌)
共著
Magnetic frustrations in the Shastry-Sutherland system ErB4
S. Michimura, A. Shigekawa, F. Iga, M. Sera, T. Takabatake, K. Ohoyama, Y. Okabe
Physica B

378-380, 596-597
2006/10/01

10.1016/j.physb.2006.01.161

Magnetization and neutron diffraction measurements on single crystal ErB4 have revealed that ErB4 orders antiferromagnetically below T-N = 15.4 K with an ordering vector k = [100]. The magnetic susceptibility is highly anisotropic, chi(c) >> chi(a). Successive metamagnetic transitions occur with a plateau at the half saturation magnetization (1/2 M-S) for both B//a and B//c. The magnetic phase diagram is presented and discussed in relation to the frustration of the Shastry-Sutherland lattice.
85 研究論文(国際会議プロシーディングス)
共著
Elastic softening mode change in Y1-xCaxTiO3 by Ca-doping
T. Suzuki, S. Morita, H. Higaki, I. Ishii, M. Takemura, F. Iga, T. Takabatake, M.Tsubota
Physica B

378-380, 332-333
2006/10/01

10.1016/j.physb.2006.01.121

We measured various elastic moduli of single-crystalline Y1-xCaxTiO3 (x = 0, and 0.2) as a function of temperature by means of an ultrasonic technique. Remarkable elastic softening has been found in transverse C-44 for the sample with x = 0 around 60 K, indicating structural symmetry lowering corresponding to the e, strain. In contrast, the sample with x = 0.2 shows large softening in C-55 in response to e(zx) around 130K. These results reveal that Y1-xCaxTiO3 (x = 0, and 0.2) undergo a structural transition and the crystal symmetry of the low temperature phase is changed by Ca-doping.
86 研究論文(学術雑誌)
共著
Lattice dynamics in the Kondo insulator YbB12
K. S. Nemkovski, P. A. Alekseev, J.-M. Mignot, A. V. Rybina, F. Iga, T. Takabatake, N. Y. Shitsevalova, T. B. Paderno, V. N. Lazukov, E. V. Nefeodova, N. N. Tiden, I. P. Sadikov
Journal of Solid State Chemistry

179, 2895-2899
2006/10/01

10.1016/j.jssc.2006.01.061

The phonon dispersion in the Kondo-insulator YbB12 and its structure analogue LuB12 has been studied in a wide energy range (up to 55 meV) by means of inelastic neutron scattering. The specific shape of phonon dispersion curves for low-frequency lattice vibrations could be described on the basis of a strong hierarchy suggested for the interactions between boron and rare-earth (RE) atoms: B-B >> B-RE >> RE-RE.
87 研究論文(国際会議プロシーディングス)
共著
Dynamics of boron nanoclusters in RB12 (R = Yb, Lu) systems
K. S. Nemkovski, P. A. Alekseev, A. V. Rybina, J. M. Mignot, K. Flachbart, P. Samuely, N. Yu. Shitsevalova, Yu. B. Paderno, F. Iga, T. Takabatake, V. N. Lazukov, E. V. Nefeodva, I. P. Sadikov, N. N. Tiden, R. I. Bewley
Crystallography Reports

51/ Suupl., S139-S143
2006/10/01

10.1134/S1063774506070200

The phonon dispersion in RB12 (R=Yb, Lu) systems is studied in detail using inelastic neutron scattering over a wide range of energies (up to 55 meV). It is demonstrated that the main features of the lattice dynamics in the RB12 crystals can be described in terms of a simple model of force constants with a strong hierarchy of interatole systems: B-B >> B-R >> R-R. An interesting feature of the low-energy portion of the phonon spectrum is the weakly dispersive optical mode. According to the model calculations, this mode is associated with the relative displacements and "breathing" vibrations of the boron nanoclusters B-12.
88 研究論文(国際会議プロシーディングス)
共著
High-field Magnetization of TmB4
S. Yoshii, T. Yamamoto, M. Hagiwara, A. Shigekawa, S. Michimura, F. Iga, T. Takabatake, K. Kindo
Journal of Physics: Conference Series

51, 59-62
2006/10/01

10.1088/1742-6596/51/1/011

The magnetization M of single crystalline TmB4 has been investigated in high magnetic fields up to 54 T and shows considerably anisotropic behavior. In the ordered antiferromagnetic state, the M for B [001] reaches the saturation magnetization Ms at about 4 T accompanying with plateaux at 1/8 Ms and 1/2 Ms. On the other hand, for B [100] and [110], the 1/2 Ms state and the saturation of M were observed at much higher field above 30 T. The results are discussed in relation to the effects of magnetic frustration and the crystalline electric field.
89 研究論文(国際会議プロシーディングス)
共著
Application of miniature pulsed magnets tosynchrotron X-ray spectroscopy and neutrondiffraction
Y. H. Matsuda, Y. Murata, T. Inami, K. Ohwada, H. Nojiri, K. Ohoyama, N. Katoh, Y. Murakami, F. Iga, T. Takabatake, A. Mitsuda, H. Wada
Journal of Physics: Conference Series

51, 490-493
2006/10/01

10.1088/1742-6596/51/1/111

Synchrotron X-ray and neutron experiments in high magnetic fields have been performed using miniature pulsed magnets. In EuNi2(Si0.18Ge0.82)2, the drastic change of the L3-edge absorption spectrum is found at the valence state transition. While, no changes of the spectra are found at the insulator-metal transitions in Kondo insulators YbB12 and Ce3Bi4Pt3. As a test experiment the high field neutron diffraction experiment of MnF2 has been carried out at JRR-3M reactor.
90 研究論文(学術雑誌)
共著
Erratum:Determination of the orbital polarization in YTiO3 by using soft X-ray linear dichroism (vol 93, art no 257507, 2004)
F. Iga, M. Tsubota, M. Sawada, H. B. Huang, S. Kura, M. Takemura, K. Yaji, M. Nagira, A. Kimura, T. Jo, T. Takabatake, H. Namatame, and M. Taniguchi
Physical Review Letters

97/ 13, 139901
2006/09/29

10.1103/PhysRevLett.97.139901

Erratum:
PACS:
75.47.Pq, 71.30.+h, 75.50.Dd, 78.70.Dm, 99.10.Cd
91 研究論文(学術雑誌)
共著
Resonant X-ray Scattering Study at Y K-edge in Y1- xCaxTiO3
H. Nakao, S. Kodama, K. Kiyoto, Y. Murakami, M. Tsubota, F. Iga, K. Uchihira, T. Takabatake, H. Ohsumi, M. Mizumaki and N. Ikeda
Journal of the Physical Society of Japan

75/ 9, 094706(1-5)
2006/09/11

10.1143/JPSJ.75.09470

Resonant X-ray scattering (RXS) in Y1-xCaTiO3 has been investigated near the Y K-edge. The resonant signal has been clearly found at a forbidden reflection, (500), and the intensity shows an azimuthal angle dependence with a twofold symmetry. The RXS intensities have almost no Ca concentration dependence, although the RXS intensities at the Ti K-edge markedly decrease with increasing Ca concentration. The origin of the RXS at the Y K-edge is clarified to be the deviation from 180 degrees of the Ti-O-Ti bond angle, namely, the tilts of the octahedra around the Y ion. We also discuss the RXS mechanism at the main edge of the Ti K-edge.
92 研究論文(国際会議プロシーディングス)
共著
Elastic anomaly around a ferromagnetic transition in Y1-xCaxTiO3 (x=0-0.2)
S. Morita, H. Higaki, I. Ishii, M. Takemura, F. Iga, T. Takabatake, M. Tsubota, T. Suzuki
Physica B

383/ 1, 43-44
2006/08/15

10.1016/j.physb.2006.03.049

We measured temperature dependence of elastic modulus for the high-quality single crystals of Y1-xCaxTiO3 (x = 0,0.1 and 0.2) using an ultrasonic technique. A step like elastic anomaly due to the volume magneto stricti on induced by a ferromagnetic transition was observed in longitudinal C-11 and C-33 for x = 0 and 0.1, although no anomaly was observed for x = 0.2. With increasing x, the magnitude of anomaly decreases, reflecting that the amplitude of the anti ferromagnetic orbital order parameter, where the orbital ordering is the origin of the ferromagnetism of YTiO3, is reduced with increasing Ca concentration.
93 研究論文(学術雑誌)
共著
Magnetic phase diagram of Ce0.7Pr0.3B6
J. M. Mignot, M. Sera, F. Iga
Physica B

383/ 1, 41-42
2006/08/15

10.1016/j.physb.2006.03.048

Low-temperature, high-field (H-[1 (1) over bar 0] <= 7.5 T), neutron diffraction experiments on single-crystal (Ce0.70Pr0.30B6)-B-11 are reported. Two successive incommensurate phases are found to exist in zero field. The appearance, for H >= 4.6 T at T = 2 K, of an antiferromagnetic structure with kAF = ((1)/(2), (1)/(2), (1)/(2)), most likely due to an underlying antiferroquadrupolar order, is discussed in connection with recent X-ray experiments.
94 研究論文(国際会議プロシーディングス)
共著
Incommensurate and commensurate quadrupole orders in Ce0.7Pr0.3B6
Y. Tanaka, M. Sera, Y. Tabata, S. Shimomura, A. Kikkawa, K. Katsumata, F. Iga, S. Kishimoto
Physica B

383/ 1, 39-40
2006/08/15

10.1016/j.physb.2006.03.047

Non-resonant X-ray diffraction experiments were performed on Ce0.7Pr0.3B6. We have found superlattice reflections for commensurate (n)/(2) (n)/(2) (n)/(2), where n are odd integers and incommensurate h +/- 26(k)/(2) l, where h and l are integers, and k are odd integers, at the low temperature phases in zero magnetic field.
95 研究論文(国際会議プロシーディングス)
共著
Nd-doping effect on phase IV of CexLa1-xB6
A. Kondo, H. Tou, M. Sera, F. Iga
Nd-doping effect on phase IV of CexLa1-xB6

383/ 1, 35-36
2006/08/15

10.1016/j.physb.2006.03.045

We have investigated the Nd-doping effect on phase IV of CexLa1-xB6. The shape of the specific heat of Ce0.7Nd0.03La0.27B6 is changed drastically from that of Ce0.7La0.3B6. The magnetic phase diagram is changed largely only by a small amount of Nd ion doping except the phase boundaries II-I and III-II. Phase IV disappears only by Nd doping of 0.03 and the ground state is changed to the antiferromagnetic one. These results indicate that Nd ions are compatible with the order parameters in phases II and III but not with that in phase IV.
96 研究論文(国際会議プロシーディングス)
共著
Neutron scattering study of spin and lattice dynamics in YbB12
J. M. Mignot, P. A. Alekseev, K. S. Nemkovski, E. V. Nefeodova, A. V. Rybina, L. P. Regnault, N. Yu. Shitsevalova, F. Iga, T. Takabatake
Physica B

383/ 1, 16-19
2006/08/15




97 研究論文(学術雑誌)
共著
Study of phonon anomalies of rare earth hexaborides RB6
M. Kohgi, K. Kuwahara, N. Ogita, M. Udagawa, F. Iga
Journal of the Physical Society of Japan

75/ 8, 085003(1-2)
2006/08/01



SHORT NOTE, KEYWORDS: neutron inelastic scattering, phonons at brillouin zone boundary, RB6, valence dependence of rare-earth ion, temperature dependence of phonon energy and intensity
98 研究論文(学術雑誌)
共著
Direct observation of an incommensurate multipolar order in CeB6 doped with Pr
Y. Tanaka, M. Sera, K. Katsumata, S. W. Lovesey, Y. Tabata, S. Shimomura, A. Kikkawa, F. Iga, S. Kishimoto
Journal of the Physical Society of Japan

75/ 7, 073702(1-4)
2006/07/01

10.1143/JPSJ.75.073702

Observations on an ordered alloy Ce0.7Pr0.3B6 by non-resonant X-ray diffraction are reported. We find incommensurate reflections (26, 1/2,0) (delta similar to 0.23) below about 3 K in zero applied magnetic field. The wave vector dependence of the intensity is compelling evidence for the first direct observation of an incommensurate multipolar ordering (MPO). The incommensurate MPO appears to coexist with a commensurate MPO with a wave vector (1/2, 1/2, 1/2), which is the ground state configuration. Additional insight to the properties is derived from extensive measurements made with the alloy placed in a magnetic field ranging up to 6T. We propose that a competition among RKKY-like exchange interactions between the Ce-Ce, Ce-Pr, and Pr-Pr multipoles is a main source for the appearance of the incommensurate MPO.
99 研究論文(学術雑誌)
共著
Magnetic structure of GdB4 from spherical neutron polarimetry
J. A. Blanco, P. J. Brown, A. Stunault, K. Katsumata, F. Iga, and S. Michimura
Physical Review B

73, 212411(1-4)
2006/06/14

10.1103/PhysRevB.73.212411

Interest in the antiferromagnetic ordering on GdB4 has been recently renewed by the observation of interference between magnetic and anisotropic charge contributions obtained from resonant x-ray scattering experiments at the Gd L-3 edge. Models accounting for the x-ray experiments with collinear and noncollinear Gd magnetic moments are almost indistinguishable from the experimental data available. However, the ambiguity between the different arrangements can be resolved directly and much more easily using spherical neutron polarimetry. The present findings show that the Gd moments order noncollinearly in a structure with the Shubnikov magnetic space group P4/m(')b(')m('), which has magnetoelectric symmetry.
100 研究論文(学術雑誌)
共著
Thermopower of CexR1-xB6 (R = La, Pr and Nd)
M.-S. Kim, Y. Nakai, H. Tou, M. Sera, F. Iga, T. Takabatake, S. Kunii
Journal of the Physical Society of Japan

75/ 6, 064704(1-4)
2006/05/25

10.1143/JPSJ.75.064704

The thermopower, S, of CexR1-xB6 (R = La, Pr, Nd) was investigated. S with a positive sign shows a typical behavior observed in the Ce Kondo system, an increase with decreasing temperature at high temperatures and a maximum at low temperatures. The S values of all the systems at high temperatures are roughly linearly dependent on the Ce concentration, indicating the conservation of the single-impurity character of the Kondo effect in a wide x range. However, the maximum value of S, Smax, and the temperature, Tmax, at which Smax is observed exhibit different x dependences between CexLa1-xB6 and CexR1-xB6 (R = Pr, Nd). In CexLa1-xB6, Tmax, which is 8K in CeB6, decreases with decreasing x and converges to ~ 1K in a very dilute alloy and Smax shows an increase below x ~ 0.1 after decreasing with decreasing x. In CexR1-xB6 (R = Pr, Nd), Tmax shows a weak x dependence but Smax shows a roughly linear decrease in x. These results are discussed from the standpoint of the chemical pressure effect and the Ce–Ce interaction. S in the long-range ordered phase shows very different behaviors between CexPr1-xB6 and CexNd1-xB6.
101 研究論文(学術雑誌)
共著
Ti 3d orbital change across metal-insulator transition in Ti2O3: Polarization-dependenct soft X-ray spectroscopy at Ti 2p edge
H. Sato, A. Tanaka, M. Sawada, F. Iga, K. Tsuji, M. Tsobota, M. Takemura, K. Yaji, M. Nagira, A. Kimura, T. Takabatake, H. Namatame, M. Taniguchi
Journal of the Physical Society of Japan

75, 053702(1-4)
2006/04/25

10.1143/JPSJ.75.053702

The Ti 3d electronic structure of Ti2O3 across metal–insulator transition (MIT) has been investigated by means of polarization-dependent soft X-ray absorption spectroscopy (XAS) at the Ti 2p edge. The XAS spectra exhibit a remarkable linear dichroism (LD) between the polarization vectors of the incident light parallel and perpendicular to the Ti ion pair along the c-axis. LD intensity is significantly reduced at 520K in the metallic phase in comparison with that at 300K in the insulating phase, indicating a large change in a_1g and e^π_g orbital occupations. The results are qualitatively consistent with a recent theory, where the MIT is due to the change of the 3d electronic configuration in the Ti pair along the c-axis driven by a change in the t_2g level splitting relative to the on-site exchange interaction.
102 研究論文(学術雑誌)
共著
High-resolution photoemission study of the temperature-dependent c-f hybridization gap in the Kondo semiconductor YbB12
Y. Takeda, M. Arita, M. Higashiguchi, K. Shimada, H. Namatame, M. Taniguchi, F. Iga, T. Takabatake
Physical Review B

73, 033202(1-4)
2006/01/24

10.1103/PhysRevB.73.033202

The temperature-dependent metal-to-insulator transition in the Kondo semiconductor YbB12 single crystal has been studied by means of high-resolution photoemission spectroscopy with a tunable photon energy. An energy gap in the valence band is gradually formed below T1 ~ 150 K, and at the same time, the Yb 4f_7/2 Kondo peak at 55 meV grows and shifts to a lower binding energy. Below T2 ~ 60 K, an additional spectral feature at 15 meV becomes apparent in the Yb 4f and Yb 5d derived spectra, indicating a strongly hybridized character. The 15 meV feature in the Yb 4f derived spectra is intense at the L point of the Brillouin zone and diminishes away from the L point. These results indicate that the energy gap is formed by the hybridization
between the Yb 4f and Yb 5d states.
103 研究論文(学術雑誌)
共著
ESR of Yb3+ ions in a single crystal of the fluctuating-valence compound YbB12
T. S. Altshuler, Y. V. Goryunov, M. S. Bresler, F. Iga, A. E. Solomey, T. Takabatake
APPLIED MAGNETIC RESONANCE

28/ 1-2, 165-171
2005



The temperature and angular dependences of electron spin resonance (ESR) spectra of Yb3+ ions in a single crystal of fluctuating-valence compound YbB12 were studied. The existence of Yb-Yb ion pairs was observed in a cubic-symmetry crystal. The ions forming the pairs are coupled by the isotropic exchange but interact also with the other pairs by the dipole and exchange coupling. The occurrence of a slight anisotropy in a cubic semiconductor may be the result of a spontaneous break of symmetry specific for the ground state of the Kondo dielectric. A strong temperature dependence of the amplitude of the ESR signals is found at 1.6-4.2 K and interpreted as a result of the capture of electrons by Yb3+ ions from electron traps with a binding energy of 18 K. ESR spectra of Yb3+ single ions in the Gamma(6) state were observed also. The decrease of temperature from 4.2 to 1.6 K indicates a tendency to the ferromagnetic ordering of Yb-Yb pairs.
104 研究論文(学術雑誌)
共著
Coupling between orbital and lattice degrees of freedom in Y1- xCaxTiO3 (0 < x < 0.75): A resonant X-ray scattering study
M. Tsubota, F. Iga, K. Uchihira, T. Nakano, S. Kura, T. Takabatake, S. Kodama, H. Nakao, Y. Murakami
Journal of the Physical Society of Japan

74/ 12, 3259-3266
2005/10/13

10.1143/JPSJ.74.3259

We have studied the orbitally ordered states in Y1-xCaxTiO3 (0 < x < 0.75) by resonant X-ray scattering (RXS) near the Ti K-edge (E ~ 5 keV) and by powder X-ray diffraction measurements. The RXS intensities at the 1s→4p transition energy (main-edge) gradually decrease with increasing x and remain finite even in the metallic phase for x > 0.4. In contrast, the intensities at the 1s→3d transition energy (pre-edge) decrease rapidly with increasing x up to 0.15 and disappear at x ~ 0.4. The x dependences of the Jahn–Teller distortion is consistent with those of the RXS intensities at pre-edge. Namely, the RXS intensity at the pre-edge energy reflects the orbital ordered state. From our results, we conclude that the magnetic order in Y1-xCaxTiO3 becomes unstable at x ~ 0.15 when the orbital order is weakened, and the metallic phase appears in x > 0.4 when the order of orbitals melts.
105 研究論文(学術雑誌)
共著
Rapid Suppression of the Commensurate Magnetic Ordered Phase of PrB6 by La Doping
M. Sera, S. Goto, T. Koshikawa, M.-S. Kim, H. Tou, F. Iga, Y. Mitsukawa, K. Kojima
Journal of the Physical Society of Japan

74/ 10, 2691-2694
2005/10/01

10.1143/JPSJ.74.2691

We have studied the magnetization, electrical resistivity and magnetic phase diagram of PrxLa1-xB6. The x dependence of the incommensurate (IC) magnetic ordering temperature, T_IC1, is similar to that of NdxLa1-xB6. The noncollinear commensurate magnetic ordering is rapidly suppressed by La doping. These suggest that the IC magnetic ordering originates from the long-range RKKY-type exchange interaction and that the short-range Oxy-type AFQ interaction, which is easily affected by the local disturbance, is associated with the formation of the commensurate magnetic ordering in the C phase. The magnetic phase diagram of Pr0.8La0.2B6 is very different from those of PrB6 and Pr0.9La0.1B6. Although there exists the C and IC1 phases at H = 0 in the latter two compounds, there exists only the IC1 phase at H = 0 in Pr0.8La0.2B6. However, the C phase appears above 6 T only for H // <111>. The unusual magnetic phase diagram for H // <111> is explained as a result of the competition between the exchange,
quadrupolar and Zeeman interactions by taking into account the domain distribution, that depends on the applied magnetic field direction.
106 研究論文(学術雑誌)
共著
Search for Orbital Excitation in YTiO3 by Neutron Scattering
S. Shamoto, M. Tsubota, F. Iga, B. Fak, T. Kajitani
Journal of Neutron Research

13, 175-181
2005/10/01




107 研究論文(学術雑誌)
共著
Experimental evidence of noncollinear magnetism in gadolinium tetraboride
J. Fernandez-Rodriguez, J. A. Blanco, P. J. Brown, K. Katsumata, A. Kikkawa, F. Iga, S. Michimura
Physical Review B

72, 052407(1-7)
2005/08/22

10.1103/PhysRevB.72.052407

Recently, the collinear or noncollinear character of the magnetic structure of gadolinium tetraboride (GdB4), laying the magnetic moments in the ab plane, becomes a matter of controversy. In the present study, the observations obtained from resistivity, specific heat, and magnetic susceptibility measurements on a single crystal sample contain no evidence for a structural phase transition in this material above or below the Neel temperature T-N. In addition to these measurements, the resonant x-ray Bragg diffraction structure factors derived from all the possible arrangements for Gd magnetic moments compatible with the room temperature tetragonal P4/mbm space group are calculated. The intensity derived with Shubnikov magnetic P4/m(')b(')m(') and P4(')/m(')b(')m space groups are found to account for the previously published resonant x-ray diffraction data on GdB4. (Both Shubnikov groups have magnetoelectric symmetry.)
108 研究論文(学術雑誌)
共著
Possible coexistence of quadrupolar and magnetic ordering in Ce0.7Pr0.3B6
S. Kishimoto, A. Kondo, M.- S. Kim, H. Tou, M. Sera, F. Iga
Journal of the Physical Society of Japan

74, 2913-2916
2005/08/19

10.1143/JPSJ.74.2913

We have investigated the physical properties of Ce0.7Pr0.3B6 in order to clarify the doping effect on the multipolar interactions in CeB6 and PrB6. As a result of the suppression of the antiferro-quadrupolar ordering temperature T-Q and the enhancement of T-N by Pr doping, six long-range ordered (LRO) phases appear. T-Q exhibits a smooth increase with increasing magnetic field as in CeB6. At a high magnetic field, above similar to 10 T, only phase II exists. As for the three LRO phases below similar to 10 T and below T-Q, there exists a possibility of incommensurate quadrupolar and magnetic ordering. A new question of how the quadrupolar ordering is compatible with the magnetic ordering appears in the present study. The coexistence of the magnetic and quadrupolar orderings in Ce0.7Pr0.3B6 indicates that the strong Ce-Pr interaction is different from the weak Ce-Nd interaction in CexNd1-xB6.
109 研究論文(学術雑誌)
共著
Evidence for Short-Range Antiferromagnetic Fluctuations in Kondo-Insulating YbB12
J.-M. Mignot, P. A. Alekseev, K. S. Nemkovski, L.-P. Regnault, F. Iga, T. Takabatake
Physical Review Letters

94, 247204(1-4)
2005/06/21

10.1103/PhysRevLett.94.247204

The spin dynamics of mixed-valence YbB12 has been studied by inelastic neutron scattering on a high-quality single crystal. In the Kondo-insulating regime realized at low temperature, the spectra exhibit a spin-gap structure with two sharp, dispersive, in-gap excitations at hω ~ 14.5 and  ~ 20 meV. The lower mode is shown to be associated with short-range correlations near the antiferromagnetic wave vector q0 = (1/2, 1/2, 1/2). Its properties are in overall agreement with those expected for a "spin-exciton" branch in an indirect hybridization gap semiconductor.
110 研究論文(国際会議プロシーディングス)
共著
High resolution soft X-ray photoernission of Kondo insulator YbB12
A. Shigemoto, S. Imada, A. Sekiyama, A. Yamasaki, A. Irizawa, T. Muro, Y. Saitoh, F. Iga, T. Takabatake, S. Suga
Journal of Electron Spectroscopy and Related Phenomena

144, 671-673
2005/06/05

10.1016/j.elspec.20005.01.144

We have measured photoemission spectra of a single crystal YbB12, which is known to be a Kondo insulator, with high energy resolution at 700 eV in the temperature range between 200 and 20 K to study the Kondo resonance behavior in the bulk. The sharp bulk Yb2+ 4f(13) peaks observed on fractured YbB12, which is only weakly influenced by the surface electronic structures, facilitate high accuracy evaluation of the temperature dependences of the Kondo peak energy and the bulk valence.
111 研究論文(国際会議プロシーディングス)
共著
High-resolution soft X-ray photoemission study of a Kondo semiconductor and related compounds
A. Sekiyama, Y. Fujita, M. Tsunekawa, S. Kasai, A. Shigemoto, S. Imada, D. T. Adroja, T. Yoshino, F. Iga, T. Takabatakae, T. Nanba, S. Suga
Journal of Electron Spectroscopy and Related Phenomena

144, 655-657
2005/06/05

10.1016/j.elspec.2005.01.099

We have performed the bulk-sensitive high-resolution soft X-ray photoemission study of a Kondo semiconductor CeRhAs and related compounds CeNiSn and CePdSn. The comparison of the spectra of polycrystalline CePdSn on the fractured and scraped surfaces show that the fracturing of the samples is much better than the scraping in order to obtain intrinsic photoemission spectra. The Cc 4d core-level spectra show clear differences in the electronic states among the materials.
112 研究論文(学術雑誌)
共著
Indirect and Direct Energy Gaps in Kondo Semiconductor YbB12
H. Okamura, T. Michizawa, T. Nanba, S. Kimura, F. Iga, T. Takabatake
Journal of the Physical Society of Japan

74, 1954-1957
2005/05/02

10.1143/JPSJ.74.1954

The optical conductivity [σ(ω)] of the Kondo semiconductor YbB12 has been measured over wide ranges of temperature (T = 8-690 K) and photon energy (hω > 1.3 meV). The σ(ω) data reveal the entire crossover of YbB12 from a metallic electronic structure at high T's to a semiconducting one at low T's. Associated with the gap development in σ(ω), a clear onset is newly found at hω = 15 meV for T < 20 K. The onset energy is identified as the gap magnitude of YbB12 appearing in σ(ω). This gap in σ(ω) is interpreted as the indirect gap, which has been predicted by the renormalized-band model of the Kondo semiconductor. On the other hand, the strong mid-infrared (mIR) peak observed in σ(ω) is interpreted as arising from the direct gap. The absorption coefficient around the onset and the mIR peak indeed show the characteristic energy dependences expected for indirect and direct optical transitions in conventional semiconductors.
113 研究論文(国際会議プロシーディングス)
共著
Raman scattering study of rare-earth hexaboride
N. Ogita, S. Nagai, M. Udagawa, F. Iga, M. Sera, T. Oguchi, J. Akimitsu, S. Kunii
Physica B

359-361, 941-943
2005/04/30

10.1016/j.physb.2005.01.270

The excitation-energy, polarization, pressure, and temperature dependences of Raman scattering spectra have been
measured for the RB6 crystals (R = Ca2+, La3+, Ce3+, Pr3+, Sm2.6+, Gd3+, Dy3+, and Yb2+). In the Raman spectra, the extra peaks have been clearly observed at around 200 cm-1; except for the Raman-active phonons and CEF excitations. The extra peaks show anomalous behavior in the temperature, polarization, and lattice parameter dependences. These anomalous behaviors originate from the vibration of the rare-earth ions in octahedral B6 cage, and the extra peaks can be assigned as the second-order Raman excitations of T1u phonon at Brillouin zone boundary. In the results of pressure dependence, the valence of all Sm ions in SmB6 becomes trivalent above 20 GPa at room temperature.
114 研究論文(国際会議プロシーディングス)
共著
Soft X-ray photoemission study of CeRhAs and related compounds: observation of a bulk pseudo-gap
A. Sekiyama, Y. Fujita, M. Tsunekawa, S. Kasai, A. Shigemoto, S. Imada, D. T. Adroja, T. Yoshino, F. Iga, T. Takabatakae, T. Nanba, S. Suga
Physica B

359-361, 115-117
2005/04/30

10.1016/j.physb.2005.01.007

We report bulk 4f andnon-4f spectral functions near the Fermi level (EF) of a Kondo semiconductor CeRhAs and its related compound CePdSn by virtue of high-resolution soft X-ray photoemission. The non-4f spectrum of CeRhAs measured at 20K shows a shoulder at the binding energy of ~ 0.18 eV and almost negligible spectral weight at EF. Although a peak similar to a tail of the Kondo resonance is observed in the 4f spectrum of CeRhAs, the intensity at EF is suppressed compared with those of metallic Ce compounds. These facts are attributed to signatures of the formation of a pseudo-gap in CeRhAs.
115 研究論文(学術雑誌)
共著
Pressure effects on the phase transitions and energy gap in CeRhAs
K. Umeo, K. Masumori, T. Sasakawa, F. Iga, T. Takabatake, Y. Ohishi, T. Adachi
Physical Review B

71, 64110(1-7)
2005/02/28

10.1103/PhysRevB.71.064110

We report on the electrical resistivity, thermal expansion, and x-ray diffraction measurements of single crystalline
sample of the so-called Kondo semiconductor CeRhAs under pressures up to 3 GPa. This compound undergoes successive structural phase transitions at T1=360, T2=235, and T3=165 K at ambient pressure. On cooling below T1, the crystal structure changes from the hexagonal LiGaGe-type to the orthorhombic ε-TiNiSi-type with a 2b x 2c superlattice. By applying pressure up to 1.5 GPa, T1 increases with a ratio of 270 K/GPa, whereas both T2 and T3 decrease with -100 K/GPa. The concurrent decrease of both the a parameter and the energy gap along the a-axis with increasing pressure contradict the c-f hybridization gap model in which the gap is enlarged by the enhancement of hybridization between the 4f electrons and conduction band. Instead, a sort of charge-density-wave transition at T1 is proposed for the origin of gap formation of this compound. The semiconducting behavior in the resistivity vanishes when the phase with the 2b x 2c superlattice decomposes into two orthorhombic phases below 100 K and above 1.5 GPa.
116 研究論文(学術雑誌)
共著
Structural, transport, and thermal properties of the single-crystalline type-VIII clathrate Ba8Ga16Sn30
D. Huo, T. Sakata, T. Sasakawa, M. A. Avila, M. Tsubota, F. Iga, H. Fukuoka, S. Yamanaka, S. Aoyagi, T. Takabatake
Physical Review B

71, 75113(1-6)
2005/02/18

10.1103/PhysRevB.71.075113

We report the electrical resistivity ρ, Hall coefficient RH, thermoelectric power S, specific heat C, and thermal conductivity κ on single crystals of the type-VIII clathrate Ba8Ga16Sn30 grown from Sn-flux. Negative S and RH over a wide temperature range indicate that electrons dominate electrical transport properties. Both ρ(T) and S(T) show typical behavior of a heavily doped semiconductor. The absolute value of S increases monotonically to 243 μV/K with increasing temperature up to 550 K. The large S may originate from the low carrier concentration n=3.7x10^19 cm-3. Hall mobility μH shows a maximum of 62 cm^2 /V s around 70 K. The analysis of temperature dependence of μH suggests a crossover of a dominant scattering mechanism from ionized impurity to acoustic phonon scattering with increasing temperature. The existence of local vibration modes of Ba atoms in cages composed of Ga and Sn atoms is evidenced by analysis of experimental data of structural refinement and specific heat, which give an Einstein temperature of 50 K and a Debye temperature of 200 K. This local vibration of Ba atoms should be responsible for the low thermal conductivity (1.1 W/m K at 150 K). The potential of type-VIII clathrate compounds for thermoelectric application is discussed.
117 研究論文(学術雑誌)
共著
Determination of the Orbital Polarization in YTiO3 by Using Soft X-Ray Linear Dichroism
F. Iga, M. Tsubota, M. Sawada, H. B. Huang, S. Kura, M. Takemura, K. Yaji, M. Nagira, A. Kimura, T. Jo,T. Takabatake, H. Namatame, M. Taniguchi(第1著者、他12名)
Physical Review Letters

93, 257207(1-4)
2004/12/17

10.1103/PhysRevLett.93.257207

直線偏光の放射光を用いて,線2色性即ちX線吸収差分スペクトルよりYTiO3の軌道秩序の軌道分極方向を決定することに成功した。
118 研究論文(学術雑誌)
共著
Search for orbitons in LaMnO3, YTiO3 and KCuF3 using high-resolution inelastic x-ray scattering
Y. Tanaka, A. Q. R. Baron, Y-J. Kim, K. J. Thomas, J. P. Hill, Z. Honda, F. Iga, S. Tsutsui, D. Ishikawa, C. S. Nelson
New Journal of Physics

6, 161(1-3)
2004/12/01

10.1088/1367-2630/6/1/161

Orbital excitations have been sought in three systems, LaMnO3, KCuF3 and YTiO3, using high-resolution inelastic x-ray scattering, with an energy resolution of 6 meV. Motivated by the recent Raman scattering result of Saitoh et al (2001 Nature 410 180), we measured the energy transfer spectra of LaMnO3 in the energy range up to 200 meV. We did not find any signal above background that could be associated with orbiton excitations. Since significant interaction between the spin and orbital degrees of freedom is expected in KCuF3 and YTiO3, energy spectra were measured above and below the respective magnetic ordering temperature. We were not able to detect any change in the excitation spectra due to the magnetic ordering temperature of these materials. We discuss the implications of the experimental findings and estimate an upper bound on the orbiton x-ray cross-section.
119 研究論文(国際会議プロシーディングス)
共著
Optical study of c-f hybridization states in mixed-valent Yb compounds: metallic YbAl3 vs semiconducting YbB12
H. Okamura, T. Michizawa, M. Matsunami, S. Kimura, T. Nanba, T. Ebihara, F. Iga, T. Takabatake
Journal of Magnetism and Magnetic Materials, supplement1

272-276, e51-e52
2004/12/01



We have measured the optical conductivity spectra σ(ω) of YbB12 and YbAl3, which are typical mixed-valent Yb compounds. The measured σ(ω) spectra in the mid-infrared region are quite similar for the two compounds, with a pronounced peak centered near 0.2 eV. This result demonstrates that the Kondo semiconductor YbB12 and the highly metallic YbAl3 share very similar electronic structures near the Fermi level, in spite of their very different transport properties at low temperatures. This observation is discussed in terms of c–f hybridization states in these compounds.
120 研究論文(学術雑誌)
共著
Phase separation in Y1-xCaxTiO3 associated with the insulator-to-metal transition: Observation by transmission electron microscopy
H. Matsuhata, F. Iga, M. Tsubota, T. Nakano, T. Takabatake, K. Kato
Physical Review B

70, 134109(1-6)
2004/10/26

10.1103/PhysRevB.70.134109

Structural changes in Y1-xCaxTiO3 (x=0.37, 0.39 and 0.41) during a transition from an insulator-to-metal were investigated by transmission electron microscopy. In a specimen for x=0.39, on cooling a transition from orthorhombic to monoclinic lattice was observed first. The onset of this transition was not certain, but the monoclinic lattice was clearly observed at 170 K. On further cooling to 140 K, approximately the metal-insulator transition temperature, an introduction of strain to this monoclinic phase was observed. Below 140 K a state of phase separation, with growth of domains, was observed. In the insulator specimen for x=0.37 the transition from orthorhombic to monoclinic lattice was observed similarly to the case of x=0.39, but no phase separation was observed in the range from room temperature to 110 K. In the case for x=0.41, the phase separation was also observed on cooling, but transition temperature of the phase separation is lower than that reported by x-ray diffraction. The phase separation observed presently for 0.39 and 0.41 is considered to be a kind of electrical phase separation, which drives the appearance of the metallic state at low temperatures. This is the first report on observation of separated domains on the electrical phase separation associated with the metal-insulator transition in paramagnetic Y1-xCaxTiO3.
121 研究論文(学術雑誌)
共著
Orbitally Ordered State in Y1-xCaxTiO3 (0 < x ≦ 0.5)
H. Nakao, M. Tsubota, F. Iga, K. Uchihira, T. Nakano, T. Takabatake, K. Kato, Y. Murakami
Journal of the Physical Society of Japan

73/ 10, 2620-2623
2004/10/01

10.1143/JPSJ.73.2620

Orbitally ordered states of a Ti t2g-electron in Y1xCaxTiO3 (0 < x < 0.5) have been systematically investigated by the resonant X-ray scattering (RXS) technique near the Ti K-edge. The RXS intensities at 1s→3d transition energy (pre-edge) reflecting the 3d-orbital ordering markedly decrease with increasing Ca concentration toward the ferromagnetic–paramagnetic phase boundary (xFP ~ 0.15). The
intensity remaining above xFP decreases gradually and almost disappears at the metal–insulator transition (xMI ~ 0.4). Namely, it is suggested that the orbital ordering is strongly suppressed toward xFP, and vanishes at xMI. The Ca concentration dependence of the Jahn–Teller distortion determined by X-ray structural analysis is consistent with the orbital state obtained by the RXS experiment.
122 研究論文(学術雑誌)
共著
Raman scattering study of CaB6 and YbB6
N. Ogita, S. Nagai, N. Okamoto, F. Iga, S. Kunii, T. Akamatsu, J. Akimitsu, M. Udagawa
Journal of Solid State Chemistry

177, 461-465
2004/09/01

10.1016/j.jssc.2003.02.014

Phonon spectra of CaB6 and RB6 (R=Yb, Ce, and Pr) have been investigated by Raman scattering. We found clear spectral difference between divalent cation hexaboride and trivalent one. E, mode shows the doublet spectra for only the divalent crystals of CaB6 and YbB6. The doublet spectra are understood by the two-dimensional charge distribution on B-6 without lattice distortion. In addition, the scattered intensities of the phonons change at around the ferromagnetic Curie temperature for YbB6 and at T similar or equal to 600 K for CaB6. These are the characteristic temperatures due to the change of the electronic system.
123 研究論文(国際会議プロシーディングス)
共著
Coupling of lattice and spin degrees of freedom in GdB6
D. F. McMorrow, K. A. McEwen, J-G. Park, S. Lee, D. Mannix, F. Iga, T. Takabatake
Physica B

345, 66-69
2004/09/01

10.1016/j.physb.2003.11.022

A detailed X-ray scattering study of the phase transitions in GdB6 has been performed. For photon energies away from resonance, peaks are observed with two distinct wave vectors of q1 = (0 0 1/2) and q2 = (1/2 1/2 0) at low temperatures.
These are shown to arise from distortions of the lattice, although an earlier study had attributed them to competing magnetic-order parameters [J. Appl. Phys. 63 (1998) 3580]. With the photon energy tuned to the Gd L2 edge, a third set of peaks with q3 = (1/4 1/4 1/2) was discovered. This is shown to be the true magnetic-order parameter. Our results reveal a surprisingly rich interaction between the lattice and spin degrees of freedom in this compound.
124 研究論文(国際会議プロシーディングス)
共著
Temperature dependence of high-energy photoemission spectra of YbB12
A. Shigemoto, S. Suga, A. Sekiyama, S. Imada, A. Yamasaki, A. Irizawa, T. Muro, Y. Saitoh, F. Iga, T. Takabatake
Physica B

351(3-4), 289-291
2004/09/01

10.1016/j.physb.2004.06.028

We have measured the photoemission spectra of a Kondo insulator YbB12 (TK = 220 K) by using synchrotron radiation at hν = 700 eV to obtain the information on Yb 4f bulk electronic structures. On the fractured surface of the single crystal, the 4f peaks from the surface Yb2+ states are suppressed and the Yb3+ multiplet structures are well explained by the atomic multiplet calculation. We have also measured the temperature dependence of the spectra. The Yb3+ peaks are found to shift toward higher binding energies by 30meV across Tmax (=75 K) as temperature is decreased.
125 研究論文(国際会議プロシーディングス)
共著
Temperature dependence of the electronic states of Kondo Semiconductor YbB12
Y. Takeda, M. Arita M, M. Higashiguchi, K. Shimada, M. Sawada, H. Sato, M. Nakatake, H. Namatame, M. Taniguchi, F. Iga, T. Takabatake, K. Takata, E. Ikenaga, M. Yabashi, D. Miwa, Y. Nishino, K. Tamasaku, T. Ishikawa, S. Shin, K. Kobayashi
Physica B

351(3-4), 286-288
2004/09/01

10.1016/j.physb.2004.06.027

The temperature dependences of the Yb 4f and 3d states of Kondo semiconductor YbB12 single crystal have been studied by high-resolution photoemission spectroscopy taken at hν= 100 and 5951 eV. The Yb2+ 4f_7/2 state at the Fermi level is enhanced on cooling. With improved energy resolution, we discover an additional quasi-particle peak at 15 meV that developes below 60 K. From the Yb 3d spectrum at 22 K, the Yb valence is estimated to be ~2.88.
126 研究論文(学術雑誌)
共著
Raman scattering investigation of Y1-xCaxTiO3
T. Tsurui, N. Ogita, M. Udagawa, M. Tsubota, F. Iga
Physical Review B

69, 024102(1-8)
2004/01/14

10.1103/PhysRevB.69.024102

Raman scattering spectra of single crystalline Y1-xCaxTiO3 (x=0, 0.10, 0.30, 0.37, 0.38, and 0.41) have been systematically studied in the temperature region from 298 to 8 K. The structural transition from the orthorhombic phase to the monoclinic one occurs below 200 K in x<0.38. The intensity increase, related to the metal-insulator transition, has been observed below 100 K in Y0.62Ca0.38TiO3. The phase separation from the monoclinic phase to the metallic orthorhombic and insulating monoclinic phase occurs below the metal-insulator transition temperature (T-MI) in Y0.62Ca0.38TiO3. The intensity-increase is explained by the appearance of the orthorhombic structure below T-MI and its anisotropic appearance is related to the random orientation of the principle axes of the orthorhombic grain.
127 研究論文(学術雑誌)
共著
Structural and electronic interplay in the gap formation in CeRhAs1-xSbx (0<x<1)
T. Sasakawa, H. Miyaoka, K. Umeo, S. Aoyagi, K. Kato, F. Iga, T. Takabatake
Journal of the Physical Society of Japan

73/ 1, 262-268
2004/01/01

10.1143/JPSJ.73.262

Both CeRhAs and CeRhSb are known to open a narrow pseudogap at the Fermi level. For the alloys CeRhAs1-xSbx (0 < x < 1), we report the magnetic, transport and x-ray diffraction measurements. For x = 0, the orthorhombic ε-TiNiSi-type crystal structure with a 2b x 2c superlattice changes to the
hexagonal LiGaGe-type upon heating above 360 K, thereby the Ce chain is straightened. The phase transitions disappear upon alloying at a small level x = 0.025, and alloys for 0.25 < x < 0.50 adopt the hexagonal YPtAs-type structure at room temperature. Over the wide range 0.025 < x < 0.94, no clear sign of gap is observed in either magnetic susceptibility, specific heat, resistivity or thermopower. The electronic specific heat coefficients for the end compounds at x = 0 and x = 1 are ten-fold increased by
alloying at x = 0.025 and x = 0.94, respectively. These results indicate that not only the coherent hybridization of the 4f wave function with the conduction bands in the ε-TiNiSi-type structure but also the superlattice structure play important roles in the gap formation in CeRhAs.
128 研究論文(学術雑誌)
共著
Nd ion doping effects on the multipolar interactions in CeB6
Y. Yoshino, S. Kobayashi, S. Tsuji, H. Tou, M. Sera, F. Iga, Y. Zenitani, J. Akimitsu
Journal of the Physical Society of Japan

73/ 1, 29-32
2004/01/01

10.1143/JPSJ.73.29

The Nd ion doping effects on the multipolar interactions in CeB6 were studied. At high magnetic fields, the antiferro-quadrupolar (AFQ) phase II is not significantly affected by Nd doping. This may be because the Nd ion is in the paramagnetic state. However, with decreasing magnetic field, the Nd doping effect becomes apparent. This may be because Nd–Nd interaction dominates the long-range ordering at low
magnetic fields. At low magnetic fields, just after TQ and TN coincide with each other when Nd ion is doped, new phases, namely, phases IIIA and V appear. Phase IIIA is the AF magnetic state where all types of quadrupolar interaction are small. The nature of phase V seems to change largely at x ~ 0.5. For x < 0. 4, phase V may be the same as the type I AF magnetic state observed in NdB6 where the O 02 -type FQ interaction is prominent. In phase V for x > 0.5, the AF magnetic state where both Ce and Nd ions contribute may be realized.
129 研究論文(学術雑誌)
共著
Raman scattering investigation of RB6 (R = Ca, La, Ce, Pr, Sm, Gd, Dy, and Yb)
N. Ogita, S. Nagai, N. Okamoto, M. Udagawa, F. Iga, M. Sera, J. Akimitsu, S. Kunii
Physical Review B

68, 224305(1-9)
2003/12/30

10.1103/PhysRevB.68.224305

The excitation-energy, polarization, and temperature dependeces of Raman scattering spectra have been measured for the RB6 crystals (R=Ca2+, La3+, Ce3+, Pr3+, Sm2.6+, Gd3+, Dy3+, and Yb2+). In the divalent crystals of CaB6 and YbB6, the Eg phonon shows doublet, while the line shape is a broad single peak for the trivalent case. In the trivalent crystals, two kinds of the extra peaks have been clearly observed at around 200 cm-1 and at around 1400 cm-1 in addition to the Raman-active phonons and crystal electric-field (CEF) excitations. The peak intensities at around 200 cm-1 show the anomalous decrease with decreasing
temperature, and this temperature dependence correlates with that of the mean-square displacement of the R ion. Furthermore, their energies decrease with the increase of the cage space due to B6. These peaks are originated from the second-order excitation of acoustic phonons. The broad peak at around 1400 cm-1 is assigned as the second-order excitation of the T2g phonon. For the T2g phonon, the energy difference and its line-shape change between the trivalent and divalent crystals suggests the existence of electron-phonon interaction. We have observed the CEF excitation in PrB6 and have obtained the Lea, Leask, and Wolf parameters
with x=0.94 and W=9.2 K.
130 研究論文(学術雑誌)
共著
Hole-doping and Pressure Effects on the Metal-Insulator Transition in Single Crystals of Y1-xCaxTiO3 (0.37 < x < 0.41)
M. Tsubota, F. Iga, T. Nakano, K. Uchihira, S. Kura, M. Takemura, Y. Bando, K. Umeo, T. Takabatake, E. Nishibori, M. Takata, M. Sakata, K. Kato, Y. Ohishi
Journal of the Physical Society of Japan

72/ 12, 3182-3188
2003/12/01

DOI: 10.1143/JPSJ.72.3182

The measurements of electrical resistivity, magnetic susceptibility, specific heat, thermopower and powder x-ray diffraction under pressure have been performed on single crystals of Y1-xCaxTiO3 (0.37 < x < 0.41) to investigate the hole-doping effect and pressure effect on the metal–insulator (MI) transition. In the low-temperature metallic state, the Ti moment is much reduced from S = 1/2 spin moment in the high-temperature insulating state. Upon applying pressure on the sample with x = 0.37 at temperatures below 200 K, the volume fraction of the orthorhombic phase exceeds that of the monoclinic phase, which is associated with the insulator to metal transition. The good correspondence between the x–T and P–T phase diagrams in the vicinity of the MI transition shows that the effect of hole doping by 1% Ca substitution is equivalent to the effect of application of pressure of 0.41 GPa. The phase separation temperature agrees well with the MI transition temperature, and both temperatures increase linearly with
increasing x or pressure. These results indicate that the MI transition in Y1-xCaxTiO3 is driven by percolation of the low-temperature orthorhombic domains.
131 研究論文(学術雑誌)
共著
Appearance of the Phase IV in CexLa1-xB6 at x〜 0.8
S. Kobayashi, Y. Yoshino, S. Tsuji, H. Tou, M. Sera, F. Iga
Journal of the Physical Society of Japan

72/ 11, 2947-2954
2003/11/01

DOI: 10.1143/JPSJ.72.2947

We have studied the transport, magnetic and elastic properties of Ce0.8La0.2B6 single crystal. The phase IV which existed for x < 0.75 in CexLa1-xB6 was found to exist in this sample but its existence region is very small. The magnetic phase diagram indicates that the phase IV appears in order to avoid a direct contact between the anti- ferromagnetic phase III dominated by the Oxy-type antiferro-quadrupolar ordering and the paramagnetic phase I. The anisotropy of the magnetization was discovered in the phase
IV above H ~ 0.3 T. There, the magnetization is large for H // <001> but small for H // <110>. From the results of the anisotropic behaviors of the magnetization in the phase IV, we proposed that there exist the three domain states, Kxy, Kyz and Kzx at H = 0 and in the Kxy domain, M under H // z is large and that under H // x or y is small. Based on the anisotropy of M and the smooth x dependence of TN and T IV-I
N through x ~ 0.8, we discussed the possibility of the magnetic ordering in the phase IV. Tβ-type antiferro- octupolar ordering whose possibility was discussed in detail by Kuramoto and Kusunose [J. Phys. Soc. Jpn. 69 (2000) 671] is also discussed.
132 研究論文(国際会議プロシーディングス)
共著
Elastic properties of ferromagnetic Mott insulator YTiO3
T. Suzuki, H. Higaki, I. Ishii, M. Tsubota, F. Iga
Physica B

329-333, 868-869
2003/09/01

10.1016/S0921-4526(02)02587-5

YTiO3 has attracted interests because of a Mott insulator with a ferromagnetic transition at Tc=29 K. We have measured temperature dependence of elastic moduli for a high-quality single-crystal of YTiO3 around Tc using an ultrasonic technique. It is plausible that the elastic anomaly at Tc arises from a usual magnetostriction effect.
133 研究論文(国際会議プロシーディングス)
共著
Raman scattering study of Y1-xCaxTiO3 single crystals
T. Tsurui, N. Ogita, M. Udagawa, M. Tsubota, K. Uchihira, F. Iga
Physica B

329-333, 833-834
2003/09/01

10.1016/S0921-4526(02)02533-4

Raman scattering spectra of single crystalline Y1-xCaxTiO3 (x = 0, 0.1, 0.3, 0.37, 0.38) have been studied in the
temperature region from 298 to 8 K. The structural phase transition from orthorhombic to monoclinic one occurs for
x<0.38 at around 200 K. The increase of the peak intensity, related to the metal–insulator transition, has been
observed below 100 K in Y0.62Ca0.38TiO3. The temperature dependence of the peak intensity shows strong polarization
dependence. The phase separation from the monoclinic phase to the monoclinic + orthorhombic phase occurs below
the metal–insulator transition temperature in Y0.62Ca0.38TiO3.
134 研究論文(国際会議プロシーディングス)
共著
Raman scattering of RB6 (R = Ca, Ce, Pr, Gd, Dy, and Yb)
N. Ogita, S. Nagai, N. Okamoto, M. Udagawa, F. Iga, M. Sera, J. Akimitsu, S. Kunii
Physica B

329-333, 661-662
2003/09/01

10.1016/S0921-4526(02)02513-9

In Raman scattering spectra of RB6, anomalous peaks have been found below 200 cm-1 for the R+3B6, except for phonons and CEF excitations. Their peak intensities decrease with decreasing temperature, and this temperature dependence correlates with that of the mean square displacement of R. Furthermore, their energies decrease with the increase of the cage size of B. This new excitation can be attributed to a local vibration of the R ion, which can be regarded as ‘rattler’ in the cage of B.
135 研究論文(学術雑誌)
共著
Magnetic and Thermoelectric Properties of Ce2NiB10-δ (δ< 0.3), a Ni-incorporated Variant of CeB6
E. Matsuoka, K. Umeo, S. Tsuji, M. Sera, F. Iga, T. Takabatake
Journal of the Physical Society of Japan

72/ 9, 2344-2349
2003/09/01

10.1143/JPSJ.72.2344

Magnetic susceptibility, electrical resistivity, thermopower, specific heat, and thermal conductivity
measurements were made on Ce2NiB10-d. The crystal structure is derived from that of CeB6 by replacing two B atoms in the B6–B6 network with one Ni atom. The sample prepared by arc melting is deficient in the boron composition, d~0.3. The magnetic susceptibility, electrical resistivity, and specific heat show no sign of magnetic ordering down to 0.5 K, but exhibit non-Fermi liquid (NFL) behavior, i.e.,
kai ∝ T^0.5,  ρ∝ T and C=T ∝ -ln T. This behavior of C(T) in zero field and its suppression by external magnetic fields are explained qualitatively by the Kondo disorder model with the average Kondo temperature of 26 K. Atomic disorder induced by boron defects is the most probable origin of the NFL behavior. The thermal conductivity of Ce2NiB9.7 is much reduced from that for CeB6, which is ascribed to the increase of phonon scattering by the lattice defect.
136 研究論文(国際会議プロシーディングス)
共著
Raman scattering study of hexaborides crystals
N. Ogita, S. Nagai, N. Okamoto, M. Udagawa, F. Iga, M. Sera, J. Akimitsu, S. Kunii
Physica B

328, 131-134
2003/08/01

10.1016/S0921-4526(02)01827-6

The valence-dependent Raman scattering spectra has been found for the RB6 crystals (R= Ca2+, Yb2+, Ce3+, Pr3+,
Gd3+, and Dy3+). In the divalent crystals of CaB6 and YbB6, Eg phonon shows doublet, while the line shape is a broad
single peak for the trivalent case. Since both doublet peaks satisfy the selection rule due to the cubic symmetry, the
doublet spectra is not explained by the structural phase transition, but by the anisotropic charge distribution on B6.
In the trivalent crystals, the two kind of the extra peaks have been commonly observed at around 200 cm-1 and at around 1400 cm-1. The peak intensities of the former decrease with decreasing temperature, and their peak energies increase with increasing lattice constant. These anomalous features are well explained by the vibration of R ion in the oversized cage space formed octahedral B6 molecule. Thus, this new excitation can be attributed to a local vibration of
the R ion, which can be regarded as‘rattler.’
137 研究論文(学術雑誌)
共著
Ion pairs and spontaneous break of symmetry in the valence-fluctuating compound YbB12
T. S. Altshuler, Yu. V. Goryunov, M. S. Bresler, F. Iga, T. Takabatake
Physical Review B

68, 014425(1-7)
2003/07/24

10.1103/PhysRevB.68.014425

Electron spin resonance (ESR) from Yb3+ ions in a single crystal of fluctuating valence compound YbB12 was studied. We have observed two main lines symmetrically displaced from a g value of 2.55 and exhibiting modulation with 5% anisotropy, when the magnetic field was rotated in the (110) plane from the cubic axis to the [110] direction. Experimental results are explained by an existence of Yb-Yb pairs which are coupled by isotropic exchange but interact also with the other pairs by dipole and exchange coupling. A strong temperature dependence of the amplitude of the ESR signals is found and interpreted as a result of capture of electrons by Yb3+ ions from electron traps with a binding energy of 18 K. The occurrence of a slight anisotropy in a cubic semiconductor may be the result of a spontaneous break of symmetry specific for the ground state of a Kondo dielectric.
138 研究論文(学術雑誌)
共著
Orbital-ordering-induced phase transition in LaVO3 and CeVO3
Y. Ren, A. A. Nugroho, A. A. Menovsky, J. Strempfer, U. Rut, F. Iga, T. Takabatake, C.W. Kimball
Physical Review B

67, 014107(1-6)
2003/01/24

10.1103/PhysRevB.67.014107

The structural phase transition in the orthovanadates LaVO3 and CeVO3 has been studied with high energy synchrotron x-ray diffraction. LaVO3 undergoes a second order phase transition at TN=143 K and a first order transition at Tt=141 K, while in CeVO3 there are phase transitions occurring at T0=154 K of second order and at TN=134 K of first order. These phase transitions are confirmed by specific heat measurements. The phase transition at Tt in LaVO3 or T0 in CeVO3 is due to a G-type orbital ordering which lowers the structure symmetry from orthorhombic Pbnm to monoclinic P21 /b11. The structure change at TN in CeVO3 is ascribed to an orbital ordering enhanced magnetostrictive distortion, while that at TN in LaVO3 is most probably due to an ordered occupation of the vanadium 3d t2g orbitals associated with an antiferromagnetic ordering. We propose that the first order phase transition at Tt in LaVO3 should be associated with a sudden change of both spin and orbital configurations, similar to the phase transition at Ts=77 K in YVO3 [Ren et al., Nature (London) 396, 441 (1998)], causing a reversal of the net magnetization. However, the ordered state above Tt in LaVO3 is identical to that below Ts in YVO3. It is found that, with increasing lanthanide ionic radius, the Neel temperature TN increases while the orbital ordering onset temperature decreases in these
orthovanadates.
139 研究論文(学術雑誌)
共著
Kondo Effect in the Long-Range Antiferromagnetic Ordered State -- CexNd1-xB6 --
S. Kobayashi, Y. Yoshino, S. Tsuji, M. Sera, F. Iga
Journal of the Physical Society of Japan

72/ 1, 25-28
2003/01/01

10.1143/JPSJ.72.25

We have studied the coexistence and the competition between the Kondo singlet state and the long-range antiferro(AF)magnetic ordering in the CexNd1-xB6 system. At high temperatures, the single impurity Kondo, effect is observed in all the samples as in the CexLa1-xB6 system. In the AF magnetic state formed mainly by Nd ions, Cc ions contribute to the AF magnetic ordering but at the same time exhibit the Kondo singlet character with a small effect from the magnetic ordering. The ground state of CexNd1-xB6 changes with x; in a small x region, the Kondo singlet state is destroyed by the exchange field from surrounding Nd moments. Above x similar to 0.3, the long-range magnetic order takes place because the Ce-Ce interaction overcomes the Kondo effect. In an intermediate x value between the above two regions, the Kondo singlet state may continue to exist down to a very low temperature.
140 研究論文(国際会議プロシーディングス)
共著
Dynamics of the internal field in RB12 (R=Er, Yb, Lu)
G. M. Kalvius, D. R. Noakes, M. Marcano, R. Wapling, F. Iga, T. Takabatake
Physica B

326, 398-402
2003/01/01

10.1016/S0921-4526(02)01644-7

The results of muSR studies on isostructural LuB12, YbB12 and ErB12 between 1.8 K and 300 K are described. LuB12 is non-magnetic. YbB12 is a Kondo insulator in which a previous muSR study reported the formation of dynamically correlated Yb moments of less than 10(-2) mu(B) below 20 K. ErB12 is an antiferromagnet (AFM) with T-N = 6.7 K. We found the muSR spectra of LuB12 and YbB12 to be quite alike at all temperatures which excludes any contribution from Yb magnetic moments. Spectral shape is determined by the field of the B-11 nuclear moments. These fields show dynamic behavior, but nuclear spin relaxation can be excluded since its rate is below the muSR time window. It is suggested that the dynamical features arise from atomic motions within the B-12, clusters and perhaps (at higher temperatures) from muon motion. In ErB12, long-range magnetic order (LRO) exists up to 6.6 K, Between 6.7 and 9 K a mixture of LRO and paramagnetic phases is present. This, together with the fact that below 6.5 K the muSR spectral parameter show little change with temperature, indicates a transition of first-order type. The local field in the LRO state is broadly distributed pointing towards a complex spin structure. At temperatures above T-N the spectra are dominated by the dynamics of the Er spins.
141 研究論文(学術雑誌)
共著
Rapid Suppression of the Metamagnetic Transition for H// <111> in NdB6 by La Doping
S. Tsuji, T. Endo, S. Kobayashi, Y. Yoshino, M. Sera, F. Iga
Journal of the Physical Society of Japan

71/ 12, 2994-3002
2002/12/01

10.1143/JPSJ.71.2994

We have studied the specific heat, magnetization and magnetoresistance of NdxLa1-xB6 (x = 0.9, 0.8, 0.7) single crystals and obtained their magnetic phase diagrams. The antiferro (AF) magnetic phase for H parallel to [111] is divided into two phases of AFI below H-cl and AFII* (or AFII) above H-cl. In the AFI phase, there exists the component of the parallel magnetic susceptibility up to Hcl The intermediate metamagnetic AFII phase in NdB6 disappears already for x = 0.9 and in place, new AFII* phase appears. The AFII* phase is considered to be the spin-canted phase characterized by the perpendicular magnetic susceptibility. However, the magnetization in the AFII* phase for x = 0.9 and 0.8 exhibits the unusual behavior that the extrapolation of the magnetization curve down to H = 0 takes a large negative magnetization value. This unusual behavior disappears at x = 0.7. These results indicate that the interaction forming the interamediate metamagnetic AFII phase is rapidly suppressed by the La doping and disappears at x similar to 0.7.
142 研究論文(学術雑誌)
共著
Effect of defects in ytterbium ion valency in YbB12
T. S. Altshuler, M. S. Bresler, Yu. V. Goryunov, F. Iga, T. Takabatake
Physics of Solid State

44, 1536-1539
2002/09/01

10.1134/1.1501354

An EPR signal from ytterbium ions with an integer valence of +3 was detected in the YbB12 intermediate-valence compound. The measurements were carried out on an YbB12 single crystal in the 1.6- to 4.2-K temperature interval. The EPR spectrum is interpreted as being due to the existence of exchange-coupled ytterbium ion pairs.
143 研究論文(学術雑誌)
共著
The metal-insulator transition in Y1-xCaxTiO3 caused by phase separation
K. Kato, E. Nishibori, M. Takata, M. Sakata, T. Nakano, K. Uchihira, M. Tsubota, F. Iga, T. Takabatake
Journal of the Physical Society of Japan

71/ 9, 2082-2085
2002/09/01

10.1143/JPSJ.71.2082

In order to explore the origin of the metal-insulator (M-I) transition, the precise crystal structures of the hole-doped Mott insulator system, Y1-xCaxTiO3 (x = 0.37, 0.39 and 0.41), are studied for the temperature range between 20 K and 300 K by synchrotron radiation (SR) X-ray powder diffraction. For both Y0.63Ca0.37TiO3 and Y0.61Ca0.39TiO3 Compositions, the orthorhombic (Pbnm) - monoclinic (P2(1)/n) structural phase transition occurs at approximately 230 K, which is much higher than their individual M-I transition temperatures, i.e., 60 K and 130 K, respectively. For these compositions, significant phase separation (low-temperature orthorhombic + monoclinic) is also found in the vicinity of the M-I transition temperature. On the other hand, Y0.59Ca0.41TiO3, which does not exhibit M-I transition and preserves a metallic behavior down to 1.5 K, is in a two phase state from 20 K to 300 K. It is concluded that the existence of the phase separation causes the M-I transition in Y1-xCaxTiO3, and the low-temperature orthorhombic phase contributes to the metallic property of this system.
144 研究論文(学術雑誌)
共著
Evidence for a ferromagnetic transition in Yb1-xLaxB6  ( 0 < x < 0.006 )
F. Iga, Y. Ueda, T. Takabatake, T. Suzuki, W. Higemoto, K. Nishiyama, H. Kawanaka
Physical Review B

65, 220408(1-4)
2002/06/19

10.1103/PhysRevB.65.220408

Magnetization and muon spin relaxation (muSR) in Yb1-xLaxB6 have been measured to study whether ferromagnetic moments appear as in La-doped CaB6. Magnetization processes of polycrystalline samples for 0less than or equal toxless than or equal to0.006 have revealed ferromagnetic hysteresis with a saturation moment of 1x10(-4)mu(B)/f.u. On cooling below 150 K, an increase of the internal field up to 0.6 mT has been observed by zero-field muSR measurements on both polycrystals and single crystals. These results indicate a ferromagnetic transition at 150 K in this system.
145 研究論文(国際会議プロシーディングス)
共著
μSR spectroscopy of the Kondo insulator Lu1-xYbxB12
G. M. Kalvius, D. R. Noakes, R. Wappling, A. Kratzer, A. Schreier, F. Iga, T. Takabatake, H.v. Löhneysen
Physica B

312-313, 210-212
2002/06/01

10.1016/S0921-4526(01)01197-8

Single crystals of Yb1xLuxB12 (x= 0, 0.125, 0.5, 1) were measured between 1.8 and 300 K. We found similar spectral
shapes in all compounds studied over the whole temperature range scanned. In zero field the spectra alter their
appearance around 20, 100 and 150 K. In a longitudinal field of 100 G, which largely suppresses the contribution from
11B nuclear moments the relaxation rate remained constant up to ~150 K where it suddenly peaks. These findings exclude magnetic correlations as the origin of muon spin relaxational behavior. The dominant features of the mSR
spectra at low temperatures arise from lattice dynamics, probably involving the B12 clusters, rather than from spin
dynamics. Higher temperature features are most likely muon related.
146 研究論文(国際会議プロシーディングス)
共著
Optical gap in diluted Kondo semiconductors Yb1-xLuxB12: lattice and single-site effect
H. Okamura, M. Matsunami, T. Inaoka, S. Kimura, T. Nanba, F. Iga, S. Hiura, T. Takabatake
Physica B

312-313, 157-158
2002/06/01

10.1016/S0921-4526(01)01074-2

We have studied the evolution of energy gap in the optical conductivity [sigma(w))] of Yb1-x,LuxB12 as functions of temperature (T) and non-magnetic Lu substitution (x). YbB12, has a well-developed energy gap in 6(c)) below 20 K, with the onset of sigma(w)) at similar to20meV and a shoulder at similar to40meV. With increasing T or x, the energy gap is rapidly filled in, which indicates the importance of coherence among the Yb 4f orbitals at different sites. The shoulder, on the other hand, remains nearly unshifted in wide ranges of T (to 70K @) and x (0-1/2), which shows that the characteristic energy for the gap is given by some single-site energy scale of Yb3+. Namely, both lattice and single-site effects play important roles in the gap formation of YbB12.
147 研究論文(国際会議プロシーディングス)
共著
Defect-induced magnetic fluctuations in YbB12
N. Shishiuchi, Y. Kato, O. M. Vyaselev, M. Takigawa, S. Hiura, F. Iga, T. Takabatake
Journal of Physics and Chemistry of Solids

63, 1231-1234
2002/06/01

10.1016/S0022-3697(02)00121-X

We present the results of nuclear spin-lattice relaxation rate (1/T-1) measurements in a typical Kondo insulator YbB12 for B-10.11 and Yb-171 nuclei. Above 20 K,1/T-1 at the B sites shows thermally activated temperature dependence with a gap of about 100 K. However, it shows anomalous enhancement below 15 K, which is partially suppressed by magnetic field up to 16 T. No such anomaly was observed at the Yb sites. The ratio of 1/T-1 for B-11 and B-10 nuclei indicates that the anomaly below 15 K is caused by dilute magnetic moments assisted by nuclear spin diffusion. The origin and the nature of the low temperature magnetic fluctuations are discussed.
148 研究論文(国際会議プロシーディングス)
共著
Ce site substitution effects on the multipolar interactions in CeB6
M. Sera, F. Iga, S. Kobayashi, H. Sugamoto, K. Nagatomo, K. Umeo, S. Tsuji, K. Kojima, J. Akimitsu, Y. Zenitani
Journal of the Physical Society of Japan

71/ Suupl., 52-55
2002/05/01




149 研究論文(国際会議プロシーディングス)
共著
Magnetic properties in phase IV of Ce0.8Nd0.2B6 studied by muon spin relaxation
H. Takagiwa, K. Ohishi, J. Akimitsu, W. Higemoto, R. Kadono, F. Iga, M. Sera
Journal of the Physical Society of Japan

71/ Suupl., 118-120
2002/05/01




150 研究論文(国際会議プロシーディングス)
共著
Electronic structure of the Kondo Semiconductor YbB12: Temperature and non-magnetic dilution effects
H. Okamura, M. Matsunami, T.i Inaoka, S. Kimura, T. Nanba, F. Iga, S. Hiura, T. Takabatake
Journal of the Physical Society of Japan

71/ Suupl., 303-305
2002/05/01




151 研究論文(国際会議プロシーディングス)
共著
Raman scattering investigation of high temperature ferromagnetic crystals
M. Udagawa, S. Nagai, N. Ogita, F. Iga, R. Kaji, K. Sumida, J. Akimitsu, S. Kunii, K. Suzuki, H. Onodera, Y. Yamaguchiv
Journal of the Physical Society of Japan

71/ Suupl.
2002/05/01




152 研究論文(国際会議プロシーディングス)
共著
Thermoelectric properties of the Kondo semiconductor:Yb1-xLuxB12
F. Iga, T. Suemitsu, S. Hiura, K. Takagi, K. Umeo, M. Sera, T. Takabatake
Journal of Magnetism and Magnetic Materials

226, 137-138
2001/12/01



We have measured electrical resistivity, thermal conductivity, and thermopower S of the Kondo semiconductor
YbLuB12 (0<x<1). On increasing x to 0.05, both the semiconducting increase in rho(T) and the enhancement of Kapper below 60K are strongly suppressed. A giant negative peak in S(T) for x=0 at 10K vanishes already at x=0.01, whereas a broad peak stays at 30K for x<0.25 and shifts to 100K for x>0.75. These results suggest that the energy gap in YbB12 is filled up by the development of a mid-gap state at the Fermi level with Lu substitution.
153 研究論文(学術雑誌)
共著
Transport and magnetic properties of a low-density carrier system SmBiPt
M. S. Kim, T. Sasakawa, T. Suemitsu, Y. Bando, K. Umeo, F. Iga, H. Mitamura, T. Goto, T. Takabatake
Journal of the Physical Society of Japan

70/ 12, 3650-3653
2001/12/01



We present the results of electrical resistivity ρ, Hall coeffcient R_H, thermoelectric power S, specic heat C and magnetic susceptibility χ on SmBiPt with the MgAgAs-type structure. ρ(T) and S(T) exhibit a broad maximum of 1.7 mΩcm at 80 K and 60 μV/K at 126 K, respectively. These facts and the change of R_H from negative to positive on cooling through 200 K indicate the semimetallic nature of SmBiPt. At around 7 K, ρ(T) and S(T) show a shallow dip and a small peak, respectively. The antiferromagnetic order at TN = 2.2 K is manifested as a sudden drop in ρ and a peak in both C and χ. The magnetic entropy at TN is 0.6 Rln4 and reaches a plateau of Rln4 at 18 K, indicating the Γ8 quartet ground state of Sm3+ in this cubic compound. The results of χ(T) and magnetization curve can be explained by considering the mixing of higher multiplet levels of J = 7/2 and 9/2 into the ground multiplet of J = 5/2.
154 研究論文(国際会議プロシーディングス)
共著
Kondo semiconductors as a novel thermoelectric-cooling material
T. Takabatake, T. Suemitsu, T. Sasakawa, Y. Bando, T. Takagi, Y. Echizen, T. Yoshino, K. Umeo, F. Iga, M. Sera, Ch. Paul, St. Berger, E. Bauer
Proceedings of 20th International Conference on Thermoelectrics

266
2001/12/01

10.1016/S0304-8853(00)00627-2

We have measured electrical resistivity rho, thermal conductivity kappa, and thermopower S of the Kondo semiconductor Yb1-xLuxB12 (0 less than or equal to x less than or equal to 1). On increasing x to 0.05, both the semiconducting increase in rho (T) and the enhancement of kappa below 60 K are strongly suppressed. A giant negative peak in S(T) for x = 0 at 10 K vanishes already at x = 0.01, whereas a broad peak stays at 30 K for x less than or equal to 0.25 and shifts to 100 K for x greater than or equal to 0.75. These results suggest that the energy gap in YbB12 is filled up by the development of a mid-gap state at the Fermi level with Lu substitution.
155 研究論文(国際会議プロシーディングス)
共著
Temperature-dependentmetal-insulator transition in d- and f-electron systems studied by high-resolution photoemission spectroscopy
K. Shimada, Y. Takeda, M.i Arita, H. Sato, A. Kimura, H. Namatame, M. Taniguchi, M. Tsubota, K. Katoh, F. Iga, T. Takabatake
Journal of Electron Spectroscopy and related Phenomena

114-116, 711-716
2001/08/01

10.1016/S0368-2048(00)00319-4

We have studied metal-insulator transition (MIT) of Y1-xCaxTiO3 (x=0.37, 0.39 and 0.41) single crystals and a Kondo semiconductor Ce3Bi4Pt3 single crystal by temperature-dependent high-resolution photoemission spectroscopy. At the phase boundary between insulating and metallic states of Y1-xCaxTiO3 (x similar to0.4), an unusual temperature-dependent spectral-weight redistribution has been observed. The x=0.39 compound showed a clear MIT at similar to 150 K: a finite spectral weight at E-F appears below similar to 150 K. A narrow V-shaped pseudogap is formed in the conduction band of Ce,Bi,Pt, below a characteristic temperature T(coh)similar to 100 K. The spectral weight of the Ce 4f state at E-F increases above T-coh on cooling, but reduces significantly below T-coh forming a V-shaped pseudogap. It is a clear evidence for the crossover from an incoherent state above T-coh to a coherent state below T-coh. In these materials, electron correlation and intersite-hybridization interactions (or a bandwidth) are both important for the temperature-dependent MIT.
156 研究論文(学術雑誌)
共著
Photoemission study of substitution effects on the conduction-band states in the Kondo insulator YbB12
T. Susaki, A. Fujimori, Y. Takeda, M. Taniguchi, M. Arita, K. Shimada, H. Namatame, S. Hiura, F. Iga, T. Takabatake
Journal of the Physical Society of Japan

70/ 3, 756-761
2001/03/01

10.1143/JPSJ.70.756

We have studied the substitution and temperature dependences of the B sp-Yb d-derived conduction-band states in Yb1−xLuxB12 (x = 0.25, 0.50, 0.75, and 1.00) by high-resolution photoemission spectroscopy. As the Yb content increases, the density of states (DOS) between
the Fermi level (EF ) and ∼ − 20 meV is reduced while a broad peak grows at ∼ −80 meV. The DOS reduction is considerably weakened at high temperatures (> ∼100 K), consistent with the gap formation in YbB12 only at low temperatures. The temperature dependence of the DOS near EF gradually becomes stronger with decreasing x but is abruptly enhanced below x = 0.25–0.50, showing the crucial contribution of Yb-Yb intersite interaction to the gap formation in YbB12. The ∼ −80meV peak survives up to large x, indicating that it is caused by a single-impurity effect.
157 研究論文(学術雑誌)
共著
Scaling and phase diagram in the vicinity of the first-order metal-insulator transition in Y1-xCaxTiO3
T. Naka, T. Matsumoto, A. Matsushita, F. Iga, Y. Nishihara
Physica B

304, 27-37
2001/01/05

10.1016/S0921-4526(01)00390-8

Magnetic susceptibility and electrical resistivity in the metal–insulator (MI) transition of a mixed-valent Ti-oxide system, Y1-xCaxTiO3, are investigated at high pressure. This system exhibits obviously the first-order temperature-induced
MI transition at x = 0.35-0.4. In the low temperature metallic phase, susceptibility, χ(T), is strongly suppressed with pressure. On the other hand, in the high temperature semiconductiong phase χ(T) obeying the Curie–
Weiss law is affected weakly by applying pressure. Below the MI transition temperature resistivity, ρ(T), shows the
characteristic Fermi liquid behavior, ρ= ρ_0 + AT^2. The coefficient of the T^2-term in resistivity, A, and the residual resistivity, ρ_0, decrease rapidly with pressure, while following the simple relation of A(x,P) ∝ ρ_0(x,P). A scaled temperature-composition phase diagram shows that the first-order transition terminates at both the lower and upper critical points and A, ρ_0 and χ are enhanced enormously at the lower critical point. We present scalings between susceptibility and resistivity in the metallic state taken into account the distribution of the Kondo temperature.
158 研究論文(学術雑誌)
共著
Electronic structure of the strongly hybridized ferromagnet CeFe2
T.Konishi, K. Morikawa, K. Kobayashi,T. Mizokawa, A. Fujimori, K. Mamiya, F. Iga, H. Kawanaka, Y. Nishihara, A. Delin, O. Eriksson
Physcal Review B

62/ 21, 14304-14312
2000/12/01

10.1103/PhysRevB.62.14304

We report on results from high-energy spectroscopic measurements on CeFe2, a system of particular interest due to its anomalous ferromagnetism with an unusually low Curie temperature and small magnetization compared to the other rare-earth iron Laves phase compounds. Our experimental results, obtained using core-level and valence-band photoemission, inverse photoemission and soft x-ray absorption techniques, indicate very strong hybridization of the Ce 4 f states with the delocalized band states, mainly the Fe 3d states. In the interpretation and analysis of our measured spectra, we have made use of two different theoretical approaches: The first one is based on the Anderson impurity model, with surface contributions explicitly taken into account. The second method consists of band-structure calculations for bulk CeFe2. The analysis based on the Anderson impurity model gives calculated spectra in good agreement with the whole range of measured spectra, and reveals that the Ce 4 f -Fe 3d hybridization is considerably reduced at the surface, resulting in even stronger hybridization in the bulk than previously thought. The band-structure calculations are ab initio fullpotential
linear muffin-tin orbital calculations within the local-spin-density approximation of the density functional. The Ce 4 f electrons were treated as itinerant band electrons. Interestingly, the Ce 4 f partial density of states obtained from the band-structure calculations also agree well with the experimental spectra concerning both the 4 f peak position and the 4 f bandwidth, if the surface effects are properly taken into account. In addition, results, notably the partial spin magnetic moments, from the band-structure calculations are discussed in some detail and compared to experimental findings and earlier calculations.
159 研究論文(学術雑誌)
共著
Optical conductivity of Yb1-xLuxB12: Energy gap and mid-infrared peak in diluted Kondo semiconductors
H. Okamura, M. Matsunami, T. Inaoka, T. Nanba, S. Kimura, F. Iga, S. Hiura, J. Klijn, and T. Takabatake
Physical Review B

62/ 20, R13265-13269
2000/11/15

10.1103/PhysRevB.62.R13265

We have measured the optical conductivity σ(ω) of Yb1-xLuxB12 (0<x<1), where the system evolves from a Kondo semiconductor at x=0 to a nonmagnetic metal at x=1. For x=0, σ(ω) exhibits a clear energy gap with an onset at ~20 meV and a shoulder at 38 meV. As x increases, the gap is rapidly filled in from the bottom, while the shoulder remains at ~40 meV up to x=1/2. These results suggest that Lu substitution into YbB12 produces an in-gap band, but the characteristic energy for the gap remains unchanged in a wide range of x. Spectral evolutions of a characteristic mid-infrared peak, which results from Yb 4f-derived states near the Fermi level, suggest a strong coupling of Yb 4f electrons with conduction electrons.
160 研究論文(国際会議プロシーディングス)
共著
Elastic quantum oscillation of LuB12
N. Okuda, T. Suzuki, I. Ishii, S. Hiura, F. Iga, T. Takabatake, T. Fujita, H. Kadomatsu, H. Harima
Physica B

281&282, 756-757
2000/11/01

10.1016/S0921-4526(99)01238-7

The quantum oscillations of elastic moduli of LuB12 have been measured by an ultrasonic technique at 0.4 K with a 16 T superconducting magnet to investigate the electronic structure of the compound. We succeeded in mapping the Fermi surfaces of LuB12. The angular dependence of the oscillation frequencies for various branches is almost identical to the theoretical one by band calculation of the LAPW method, which revealed that the Fermi surfaces of LuB12 consist of the 27th and 28th bands originating from the hybridization between B-2p and Lu-5d orbits.
161 研究論文(国際会議プロシーディングス)
共著
Low-field magnetic anisotropy in Mott-insulating ferromagnet Y1-xCaxTiO3(x<0.1)
M. Tsubota, F. Iga, T. Takabatake, N. Kikugawa, T. Suzuki, I. Oguro, H. Kawanaka, H. Bando
Physica B

281&282, 622-624
2000/11/01

10.1016/S0921-4526(99)01035-2

YTiO3 has been known as a Mott-insulating ferromagnet with the Curie temperature Tc=30 K. We have measured the magnetization of single crystals of Y1-xCaxTiO3 (x=0, 0.01, 0.02, 0.05, 0.10) grown by a floating-zone method. In YTiO3, a metamagnetic-like transition was observed for B//a and B//c (easy axis) at 1.5 and 7.5 mT, respectively, only after zero-field cooling. For B//b (hard axis), however, no metamagnetic behavior appears. From the magnetization measurements of alloys, it is found that the anisotropy in magnetization for B//a and B//c vanishes with increasing x up to 0.1.
162 研究論文(国際会議プロシーディングス)
共著
High-resolution temperature-dependent photoemission study of metal-insulator transition of Y1-xCaxTiO3
M. Arita, Y. Takeda, Y. okamura, H. Sato, K. Shimada, K. Mamiya, H. Namatame, M. Taniguchi, M. Tsubota, F. Iga, T. Takabatake
Physica B

281&282, 617-618
2000/11/01

10.1016/S0921-4526(99)01147-3

Temperature-dependent high-resolution photoemission spectroscopy has been performed on Y1-xCaxTiO3 (x=0.37, 0.39, and 0.41) single crystals. Ti 3d photoemission spectra have an incoherent part, indicating electrons in the Ti 3d bands are strongly correlated. Although the temperature dependence of the incoherent part at 150 K for the present samples is continuous and nearly the same, the spectral intensity near EF of the x=0.39 composition clearly shows the metal-insulator transition at &150 K, indicating the importance of the carrier density in the Ti 3d bands.
163 研究論文(国際会議プロシーディングス)
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Strong depression of the magnetic ordering temperature in CeSb by Ni incorporation
M. H. Jung, D. T. Adroja, F. Iga, T. Suzuki, T. Fujita, T. Takabatake
Physica B

281&282, 443-444
2000/11/01

10.1016/S0921-4526(99)01049-2

We have studied the effect of Ni incorporation into CeSb by the measurements of magnetic susceptibility and electrical resistivity for polycrystalline samples of CeSbNix (0<x<0.15). It is found that the Neel temperature TN drops from 16 K for x=0 to 9 K for x=0.05 and gradually increases with x for x=0.05. The drastic depression of TN is ascribed to the filling of the 5p hole band with Ni 3d electrons, resulting in the collapse of p}f mixing. Below TN, the electrical resistivity decreases for x<0.05, whereas it increases for x>0.08. The rise in the resistivity below TN indicates the formation of a superzone gap in a simple antiferromagnetic phase for x>0.08.
164 研究論文(国際会議プロシーディングス)
共著
High-resolution temperature-dependent photoemission study of Kondo semimetal CeNiSn
Y. Takeda, M. Arita, K. Shimada, H. Sato, H. Namatame, M. Taniguchi, Y. echizen, F. Iga, T. Takabatake
Physica B

281&282, 286-287
2000/11/01

10.1016/S0921-4526(99)01195-3

The temperature dependence of the electronic states of Kondo semimetal CeNiSn has been studied by high-resolution photoemission spectroscopy. The results have been compared with those of LaNiSn. The temperature-dependent spectral weight reduction of CeNiSn is observed below 30 K at binding energies between ~20 meV and EF, and is similar to that of the Kondo insulator Ce3Bi4Pt3. This behavior is not observed for LaNiSn. The spectral intensity of Ce 4f derived state near the Fermi level (EF) is enhanced at temperatures above a characteristic temperature, but the spectral intensity at EF is reduced at 7 K.
165 研究論文(国際会議プロシーディングス)
共著
Photoemission study of Kondo insulator YbB12
T. Susaki, Y. Takeda, M. Arita, A. Fujimori, K. Shimada, H. Namatame, M. Taniguchi, S. Hiura, F. Iga, T. Takabatake
Physica B

281&282, 282-283
2000/11/01

10.1016/S0921-4526(99)00854-6

We have studied the temperature and Lu-substitution dependence of the B p-derived conduction band in YbB12 by high-resolution photoemission spectroscopy. As the Yb content increases, the density of states (DOS) between the Fermi level (EF) and ~ -20 meV is reduced and simultaneously a broad peak grows at ~ -80 meV. The temperature dependent
loss of DOS near EF becomes pronounced only for small x, showing an effect beyond a single-site mechanism in the gap formation of YbB12.
166 研究論文(国際会議プロシーディングス)
共著
Energy gap evolution in the optical spectra of Kondo alloy system Yb1-xLuxB12
H. Okamura, T. Inaoka, S. Kimura, T. Nanba, F. Iga, S. Hiura, J. Klijn, T. Takabatake
Physica B

281&282, 280-281
2000/11/01

10.1016/S0921-4526(99)01194-1

We have measured the optical conductivity spectra [σ(ω)] of Yb1-xLuxB12 (x=0, 0.25, 0.5, 0.75, 1) single crystals, where the system evolves from a Kondo semiconductor (YbB12, x=0) to a non-magnetic metal (LuB12, x=1). For YbB12, σ(ω) has a clear energy gap at low temperatures, with the onset of σ(ω) at ~20 meV and a 'shoulder' at ~40 meV. As x increases, the gap is progressively filled in from the bottom, while the position of the shoulder remains almost the same. This spectral evolution suggests that a Lu-related band grows within the gap, and that the magnitude of the intrinsic Kondo semiconducting gap is basically unaffected by the Lu substitution.
167 研究論文(国際会議プロシーディングス)
共著
Energy gap in Lu-substituted YbB12 proved by break junction
T. Ekino, H. Umeda, J. Klijn, F. Iga, T. Takabatake, H. Fujii
Physica B

281&282, 278-279
2000/11/01

10.1016/S0921-4526(99)01193-X

Tunneling measurements of Yb1-xLuxB12 have been carried out with break junction. The gap magnitude rapidly reduces with increasing x up to ~0.5, but the decreasing rate becomes weak for x>0.5. By contrast, the zero-bias conductance increases rather smoothly with increasing x up to ~0.8. These results clarify the nontrivial relationship between the Lu substitution and the gap suppression in Yb1-xLuxB12.
168 研究論文(国際会議プロシーディングス)
共著
171Yb NMR in the Kondo semiconductor YbB12
K. Ikushima, Y. Kato, M. Takigawa, F. Iga, S. Hiura, T. Takabatake
Physica B

284&282, 274-275
2000/11/01

10.1016/S0921-4526(99)00815-7

The 171Yb nuclear magnetic resonance (NMR) is observed below 10K in the single crystal of the Kondo semiconductor YbB12 with the large shift of 66%. The hyperfine coupling constant of 115 T/μB agrees with the calculated value for the J=7/2 state of free Yb3+ ions, indicating that the the magnetic susceptibility at the low-temperature (T) limit is the Van Vleck contribution within the J=7/2 multiplet. The nuclear spin-lattice relaxation rate at the Yb sites shows an activated temperature dependence below 15K with the activation energy of 87 K, which however is completely different from the behavior at the B sites.
169 研究論文(国際会議プロシーディングス)
共著
High-field magnetization and magnetoresistance of single crystal Yb1-xLuxB12 (x<1/4)
S. Hiura, F. Iga, N. Takamoto, K. Kindo, I. Oguro, T. Takabatake
Physica B

281&282, 271-273
2000/11/01

10.1016/S0921-4526(99)01056-X

For single crystals of Yb1-xLuxB12 (x=0,1/8, 1/4), we have measured the magnetization and magnetoresistance up to 60 and 15 T, respectively. The first-order metamagnetic transition for x=0 becomes a second-order type for x=1/8, keeping the critical field value for B//<1 1 1> at 53 T. The longitudinal magnetoresistance for x=0 is negative for B//<1 1 0> and <1 1 1> at all temperatures, whereas that for x=1/4
at 1.5 K exhibits a positive maximum at around 5 T, followed by a change to negative values. These results suggest that Lu substitution results in the growth of a mid-gap state and weakens the anisotropy of the c-f mixing in YbB12.
170 研究論文(国際会議プロシーディングス)
共著
Temperature dependence of metamagnetic transition in YbB12
S. Kawasaki, N. Takamoto, Y. Narumi, K. Kindo, S. Hiura, F. Iga, T. Takabatake
Physica B

281&282, 269-270
2000/11/01

10.1016/S0921-4526(99)00976-X

Kondo semiconductor YbB12 shows metamagnetic transition with a large jump of magnetization at the "eld of 47 T along <1 0 0> and at 53 T along <1 1 0> and <1 1 1>. Resistivity decreases with increasing field and shows steep decrease at the transition field. By lowering temperature down to 88 mK, a few peaks and shoulders appear in the magnetoresistance curve along <1 0 0> and <1 1 1> below the transition field which may indicate that the density of state is not a simple structure in the band gap. The anisotropic magnetoresistance in YbB12 at various temperatures is reported.
171 研究論文(学術雑誌)
共著
Anisotropic transport and magnetic properties and magnetic-polaron-like behavior in CeTe2-x
M. H. Jung, B. H. Min, Y. S. Kwon, I. Oguro, F. Iga, T. Fujita, T. Ekino, T. Kasuya, T. Takabatake
Journal of the Physical Society of Japan

69/ 3, 937-944
2000/03/01

10.1143/JPSJ.69.937

Highly anisotropic properties of CeTe2-x with the layered Cu2Sb-type structure have been found by the measurements of electrical resistivity ρ(T), magnetic susceptibility χ(T) and magnetization M(B) on single crystals for x = 0.15–0.18. Both M(B//c) and M(B ⊥ c) at 2K exhibit a metamagnetic transition at 0.04 T. M(B//c) saturates rapidly to a value of 1.06 μB/f.u. at around B = 0.2T, whereas M(B ⊥ c) gradually increases to a value of 0.75 μB/f.u. at 5.5 T. The absence of hysteresis and remanence in M(B) indicates an antiferromagnetic ground state. The resistivity ratio ρc/ρ⊥c increases from 3 to 150 on cooling from 300K to 1.5 K. Both ρc and ρ⊥c exhibit a sharp peak at 6K before of long-range magnetic order at TN = 4.3K, where χ(T) exhibits a sharp peak. Measurements of the Hall coefficient RH(T), specific heat C(T), magnetoresistance ρ(B) and electron tunneling have been done for polycrystals with x = 0.13 and 0.18. Electron-tunneling spectra at 77K revealed the presence of a charge density wave with a pseudogap of 0.35 eV. It is found that C(T) exhibits a peak at TN with a significant tail up to 10K, whereas RH(T) continuously increases. The magnetic entropy at TN is only 0.7Rln 2, being indicative of a short-range order above TN. At 1.5K, ρ(B) shows a peak at 0.075T and then decreases rapidly for B ≤ 1T, resulting in a negative magnetoresistance of 20%. These results are analogous to those found in EuTe0.9, GdAs0.95 and CeSb, where low-density carriers form a pseudo-localized state and align the 4f spins within a magnetic polaron.
172 研究論文(学術雑誌)
共著
Phonon Raman scattering study of a Kondo insulator YbB12
Y. Fujita, N. Ogita, N. Shimizu, F. Iga, T. Takabatake, M. Udagawa
Journal of the Physical Society of Japan

68/ 12, 4051-4052
1999/12/01

10.1143/JPSJ.68.4051

Short note
173 研究論文(国際会議プロシーディングス)
共著
Optical spectra of RBx (R=Rare-Earth, x=4,6,12)
S. Kimura, H. Okamura, T. Nanba, M. Ikezawa, S. Kunii, F. Iga, N. Shimizu, T. Takabatake
Journal of Electron Spectroscopy

101-103, 761-764
1999/10/01

10.1016/S0368-2048(98)00363-6

Reflectivity spectra of rare-earth borides (RBx; R=rare-earth, x=4, 6, 12) in the VUV energy range and boron 1s absorption spectra of YB4, YB6 and LuB12 were systematically measured in order to investigate the role of the B-x-cluster in the optical spectra and the physical properties. In the optical spectra, the bonding states of B-6- and B-12-cluster are dominant in RBx (x=4, 6) and RB12, respectively. The electronic structures of the B-6- and the B-12-cluster are discussed.
174 研究論文(国際会議プロシーディングス)
共著
Temperature-dependent high-resolution photoemission study of the Kondo insulator Ce3Bi4Pt3
Y. Takeda, M. Arita, H. Sato, K. Shimada, H. Namatame, M. Taniguchi, K. Katoh, F. Iga, T. Takabatake
Journal of Electron Spectroscopy

101-103, 721-724
1999/10/01

10.1016/S0368-2048(98)00406-X

Temperature-dependent high-resolution photoemission spectroscopy was performed on the Kondo insulator Ce3Bi4Pt3. The spectral weight near the Fermi level (E-F) is reduced as temperature decreases, especially below similar to 100 K. The temperature-dependence of the Ce 4f states was obtained by taking the difference between the He II and He I spectra. The Ce 4f spectrum near E-F at 6 K shows a clear dip structure, indicating the opening of a (pseudo)gap at low temperature.
175 研究論文(国際会議プロシーディングス)
共著
Magnetic excitations in a single crystal of the Kondo semiconductor YbB12
F. Iga, A. Bouvet, L.P. Regnault, T. Takabatake, A. Hiess, T. Kasuya
Journal of Physics and Chemistry of Solids

60, 1193-1196
1999/10/01

10.1016/S0022-3697(99)00081-5

Inelastic neutron-scattering measurements have been performed on a single crystal of the Kondo semiconductor YbB12 by using a triple-axis spectrometer. We have found two magnetic excitations; one is dispersive from 14 to 20 meV and the other centered at 37 meV is less dispersive. The excitation energy of the former agrees with the gap energy derived from the specific heat and transport measurements. With increasing temperature, both excitations disappear at 60 K, above which a quasi-elastic peak and another peak centered around 25 meV appear.
176 研究論文(国際会議プロシーディングス)
共著
Low-temperature high-resolution resonant photoemission study of Kondo insulator Ce3Bi4Pt3
Y. Takeda, M. Arita, Y. Okamura, H. Sato, K. Shimada, K. Mamiya, H. Namatame, M. Tanigichi, K. Katoh, F. Iga, T. Takabatake
Japanese Journal of Applied Physics

38, 209-211
1999/10/01




177 研究論文(国際会議プロシーディングス)
共著
Temperature-dependent high-resolution photoemission study of Y1-xCaxTiO3
M. Arita, Y. Takeda, Y. Okamura, H. Sato, K. Shimada, K. Mamiya, H. Namatame, M. Tanigichi, M. Tsubota, F. Iga, T. Takabatake
Japanese Journal of Applied Physics

38, 206-208
1999/10/01




178 研究論文(学術雑誌)
共著
Antiferromagnetic Kondo-lattice systems Ce2Rh3Ge5 and Ce2Ir3Ge5 with moderately heavy-fermion behavior
Z. Hossain, H. Ohmoto, K. Umeo, F. Iga, T. Suzuki, T. Takabatake
Physical Review B

60/ 12, 10383-10387
1999/10/01

10.1103/PhysRevB.60.10383

The results of electrical resistivity, magnetic susceptibility, specific heat, isothermal magnetization, and magnetoresistance measurements on Ce2Rh3Ge5 and Ce2Ir3Ge5 are reported. For both systems, the magnetic contribution to the resistivity shows the logarithmic T dependence in two separate T ranges being typical of a Kondo-lattice system in the presence of crystalline field. A marked peak in the susceptibility and a sharp drop in the resistivity are observed at TN=5.5 and 9.5 K for Ce2Rh3Ge5 and Ce2Ir3Ge5, respectively. The total drop in resistivity is as large as 300 μΩcm. Isothermal magnetization curves at 1.3 K reveal a field induced transition at 23 T in Ce2Rh3Ge5 and two transitions at 10 and 15 T in Ce2Ir3Ge5. The saturation moment of 0.8μB /Ce is suggestive of the Γ7 doublet ground state in both compounds. The specific heat exhibits a λ-type anomaly around TN , while the values of magnetic entropy at TN are reduced to 50 and 65% of R ln 2 for Ce2Rh3Ge5 and Ce2Ir3Ge5, respectively. The linear coefficients of specific heat are 148 and 152 mJ/K2 mol Ce above TN. Thus both compounds are classified into moderate heavy-fermion antiferromagnets.
179 研究論文(国際会議プロシーディングス)
共著
Optical study of the gap formation and low-energy excitations in YbB12
H. Okamura, S. Kimura, H. Shinozaki, T. Nanba, F. Iga, N. Shimizu, T. Takabatake
Physica B

259-261, 317-318
1999/09/01

10.1016/S0921-4526(98)00628-0

Optical reflectivity experiments have been made on single crystals of the Kondo insulator YbB12 at various temperatures. The obtained optical conductivity spectra reveal that an energy gap of ~25 meV develops upon cooling below 70 K. Detailed temperature-evolution of the gap is presented. In addition, two prominent low-energy excitation peaks are observed, which are attributed to optical transitions involving the Yb 4f-derived states near the Fermi level.
180 研究論文(国際会議プロシーディングス)
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Tunneling spectroscopy of the Kondo-semiconducting gap in YbB12
T. Ekino, H. Umeda, F. Iga, N. Shimizu, T. Takabatake, H. Fujii
Physica B

259-261, 315-316
1999/09/01

10.1016/S0921-4526(98)01141-7

We have carried out the tunneling measurements of YbB12 single crystals using break junction. The differential conductance shows very sharp gap-edge peaks with quite low leakage at zero bias. The representative gap magnitude of 2Δ~220-260 meV at 4.2 K is unexpectedly larger than that from the transport measurements. The resultant ratio 2Δ/kBTχ (the susceptibility-maximum temperature) ~37 is three times larger than that of the Ce-based Kondo semiconductors.
181 研究論文(国際会議プロシーディングス)
共著
Kondo-semiconductor to Kondo-impurity transitionin the heat capacity of Yb1-xLuxB12
F. Iga, S. Hiura, J. Klijn, N. Shimizu, T. Takabatake, M. Ito, Y. Matsumoto, F. Masaki, T. Suzuki, T. Fujita
Physica B

259-261, 312-314
1999/09/01

10.1016/S0921-4526(98)01096-5

We have measured the heat capacity and magnetic susceptibility of Yb1-xLuxB12 single crystals in the temperature range from 4 to 300 K. For x<1/2, formation of an energy gap is manifested by a strong decrease in the magnetic susceptibility below 100 K. The heat capacity exhibits a Schottky anomaly with an activation energy of about 170 K for x<1/2, whereas for x>1/2 single-impurity Kondo behavior with TK~300 K. For 0<x<3/4, the magnetic entropy saturates to a value of R ln 4/molYb around 300 K, indicating that the crystal-field ground state of Yb3+ is a quartet in the whole series of alloys.
182 研究論文(国際会議プロシーディングス)
共著
Optical spectra of the Kondo semiconductor YbB12
H. Okamura, S. Kimura, H. Shinozaki, T. Nanba, F. Iga, N. Shimizu, T. Takabatake
Japanese Journal of Applied Physics

series 11, 85-87
1999/08/01




183 研究論文(国際会議プロシーディングス)
共著
Effects of high pressure and high-magnetic field on the energy gap in a Kondo semiconductor YbB12
F. Iga, T. LeBihan, D. Braithwate, J. Flouquet, M. Idiri, K. Kindo, N. Takamoto, I. Oguro, N. Shimizu, S. Hiura, T. Takabatake
Japanese Journal of Applied Physics

series 11, 88-90
1999/08/01




184 研究論文(国際会議プロシーディングス)
共著
High-resolution photoemission study of the Kondo insulator YbB12
T. Susaki, Y. Takeda, M. Arita, K. Mamiya, A. Fujimori, K. Shimada, M. Taniguchi, F. Iga, T. Takabatake, M. Kasaya, S. Suga, H. Harima
Japanese Journal of Applied Physics

series 11, 94-96
1999/08/01




185 研究論文(国際会議プロシーディングス)
共著
Electron-tunneling studies of the cubic Kondo semiconductors
T. Ekino, K. Katoh, F. Iga, T. Takabatake
Japanese Journal of Applied Physics

series 11, 97-99
1999/08/01




186 研究論文(学術雑誌)
共著
Temperature-dependent high-resolution photoemission study of the Kondo insulator YbB12
T. Susaki, Y. Takeda, M. Arita, K. Mamiya, A. Fujimori, K. Shimada, H. Namatame, M. Taniguchi, N. Shimizu, F. Iga, T. Takabatake
Physical Review Letters

82/ 5, 992-995
1999/02/01

10.1103/PhysRevLett.82.992

We have performed a detailed temperature-dependent photoemission study of the conduction-band states in the Kondo insulator YbB12 using single crystalline samples. Measurements with improved energy resolution have revealed (pseudo)gaps of two energy scales. The size of the narrower gap, ~10 meV and its disappearance at high temperatures are consistent with the temperature dependence of the electrical resistivity and the magnetic susceptibility. The larger pseudogap of ~100 meV depends on temperature even at room temperature. We discuss how the characteristics of the Kondo insulator are reflected in the conduction-band photoemission spectra.
187 研究論文(学術雑誌)
共著
Optical condctivity of the Kondo insulator YbB12: Gap formation and low-energy excitations
H. Okamura, S. Kimura, H. Shinozaki, T. Nanba, F. Iga, N. Shimizu, T. Takabatake
Physical Review B

58/ 12, R7496-R7499
1998/09/15

10.1103/PhysRevB.58.R7496

Optical reflectivity experiments have been conducted on single crystals of the Kondo insulator YbB12 in order to obtain its optical conductivity, σ(ω). Upon cooling below 70 K, a strong suppression of σ(ω) is seen in the far-infrared region, indicating the opening of an energy gap of ~25 meV. This gap development is coincident with a rapid decrease in the magnetic susceptibility, which shows that the gap opening has significant influence on magnetic properties. A narrow, asymmetric peak is observed at ~40 meV in σ(ω), which is attributed to optical transitions between the Yb 4 f -derived states across the gap. In addition, a broad peak is observed at ~0.25 eV. This peak is attributed to transitions between Yb 4f-derived states and the p-d band, and is reminiscent of similar peaks previously observed for rare-earth hexaborides.
188 研究論文(学術雑誌)
共著
Pressure effect on metal-Insulator transition in Y1-xCaxTiO3
T. Naka, T. Matsumoto, F. Iga, Y. Nishihara
Rev. High Pressure Sci. Technology

7, 608-610
1998/09/01



The magnetic susceptibility and electrical resistivity in the metal-insulator (MI) transition of a mixed-valent Ti-oxide system, Y1-xCaxTiO3, is investigated as a function of pressure. This system exhibits obviously the temperature-induced MI transition at x=0.3-0.4. At the low temperature metallic phase, susceptibility, χ(T), is strongly suppressed with pressure. On the other hand, in the high temperature semiconductiong phase χ(T) obeying the Curie-Weiss law is not affected strongly with pressure. The coefficient of the T^2-term in resistivity, A, and the residual resistivity, p(O), are decreases rapidly with pressure. At x=0.375 and 0.39, we recognize the relation of A∝χ(10 K)^2 at the low temperature metallic state.
189 研究論文(学術雑誌)
共著
Magnetic excitations observed by means of inelastic neutron scattering in polycrystalline YbB12
A. Bouvet, T. Kasuya, M. Bonnet, L. P. Regnault, J. Rossat-Mignod, F. Iga, B. Fåk, A.Severing
Journal of Physics: Condensed Matter

10/ 25, 5667-5677
1998/06/29

10.1088/0953-8984/10/25/016

Neutron scattering measurements have been performed with time-of-flight spectrometers on an YbB12 polycrystal. The spectra show clearly the presence of magnetic excitations and a gap at around 14 meV. At low temperature, two broad signals (FWHM similar to 7 meV), one asymmetric, the other one of Lorentzian shape, are well defined. After subtracting the phonon contribution, the former signal is further decomposed into a sharp 15.5 meV peak and a broader 20 meV peak. The 15.5 meV peak behaves similarly to that of the exciton at 14.5 meV for SmB6 as regards both the Q- and T-dependencies. The 20 meV peak was identified as manifesting a transition from a localized Kondo singlet with a gap to a magnetic state. With decreasing temperature, the gap disappears and the scattering becomes quasi-elastic, like for the metallic Kondo system. The second signal, centred at 37 meV, was assigned previously to a crystal-field excitation. However, the observed behaviour is anomalous, indicating a shift of the intensity from the lower peaks to the higher peak, suggesting a change in the T-K-value due to lattice distortion.
190 研究論文(国際会議プロシーディングス)
共著
High-resolution and low-temperature photoemission study of a Kondo insulator
Y. Takeda, M. Arita, H. Sato, K. Shimada, H. Namatame, M. Taniguchi, K. Katoh, F. Iga, T. Takabatake
Journal of Electron Spectroscopy and Related Phenomena

88-91, 391-394
1998/03

10.1016/S0368-2048(97)00272-7

High-resolution and low-temperature photoemission spectroscopy has been carried out on the Kondo insulator Ce3Bi4Pt3. The Ce 3d spectrum indicates that the Ce ion is close to trivalent at low temperature, corresponding to the Kondo regime for Ce3Bi4Pt3. The Ce 4f spectral weight was obtained by difference, and it showed a sharp peak at the Fermi level and broad structures at 120 and 300 meV. We compared the Ce 4f spectrum with the Ce 4f partial density of states given by the LDA calculation and found reasonable agreement.
191 研究論文(国際会議プロシーディングス)
共著
Photoemission and inverse-photoemission study of ferromagnetic valence fluctuating system CeFe2
T. Konishi, K. Mamiya, K. Morikawa, K. Kobayashi, T. Mizokawa, A. Fujimori, F. Iga, H. Kawanaka, Y. Nishihara, A. Delin, O. Eriksson
Journal of Electron Spectroscopy and Related Phenomena

88-91, 303-307
1998/03

10.1016/S0368-2048(97)00144-8

Various electron-spectroscopic results including core-level X-ray photoemission, X-ray absorption, Ce 3d-4f and 4d-4f resonant photoernission, and inverse-photoemission spectroscopy on the valence fluctuating ferromagnet CeFe2 are presented and analyzed with model calculations using the Anderson-impurity Hamiltonian. Surface effects on the spectra are taken into account in the analysis. The valence-band spectra are also compared with the band-structure density of states calculated using the local-spin-density approximation. The validity of describing the Ce 4f states as a localized impurity level and in the 4f band picture is discussed.
192 研究論文(国際会議プロシーディングス)
共著
Photoemission of CeX2Y2 and Ce(Pd1-xCux)3
S. Imada, M. Tsunekawa, T. Iwasaki, S.Suga, Y. Onuki, F. Iga, M. Kasaya, Y. Kawasaki, K. Ishida, Y. Kitaoka, K. Asayama
Journal of Magnetism and Magnetic Materials

177-181, 387-388
1998/01

10.1016/S0304-8853(97)00510-6

A high-resolution valence-band photoemission study on various Ce compounds is presented. The intensity of the features just below the Fermi energy is quite sensitive to the Kondo temperature. On the other hand, the overall shape of the whole 4f spectrum is strongly affected by the overall hybridization between the 4f state and the valence band.
193 研究論文(国際会議プロシーディングス)
共著
Spectroscopic study of Kondo insulator YbB12 using a free electron laser
H. Ohta, T. Nanba, K. Lehnert, S. J. Allen, M. Motokawa, F. Iga, M. Kasaya
Journal of Magnetism and Magnetic Materials

177-181, 341-342
1998/01

10.1016/S0304-8853(97)00401-0

The bolometric response of Kondo insulator YbB12 has been studied for the first time in the energy region from 4 to 130 cm-1 at 1.5 K using the free-electron laser (FEL) at University of California at Santa Barbara. The increases of response are observed at around 50 and 130 cm-1 and they are discussed in connection with the gap structure proposed by Sugiyama et al. previously.
194 研究論文(国際会議プロシーディングス)
共著
Single crystal growth and physical properties of Kondo insulator YbB12
F. Iga, N. Shimizu, T. Takabatake
Journal of Magnetism and Magnetic Materials

177-181, 337-338
1998/01

10.1016/S0304-8853(97)00493-9

YbB12 is the only Yb-based Kondo insulator. We have succeeded for the first time in growing a large single crystal (6 mmφ x 50 mm) of YbB12 by floating zone method using a newly developed image-furnace with four xenon lamps. The results of the electrical resistivity, Hall effect and magnetic susceptibility measurements indicate that the quality of this crystal is better than that of the best polycrystal ever reported.
195 研究論文(国際会議プロシーディングス)
共著
Ce- and Yb- based Kondo semiconductors
T. Takabatake, F. Iga, T. Yoshino, Y. Echizen, K. Katoh, K. Kobayashi, M. Higa, N. Shimizu, Y. Bando, G. Nakamoto, H. Fujii, K. Izawa, T. Suzuki, T. Fujita, M. Sera, M. Hiroi, K. Maezawa, S. Mock, H.v.Löhneysen, A. Brückl, K. Neumaier, K. Andres
Journal of Magnetism and Magnetic Materials

177-181, 277-282
1998/01

10.1016/S0304-8853(97)00842-1

Kondo semiconductors are a class of strongly correlated f-electron materials whose low-energy excitations exhibit a (pseudo)gap at low temperatures. The significant difference between the pseudogap in orthorhombic CeNiSn and CeRhSb and the real gap in cubic Ce3Bi4Pt3 and YbB12 is highlighted by a comparison of transport properties. Low-temparature measurements of the magnetoresistance, Hall coefficient and specific heat of CeNiSn have revealed a field-induced excitation of the coherent low-carrier state. A systematic study of CeNi1-xTxSn (T = Co, Cu and Pt) has shown that the residual carriers in CeNiSn are immobilized by 1% substitution irrespective of the substitutes. Further substitution with 5% Cu induces a magnetic instability at T = 0.
196 研究論文(学術雑誌)
共著
Evolution of electronic states in the Kondo alloy system Yb1-xLuxB12
T. Susaki, T. Konishi, A. Sekiyama, T. Mizokawa, A. Fujimori, T. Iwasaki, S. Ueda, T. Matsushita, S. Suga, H. Ishii, F. Iga, M. Kasaya
Physical Review B

56/ 21, 13727-13730
1997/12/01

10.1103/PhysRevB.56.13727

We have studied the effect of Lu substitution on the Kondo insulator YbB12 by high-resolution photoemission. Comparison of the spectra of YbB12, Yb0.5Lu0.5B12, and LuB12 reveals that the density of states (DOS) of the B sp-derived conduction band near the Fermi level is reduced in YbB12 over a rather wide (similar to 40 meV) energy region. Lu substitution (i) recovers the reduced B sp DOS, (ii) shifts the Yb 4f-derived Kondo peak towards higher binding energy, and (iii) decreases the Yb valence. These results are consistently analyzed using the Anderson-impurity model, and imply interaction between the Yb 4f ions mediated by the Yb 4f-B sp hybridization in YbB12.
197 研究論文(学術雑誌)
共著
Microstructural study of the Ni-doped 2212-phase of Bi-Sr-Ca-Cu-O superconductor single crystals
H. Matsuhata, D. H. Ha, K. Oka, F. Iga, Y. Nishihara
Physica C

290, 122-131
1997/10/15

10.1016/S0921-4534(97)01606-7

The microstructure of Bi2SreCaiCu 2 ,Ni~O,. (x = 0.0, 0.02, 0.04) single crystals grown by the travelling solvent floating zone method have been investigated using transmission electron microscopy. High-resolution observations and diffraction patterns at the [100] zone axis show a very small change in the direction of the modulation wave with increasing of Ni concentration. Streak-shaped diffuse scattering along the row of modulation reflections was observed in all the specimens. Convergent beam diffraction patterns at the [001] zone axis showed two distinct symmetries: either a very strong asymmetry or a weak asymmetry in the b-plane. The strong asymmetry was attributed to the presence of intergrowth layers on the c-plane. The positions of Ni atoms within the structure of the crystal and their influence on the electric resistivity was discussed by considering the present experimental observation results.
198 研究論文(国際会議プロシーディングス)
共著
Magnetic phase transitions of competing system CePtAl
H. Kitazawa, S. Nimori, J. Tang, F. Iga, A. Dönnï, T. Matsumoto, G. Kido
Physica B

237-238, 212-214
1997/07

10.1016/S0921-4526(97)00104-X

Magnetic properties of CePtAl single crystals with orthorhombic TiNiSn-type crystal structure were investigated for each axis by measurements-of magnetic susceptibility, high-field magnetization up to 30 T and magnetoresistance up to 10 T. The pronounced anisotropy observed indicates that magnetism in CePtAl is dominated by a ferromagnetic component along the magnetically easy a-axis and that the magnetically hard axis changes from the c-axis (T > 39 K) to the b-axis (T < 39 K). The results are consistent with the magnetic structure model proposed by neutron diffraction.
199 研究論文(国際会議プロシーディングス)
共著
Optical spectra of the correlated metallic system Ca1-xSrxVO3
H. Makino, I. H. Inoue, M. J. Rozenberg, F. Iga, Y. Aiura, S. Onari
Physica B

237-238, 56-58
1997/07

10.1016/S0921-4526(97)00047-1

Optical reflectivity spectra are studied for single crystals of a perovskite-related 3d(1) correlated metallic system Ca1-xSrxVO3. The measurement is made at room temperature between 0.03 and 40 eV. In the optical conductivity spectra a Drude-like contribution peaked at omega = 0 and other two contributions, which may be associated with Hubbard band and a quasiparticle band at E-F, are observed below 5 eV. The infinite-dimension Hubbard model seems to reproduce the observed features in CaVO3. However, the evolution of these features on varying the value of x is not exactly what we expect from the model.
200 研究論文(国際会議プロシーディングス)
共著
Specific heat and thermoelectric power of Mott transition compound Y0.61Ca0.39TiO3
F. Iga, Y. Nishihara, J. Sakurai, M. Ishikawa
Physica B

237-238, 14-15
1997/07

10.1016/S0921-4526(97)00021-5

Specific heat and thermoelectric power of the Y1-xCaxTiO3 alloy system have been measured, especially near the metal-insulator (MI)-transition region around x = 0.35, in order to clarify the characteristics of a first-order transition in this region. The results are consistent with the previous electrical resistivity and magnetic susceptibility measurements in the same concentrations. This system may be interpreted as a typical example in a crossover region between a Kondo-like state and a Mott-Hubbard phase with strong correlation effect.
201 研究論文(学術雑誌)
共著
Intrinsic photoemission spectra for YbB12 - Reply
T. Susaki, A. Sekiyama, K. Kobayashi, T. Mizokawa, A. Fujimori, M. Tsunekawa, T. Muro, T. Matsushita, S. Suga, H. Ishii, T. Hanyuu, A. Kimura, H. Namatame, M. Taniguchi, T. Miyahara, F. Iga, M. Kasaya, H. Harima
Physical Review Letters

78/ 9, 1832
1997/03/03

10.1103/PhysRevLett.78.1832

A Reply to the Comment by J. J. Joyce and A. J. Arko.
202 研究論文(学術雑誌)
共著
Dimensionality of vortex state magnetization in tilted-field to c-axis of Bi2Sr2CaCu2Oy
Y. Yamaguchi, H. Tomono, F. Iga, Y. Nishihara
Physica C

273/ 3-4, 261-267
1997/01/01

10.1016/S0921-4534(96)00629-6

The dimensionality of vortex states of Bi(2)Sr(2)CaCu(2)Oy has been investigated via DC-magnetization M for an applied field H tilted an angle theta to the c-axis, Magnetization curves at various tilt angles were scaled onto M/cos theta versus Hcos theta. The second-peak field H-p2(theta) at 30 K and the melting-transition field H-M(theta) above 60 K were scaled to H cos theta within a few percent for theta < 45 degrees, although they deviated a little more for theta > 70 degrees. For M/cos theta, a large deviation below H-p2 and a small deviation around H-M is observed. The deviation is considered to be evidence of three-dimensional coupling between pancake vortices. The behavior at 30 K supports a model of crossover from three-dimensional to two-dimensional coupling of vortices across H-p2. The small deviation from the cos a scaling above 60 K suggests a weak three-dimensional coupling. The transition at H-M is supposed to be a lattice melting not accompanying the decoupling effect.
203 研究論文(学術雑誌)
共著
Paramagnetic Meissner effect in superconducting single crystals of Ba1-xKxBiO3
H. T. Kim, H. Minami, W. Schmidbauer, J. W. Hodby, A. Iyo, F. Iga, H. Uwe
Jouranal of Low Temperature Physics

105/ 3-4, 557-562
1996/11

10.1103/PhysRevLett.77.4269

The paramagnetic Meissner effect (PME) in the field cooled susceptibility has been observed in superconducting single crystals of Bal_xKxBi03 using magnetometers of Quantum-Design Co.. The PME is similar to that observed in cuprates and Nb superconductors. For the Bao.63Ko.37 Bi03 crystal the PME is observed in cooling process under the field up to 750 Oe. The PME was not found in the virgin-charged superconducting magnet. The PME is found in a superconductor with trapped flux and may arise from a remnant field, a temperaturc inhomogeneity near To, and a potassium-concentration inhomogeneity.
204 研究論文(学術雑誌)
共著
Low-energy electronic structure of the Kondo insulator YbB12
T. Susaki, A. Sekiyama, K. Kobayashi, T. Mizokawa, A. Fujimori, M. Tsunekawa, T. Muro, T. Matsushita, S. Suga, H. Ishii, T. Hanyuu, A. Kimura, H. Namatame, M. Taniguchi, T. Miyahara, F. Iga, M. Kasaya, H. Harima
Physical Review Letters

77/ 20, 4269-4272
1996/11/11

10.1103/PhysRevLett.77.4269

We have studied the low-energy electronic structure of a Kondo insulator YbB12 by high-resolution photoemission spectroscopy. A “Kondo peak” is observed ,25 meV below the Fermi level, which agrees well with the Kondo temperature, whereas the gap at the Fermi level is found much smaller, indicating that the magnetic properties at higher temperatures (i75 K) are indeed determined by the Kondo effect in spite of the gap formation at lower temperatures. A renormalized band picture is presented to describe the coexistence of the Kondo peak and the transport gap as well as the highly asymmetric line shape of the Kondo peak.
205 研究論文(国際会議プロシーディングス)
共著
Study of first-order metal-insulator transition in the strongly correlated electron system Y1-xCaxTiO3
F. Iga, T. Naka, T. Matsumoto, N. Shirakawa, K. Murata, Y. Nishihara
Physica B

223&224/ 1-4, 526-528
1996/06

10.1016/0921-4526(96)00163-9

The metal-to-insulator (MI) transition in the Y1-xCaxTiO3 system is known to occur at x = 0.35. Furthermore, near this critical metallic region, electrical resistivity and magnetic susceptibility show a broad peak around 170 K and are accompanied by clear thermal hysteresis. This thermal hysteresis in magnetic susceptibility is found to vanish at 1.05 GPa. These phenomena can be interpreted as some aspects of a first-order type metal insulator transition.
206 研究論文(学術雑誌)
共著
Low frequency spectroscopy of the correlated metallic system Ca1-xSrxVO3
M. J. Rozenberg, I. H. Inoue, H. Makino, F. Iga, Y. Nishihara
Physical Review Letters

76/ 25, 4781-4784
1996/06/17

10.1103/PhysRevLett.76.4781

We study the photoemission and optical conductivity response of the strongly correlated metallic system CaxSr12xVO3. We find that the basic features of the transfer of spectral weight in photoemission experiments and the unusual line shape of the optical response can be understood by modeling the system with a one-band Hubbard model close to the Mott-Hubbard transition. We present a detailed comparison between the low frequency experimental data and the corresponding theoretical predictions obtained within the local impurity self-consistent approximation that is exact in the limit of large lattice connectivity.
207 研究論文(学術雑誌)
共著
Magnetization anomaly and anisotropy of Bi2Sr2CaCu2-xNixO8 single crystals
D. H. Ha, K. W. Lee, K. Oka, Y. Yamaguchi, F. Iga, Y. Nishihara
Physica C

260/ 3-4, 242-248
1996/04/10

10.1016/0921-4534(96)00044-5

We have observed the anomalous magnetization of Bi2Sr2CaCu2_xNixO 8 (x = 0 and 0.02) single crystals. Anisotropy decreases with iodine intercalation although it expands the space between CuO 2 layers. Iodine intercalation seems to suppress the magnetization anomaly for Ni - 0% crystals, but not for Ni = 1% substituted crystals. We have discussed these results in terms of the increase of anisotropy by Ni substitution and the dimensional crossover of flux lines. Effects of both oxygen concentration and substitution of a magnetic element for the Cu site on the anisotropy of Bi2Sr2CaCu208 crystals show the same tendency as the case of the YBa2Cu307 superconductor.
208 研究論文(国際会議プロシーディングス)
共著
Study of strongly correlated electron alloy system Y1-xCaxTiO3 
F. Iga, T. Nishiguchi, Y. Nishihara
Physica B

206&207, 859-861
1995



The metal-insulator transition in the Y1-xCaxTiO3 alloy system occurs about x = 0.35. Various physical property measurements suggest that this M-I transition cannot be understood as a simple change in the carrier number in the Hubbard band. In this paper, detailed magnetic property and electrical resistivity behaviours around M-I transition are discussed.
209 研究論文(国際会議プロシーディングス)
共著
Vortex phase diagram near TC via dc and ac magnetization of BSCCO-2212 crystal
Y. Yamaguchi, A. K. Grover, F. Iga, Y. Nishihara
Advances in Superconductivity VII, K. Yamafuji and T. Morishita (Eds.)

7, 205-208
1995




210 研究論文(学術雑誌)
共著
Scaling of negative magnetoresistance and extraordinary Hall effect in CaVO3-d
N.I. Shirakawa, K. Murata, H. Makino, F. Iga, Y. Nishihara
Journal of the Physical Society of Japan

64/ 12, 4824-4833
1995/12

10.1143/JPSJ.64.4824

We have investigated the detailed temperature- and magnetic field dependence of magnetoresistance and Hall resistance in single-crystal CaVO3-d (d~0.2). We have found that the low-temperature magnetoresistance and extraordinary contribution to the Hall resistance are both expressed well in terms of a Brillouin function BJ[gJμBH/kB(T-θ)], θ=-2K. From thwe magnetization vs magnetic field data, it may be deduced that 4% of vanadium ions have a localized magnetic moment of S=3/2 or that 6% have S=1. However a resistance minimum (the Kondo effect) was not obseved in this system and its possible reasons are discussed.
211 研究論文(学術雑誌)
共著
Spectral weight transfer and mass renormalization in Mott-Hubbard system SrVO3 and CaVO3: Influence of long-range Coulomb interaction
K. Morikawa, T. Mizokawa, K. Kobayashi, A. Fujimori, H. Eisaki, S. Uchida, F. Iga, Y. Nishihara
Physical Review B

52/ 19, 13711-13714
1995/11/15

10.1103/PhysRevB.52.13711

We have studied metallic SrVO3 and CaVO3 by inverse photoemission and high-resolution photoemission. In going from Sr to Ca, considerable spectral weight is transferred from the coherent band to the upper and lower Hubbard bands. Meanwhile, the overall intensity rather than the width of the coherent band decreases, implying that the bandwidth remains finite as the system approaches the Mott transition. The result implies that the effect of long-range Coulomb interaction as well as short-range interaction becomes increasingly important towards the transition.
212 研究論文(学術雑誌)
共著
Vortex phase diagram via magnetization measurements for H // c in Bi2Sr2CaCu2O8
Y. Yamaguchi, N. Aoki, F. Iga, Y. Nishihara
Physica C

246/ 3-4, 216-222
1995/05/10

10.1016/0921-4534(95)00152-2

DC and AC magnetization for H parallel to c-axis of an air-annealed Bi2Sr2CaCu2O8 single crystal have been investigated in detail. In a DC magnetization (M-H) curve above 30 K a distinct kink has been observed at H-k close to the irreversibility field (H-irr), where a distinct change also in AC loss (chi '') has been observed. Below 40 K a prominent second peak has been exhibited in the M-H curve. It has been shown that the field (H-m) at the maximum \dM/dH\, where the second peak grows rapidly, is almost temperature independent. The chi '' versus H-DC curve has exhibited a sharp dip near the H-m. Also has been observed another characteristic field H-t where a crossover in the magnetic relaxation occurs. These characteristic fields (H-k, H-m and H-t) give a well defined vortex phase diagram, which is discussed in relation to a flux-melting and a possible vortex/Bose glass transition.
213 研究論文(学術雑誌)
共著
Irreversibility line and the hierarchy of weak links in Bi2Sr2CaCu2O8+d  
F. Iga, A. K. Grover, Y. Yamaguchi, Y. Nishihara, N. Goyal, S.V. Bhat
Physical Review B

51/ 13, 8521-8528
1995/04/01

10.1103/PhysRevB.51.8521

The irreversibility line for H∥c in a single crystal specimen of Bi2Sr2CaCu2O8+Δ (Bi2212) has been determined via vanishing of hysteresis in isothermal dc magnetization measurements. The hysteresis loops (H∥c) in Bi2212 appear to show signatures of two-component magnetic response in several temperature regions where the temperature dependence of irreversibility field charges sharply. It is proposed that the observed behavior may be a consequence of existence of weak links of varying strength in Bi2212.
214 研究論文(学術雑誌)
共著
Magnetic phase diagram of anisotropic layered superconductors via magnetization measurements for H // c in Bi2212
A.K. Grover, F. Iga, Y. Yamaguchi, N. Aoki, K. Ino, N. Goyal, Y. Nishihara
PRAMANA-Journal of Physics

42/ 3, 193-222
1994

10.1007/BF02847681

The Bi2Sr2CaCu2O8 system is viewed as an archetypal of superconductors modelled as Josephson coupled CuO2 bilayers. The isothermal and temperature dependent DC and AC magnetization measurements for H parallel-to c in a single crystal of Bi2212 have been performed. Qualitative changes are observed to occur over a narrow range of temperature values before reaching the superconducting-normal transition. The observed behaviour can be ascribed to the rapid variation in the strength of the coupling between the superconducting CuO2 planes (i.e., bilayers in the case of Bi2212). Strongly coupled planes behave like a 3D superconductor, whereas weakly coupled planes have a two component response attributable to 2D planes and interplanar couplings. We believe that this paper is a plethora of new findings. Our observations imply that resistivity across the planes becomes zero earlier than that within the planes. A new line (designated as H2D(T)) above which the change in the electromagnetic response is dominated by quasi 2D-planes has been determined for the first time. This paper also contains the first observation of Differential Diamagnetic Effect (DDE) in the In-phase AC susceptibility data which signals the onset (at T2D(H)) of dominance of response from 2D-planes. In addition to a host of interesting thermomagnetic history effects which are a consequence of interplay between the diamagnetic responses from the two components, a comparison of irreversibility lines (of the 3D state) determined by different methods on the same specimen of a HTSC is also being presented for the first time. We have come across Paramagnetic Meissner Effect (PME), first recognized in ceramic samples of Bi2212, in the temperature region of dimensional crossover in our single crystal sample, which inter-alia confirms our labelling of the two component behaviour. A schematic phase diagram summarizing the various transformations that can occur near T(c) in the electromagnetic response of an anisotropic layered system has been drawn.
215 研究論文(国際会議プロシーディングス)
共著
Ac-loss anomaly near second-peak in magnetization of a Bi2Sr2CaCu2O8 single crystal
Y.Yamaguchi, N. Aoki, K. Ino, F. Iga, Y. Nishihara
Physica C

235-240/ part 5, 3233-3234
1994/12

10.1016/0921-4534(94)91143-6

A sharp kink in chi ''-vS-Hdc curve for H//c-axis of a Bi2Sr2CaCu2O8 single crystal was observed near the second peak in the tic-magnetization curve around 35K. An anomaly was also observed near the irreversibility field at higher temperatures where the dc-magnetization curve has no second peak. These anomalies are explained from a change in relaxation of the magnetization which has probably close relation to vortex transitions.
216 研究論文(国際会議プロシーディングス)
共著
Anomalies in low field magnetic response near TC in layered high temperature superconductors
A.K. Grover, F. Iga, Y. Yamaguchi, N. Aoki, K. Ino, Y. Nishihara
Physica C

235-240, 3281-3282
1994/12

10.1016/0921-4534(94)91167-3

Magnetic response at low fields is observed to be hysteretic and anomalous in a JCLS for T* <T<T-c(0). These results inter alia confirm that rho(c)= 0 and rho(ab) not equal 0 in this temperature interval.
217 研究論文(学術雑誌)
共著
Anisotropy in ac losses near the irreversibility field of Bi2Sr2CaCu2O8
Y. Yamaguchi, D.H. Ha, F. Iga, Y. Nishihara
Journal of Superconductivity

7/ 2, 291-293
1994/07/10

10.1007/BF00724555

The ac susceptibility under a biased dc field near the irreversibility field (H(irr)) of a Bi2Sr2CaCu2O8 single crystal has been measured. The frequency dependence, the ac-power dependence, and the nearly lossless character of the chi'' vs. H(dc) curve for H \\ a-axis have been roughly explained from a reversible (elastic) fluxoid motion, while those for H \\ c-axis have been explained from a thermally assisted flux-flow (TAFF) model. The obtained parameters are discussed in relation to anisotropic flux-pinning mechanisms in the layered structure of this compound.
218 研究論文(学術雑誌)
共著
Crossover in ac loss mechanism of a Bi2Sr2CaCu2O8 single crystal
Y. Yamaguchi, N. Aoki, F. Iga, Y. Nishihara
Physica C

228/ 1-2, 141-148
1994/07/10

10.1016/0921-4534(94)90185-6

The AC susceptibility (chi', chi'') undcr biased DC field (H(DC)) swept near the irreversibility field (H(irr)) of a Bi2Sr2CaCu2O8 single crystal for the H//c-axis has been investigated. The frequency and AC power dependence of chi'' vs. H(DC) curves at low temperature have been analyzed on the basis of the thermally assisted flux flow (TAFF) model, while those at high temperature and low magnetic field have been analyzed assuming hysteresis loss and reversible (elastic) flux motion. The obtained parameters are discussed in relation to the flux pinning mechanisms of this compound.
219 研究論文(国際会議プロシーディングス)
共著
The study of high field magnetization in the Kondo alloy system Lu1-xYbxB12
F. Iga, Y. Kanamori, K. Sugiyama, M. Date, M. Kasaya
Physica B

199&200, 529-531
1994/04

10.1016/0921-4526(94)91894-5

The high field magnetization measurements in Lu1-xYbxB12 have been performed. In the dilute Yb region (0 < x < 0.5), the molar RB12 high field magnetic susceptibility values at 4.2 K show a linear increase with x and this behavior seems to be of a single-impurity Kondo state. In the more dense Yb region, the molar magnetic susceptibility values at 4.2 K remain almost constant with no relation to x. This behavior may be due to the formation of an energy gap.
220 研究論文(学術雑誌)
共著
Annealing effects on transport and magnetic properties of perovskite-type vanadium oxide CaVO3
A. Fukushima, F. Iga, I. H. Inoue, K. Murata, Y. Nishihara
Journal of the Physical Society of Japan

63/ 2, 409-412
1994/02

10.1143/JPSJ.63.409

We report transport and magnetic properties for single-crystalline CaVO3, which are seriously affected by oxygen off-stoichiometry. The electrical resistivity of a stoichiometric sample is well fit by the relation rho-rho0+AT2 in an extremely wide temperature range. Oxygen off-stoichiometry causes the systematic increment of both rho0 and A. Magnetoresistance data also change drastically due to the oxygen off-stoichiometry; the stoichiometric sample shows positive magnetoresistance with B2 dependence, while a sample with a small number of oxygen vacancies shows an unusual negative magnetoresistance. Magnetic susceptibility of the latter sample shows Curie-Weiss behavior, while that of the former one shows Pauli paramagnetic behavior.
221 研究論文(国際会議プロシーディングス)
共著
Structural and electromagnetic properties of Bi-Sr-Ca-Cu-Ni-O single crystals
D.-H. Ha, K. Oka, H. Matsuhata, F. Iga, Y. Nishihara
Physica B

194/ part 2, 2203-2204
1994/02



The structural and physical properties have been investigated in Bi2Sr2Ca1Cu2-xNixO8+delta (X=0, 0.02 and 0.04) single crystals. Powder X-ray diffraction pattern and high resolution electron microscope image confirm all of the single crystals to be of Bi(2212) phase with a superstructure along the b-axis. Although the superconducting transition temperature and the ratio of rho(b)/rho(a) decrease linearly with Ni concentration, we couldn't observe any structural change in single crystals by high resolution image and electron diffraction measurement. Magnetic hysteresis curves for X=0 and 0.04 at 35 K are the same each other. These results suggest that Ni atoms don't form clusters but distribute uniformly in CuO2 plane.
222 研究論文(国際会議プロシーディングス)
共著
Magnetic and optical properties of CaVO3-y
K. Morikawa, F. Iga, I. H. Inoue, S. Onari, Y. Nishihara
Physica B

194/ part 1, 1205-1206
1994/02

10.1016/0921-4526(94)90932-6

The physical properties of CaVO3-y change from metallic to insulating with increasing oxygen concentration for slightly off-stoichiometric samples. A detailed study of magnetic properties for the off stoichiometric chemical composition has been investigated for Ca(x)VO3-y (x=0.97, 1.00 and 1.03) with various oxidation treatments. The best residual resistance ratio (RRR) is obtained for the sample with stoichiometric Ca and V concentration. Ca0.97VO3-y specially shows growing antiferromagnetic-like anomaly at 110K and low temperature enhancement with increasing heat treatment temperature. In addition, optical reflectivity spectra have been measured for these metallic and insulating samples.
223 研究論文(国際会議プロシーディングス)
共著
Shuvnikov-de Haas effect in CaVO3-y
A. Fukkushima, K. Murata, K. Morikawa, F. Iga, G. Kido, Y. Nishihara
Physica B

194/ part 1, 1161-1162
1994/02

10.1016/0921-4526(94)90910-5

We have observed magnetoresistance oscillations in CaVO3-y single crystals below 4.2 K and in fields above 10 Telsa. The oscillation frequency is about 300 Tesla. This frequency corresponds to a surface cross section of 3 x 10(14) cm-2 in the reciprocal lattice space for cubic CaVO3. This is 1% of the cross section of the first Brillouin zone.
224 研究論文(国際会議プロシーディングス)
共著
Metal-to-insulator transition in CaVOy
I.H.Inoue, K. Morikawa, H. Fukuchi, T. Tsujii, F. Iga, Y. Nishihara
Physica B

194/ part 1, 1067-1068
1994/02

10.1016/0921-4526(94)90863-X

A metal-to-insulator transition in CaVOy by varying the oxygen stoichiometry is reported. Stoichiometric
CaVOy (y--3.00) shows good metallic conductivity but slight oxidation (y=3.05) causes the system to be insulating without any significant structural changes. This behavior is contrary to that seen in another d 1 system LaTiOy, which is a Mott insulator for y=3.00 but becomes metallic by oxidation. We suggest that, in CaVOy, oxygen defects primarily induce additional scattering which causes the metallic CaVO3 to be insulating rather than add extra carriers.
225 研究論文(国際会議プロシーディングス)
共著
Metal insulator transition in (La1-xYx)NiO3
H. Kawanaka, F. Iga, Y. Nishihara
Physica B

194/ part 1, 447-448
1994/02

10.1016/0921-4526(94)90553-3

We are investigating the metal-insulator transition in (La1-xY(x))NiO3. LaNiO3 is one of the very few oxides that exhibit metallic conductivity. It was reported that the substitution of Ln (Ln=Pr,Nd,Sm, and Eu) for La are seen metal-insulator transition in conductivity measurements. This fact is expected that metal-insulator transition is caused by the substitution of Y for La, because Ni-O distance became long for Y3+ is smaller than La3+. We prepared the sample of the partial substitution of Y for La for the first time. In this study we found that a little substitution of Y caused resistivity enlarge. It is expected that metal-insulator transition is caused by more substitution of Y.
226 研究論文(国際会議プロシーディングス)
共著
NMR of 51V in CaVO3-y
H. Nishihara, F. Iga, Y. Nishihara, T. Machida, S. Matsumoto, Y. Nakamura
Physica B

194/ part 1, 235-236
1994/02

10.1016/0921-4526(94)90447-2

NMR experiments of V-51 are reported for a perovskite oxide CaVO3-y, whose electronic and magnetic properties are recently found to depend strongly on oxygen stoichiometry. A sharp free decay signal has been observed for a powdered sample of CaVO3.0 with a metallic Knight shift of K = 0.17 +/- 0.02% and a Korringa value (T1T)-1 = 3.2 +/- 0.5 (sK)-1 which confirms the material is a simple Pauli paramagnet without any appreciable importance of electron-electron correlation.
227 研究論文(学術雑誌)
共著
Crystal growth of Bi2Sr2CaCu2O8 by the top-seeded solution-growth method
K. Oka, T.-S. Han, D.-H. Ha, F.i Iga, H. Unoki
Physica C

215/ 3-4, 407-410
1994/02/10

10.1016/0921-4534(93)90243-J

Single crystals of BiSr2CaCu2O8 have been grown by the top-seeded solution-growth (TSSG) method with Bi2.4Sr1.5Ca1.0Cu1.8Ox as the solvent. Crystals of approximately 10 phi X 6 mm in size and about 4 g weight were obtained. The susceptibility of crystals grown by the TSSG method has been measured.
228 研究論文(学術雑誌)
共著
Elucidation of 3D-to-2D transformation by magnetization studies for H // c in Bi2Sr2CaCu2O8
A.K. Grover, F. Iga, Y. Yamaguchi, N. Aoki, K. Ino, Y. Nishihara
Physica C

220/ 3-4, 353-358
1994/02/10

10.1016/0921-4534(94)90923-7

New magnetization results (for H parallel-to c) on a single crystal of Bi2212 elucidating the transition from a one-component to a two-component response across the 3D-to-2D transformation are reported. The two-component response is identified to emanate from the 2D planes and interplanar links. The magnetization response above the transformation temperature T(s) and very close to T(c) is found to be irreversible in contrast to the reversible behavior So far believed in. A new phase diagram for the mixed state of layered superconductors near T(c) is proposed.
229 研究論文(国際会議プロシーディングス)
共著
Effect of oxygen to the transport properties of Bi2Sr2CaCu2O8+d, YBa2Cu3O7-d and Nd2-xCexCuO4-d
K. Murata, K. Ino, D.-H. Ha, N. A. Fortune, F. Iga, Y. Yokoyama, Y. Nishihara
Advances in Superconductivity V, Y. bando and H. Yamauchi (Eds.)

101-106
1993




230 研究論文(国際会議プロシーディングス)
共著
Lower critical field and irreversibility line in dc and ac magnetization of Bi2Sr2CaCu2O8 single crystal
Y. Yamaguchi, D.- H. Ha, F. Iga, Y. Nishihara
Advances in Superconductivity V, Y. bando and H. Yamauchi (Eds.)

183-186
1993




231 研究論文(国際会議プロシーディングス)
共著
Effect of Ni substitution on the superconducting properties in Bi2Sr2CaCu2O8
D.-H. Ha, K. Oka, F. Iga, H. Unoki, Y. Nishihara
Advances in Superconductivity V, Y. bando and H. Yamauchi (Eds.)

323-326
1993




232 研究論文(国際会議プロシーディングス)
共著
Crystal growth of La2CuO4, YBa2Cu3O7 and Bi2Sr2CaCu2O8 by the TSSG method
K. Oka, H. Unoki, A. M. Moe, F. Iga, K. Kaneko, D.-H. Ha, T.-S. Han
Advances in Superconductivity V, Y. bando and H. Yamauchi (Eds.)

383-386
1993




233 研究論文(国際会議プロシーディングス)
共著
Metal-insulator transition in nearly stoichiometric CaVO3 
I. H. Inoue, K. Morikawa, H. Fukuchi, T. Tsujii, F. Iga, Y. Nishihara
Japanese Journal of Applied Physics

32/ 3, 451-452
1993



A metal-to-insulator (MI) transition in perovskite-type vanadium oxide CaVO(y) in the vicinity of oxygen stoichiometry is reported. In spite of good metallic conductivity for stoichiometric CaVO(y) (y = 3.00), very slight oxidation (y congruent-to 3.05) causes the system to be insulating but does not induce any significant structural changes. This type of MI transition is contrary to that reported on another d1 system LaTiO(y), which is a Mott-Hubbard insulator for y = 3.00 but becomes metallic by oxidation. We suggest that in CaVO(y), extra oxygens primarily induce additional scattering which causes the metallic CaVO3 to be insulating.
234 研究論文(国際会議プロシーディングス)
共著
Magnetism and electrical properties in the perovskite-type Y1-xCaxTiO3  
F. Iga, H. Fukuchi, I. H. Inoue, Y. Nishihara
Japanese Journal of Applied Physics

32/ 3, 332-333
1993



The perovskite-type alloy system Y1-xCaxTiO3 has the magnetic transition near x = 0.l and the metal to insulator (M-I) transition near x = 0.3. This system is found to be classified into the following three categories by the measurements Of magnetic and electrical properties. (i) In the Y-rich region (0 < x < 0.1), this alloy system shows ferromagnetic and semiconducting behaviour. (ii) In the middle region (0. 1 < x < 0.3), the temperature dependence of magnetic susceptibility shows Curie-Weiss type paramagnetic behaviour but transport property shows semiconducting behaviour. (iii) In the Ca-rich region (0.3 < x< 1), the alloy shows metallic transport and Pauli-paramagnetic behaviours.
235 研究論文(学術雑誌)
共著
Electronic structure of Ce(Pd1-xCux)3 (0<x<0.133) studied by photoemission and Bremsstrahlung Isocromat Spectroscopy
S. Ogawa, S. Suga, A.E. Boucquet, F. Iga, M. Kasaya, T. Kasuya, A. Fujimori
Journal of the Physical Society of Japan

62/ 10, 3573-3589
1993/10

10.1143/JPSJ.62.3575

We report on X-ray photoemission (XPS), Bremsstrahlung isochromat (BIS) and 4f resonant ultraviolet photoemission (4f RPES or UPS) spectroscopy of Ce(Pd1-xCux)3 (0 less-than-or-equal-to x less-than-or-equal-to 0.133). With increasing x, the 4f1 BIS peak clearly observable in CePd3 is strongly depressed, the 4f2 BIS peak is found to shift to lower energies and the intensity of the 3d(9)4f0 component of the Ce 3d core XPS is found to become weaker. These experimental results are discussed in comparison with the spectra calculated by using the impurity Anderson model. It is found that lowering of the 4f level energy epsilon(f) along with reduction of the 4f hybridization strength V with x can qualitatively reproduce the experimental result. It is also strongly suggested that the 4f-4f Coulomb energies are different for the 4f BIS and 3d XPS final states.
236 研究論文(国際会議プロシーディングス)
共著
Homogeneity of Bi2Sr2Ca1Cu2O8+d Crystal boules grown by the travelling solvent floating zone method
D. H. Ha, K. Oka, F. Iga, Y. Nishihara
Japanese Journal of Applied Physics

32/ 6A, L778-L781
1993/06/01

10.1143/JJAP.32.L778

We have grown Bi2Sr2Ca1Cu2O8+delta single crystals by the travelling solvent floating zone (TSFZ) method and measured the superconducting properties of various parts of grown crystal boules through Laue photographs, X-ray diffraction, resistivity and magnetization measurement. Generally, a part grown in an early stage shows metallic resistivity behavior, while a later part shows semiconducting behavior in a normal state and a lower superconducting transition temperature. However, after an,appropriate one-day annealing under nearly the same conditions as when it was grown, we could obtain a homogeneous Bi2Sr2Ca1Cu2O8+delta crystal boule by the TSFZ method.
237 研究論文(国際会議プロシーディングス)
共著
Tranport properties under high pressure of the dense Kondo compounds CePdSn and YbB12
F. Iga, M. Kasaya, H. Suzuki, Y. Okayama, H. Takahashi, N. Mori
Physica B

186-188, 419-421
1993/05

10.1016/0921-4526(93)90591-S

We have measured the electrical resistance of the metallic dense Kondo compound CePdSn and the semiconducting dense Kondo compound YbB12 under hydrostatic pressure up to 80 kbar. The pressure dependence of the resistance and its implications for the pressure dependence of the gap energy is discussed.
238 研究論文(国際会議プロシーディングス)
共著
Study on the hexagonal BaFeO3-y system by the Mössbauer effect
H. Kobayashi, F. Iga, Y. Nishihara
Nuclear Instruments and Methods in Physics Research B

76/ 1-4, 258-259
1993/04

10.1016/0168-583X(93)95201-F

We have measured the Fe-57 Mossbauer spectra for the hexagonal BaFeO3-y system with different oxygen contents. The Fe sites are subdivided into two electronically nonequivalent sites, that is, one is a tetravalent site and the other a trivalent one. The Fe4+ concentration decreases not only with increase of oxygen vacancies but also with temperature. In BaFeO2.88, the magnetic order occurs at 130 K and the temperature dependence of Fe4+ concentration changes around 170 K.
239 研究論文(国際会議プロシーディングス)
共著
Mössbauer spectra of 57Fe in CaVO3
F. Iga, H. Kobayashi, Y. Nishihara
Nuclear Instruments and Methods in Physics Research B

76/ 1-4, 74-75
1993/04

10.1016/0168-583X(93)95137-T

We have measured Fe-57 Mossbauer spectra of (CaV0.99Fe0.01O3)-Fe-57 in the region from room temperature to liquid helium temperature for a detailed research of magnetic properties of CaVO3. The magnetic susceptibility of this oxygen stoichiometric sample indicates a small antiferromagnetic-like kink around 170 K but the Mossbauer spectra have two paramagnetic contributions in the entire temperature region. One is a strong single line attributed to Fe3+ and the other is a weak doublet with a large eqQ splitting attributed to Fe2+. The results suggest that the magnetic susceptibility anomaly at 170 K is not related to a magnetic transition but to some kind of structural transition.
240 研究論文(学術雑誌)
共著
Effect of oxygen vacancies on electronic states of CaVO3-d and SrVO3-d: A photoemission study
Y. Aiura, F. Iga, Y. Nishihara, H. Ohnuki, H. Kato
Physical Review B

47/ 11, 6732-6735
1993/03/15

10.1103/PhysRevB.47.6732

There are two structures in the photoemission spectra of CaVO3-delta and SrVO3-delta, in the d-band region. One peak is within approximately 1 eV of the Fermi level and has a sharp Fermi cutoff. The other is a broad peak centered at approximately 1.6 eV. From our photoemission spectra of CaVO3-delta and SrVO3-delta both with oxygen vacancies (delta = approximately 0.1) and without vacancies (delta = 0), it is shown that the relative intensity of the peak near the Fermi level for CaVO3-delta drastically decreases with decreasing oxygen vacancy concentration (delta). In contrast, no such drastic change in intensity is observed for SrVO3-delta. This difference in the effect of oxygen vacancies between SrVO3-delta and CaVO3-delta reflects the fact that a metal-insulator phase transition induced by bulk oxygen vacancies takes place in CaVO3-delta but not in SrVO3-delta.
241 研究論文(国際会議プロシーディングス)
共著
Influence of magnetic ordering on transport properties of Pr2-xCexCuO4
N. A. Fortune, K. Murata, F. Iga, Y. Yokoyama, Y. Nishihara, Y. Hidaka
Advances in Superconductivity IV, H. Hayakawa and N. Koshizuka (Eds.)

93-96
1992




242 研究論文(国際会議プロシーディングス)
共著
Magnetic and transport properties of BaFeO3-y
F. Iga, Y. Nishihara, T. Katayama, K. Murata, Y. Takeda
Journal of Magnetism and Magnetic Materials

104-107, 1973-1975
1992



We have measured the magnetic susceptibility at low fields by SQUID, the transport properties and the specific heat. In
order to study the electronic state of BaFeO 3_y. We found that the quenching of the magnetic moment has a correlation
with the transport properties. In the low-temperature region the temperature dependence of the magnetization shows a
spin-glass-like large thermal hysteresis and the resistivity becomes very large.
243 研究論文(国際会議プロシーディングス)
共著
Mössbauer effect and high-field magnetization of BaFeO3-y
F. Iga, Y. Nishihara, G. Kido, Y. Takeda
Journal of Magnetism and Magnetic Materials

104-107, 1969-1972
1992



The Maessbauer effect of 57Fe and high-field magnetization up to 24 T reveal that the magnetic state between 160 and 220 K is paramagnetic, though the inverse susceptibility extrapolated from 350 K becomes zero below ~ 220 K. The Maessbauer spectrum broadens below 160 K. Below 160 K the magnetization shows existence of a phase transition around 6 kOe from the antiferromagnetic phase to the paramagnetic phase with increasing applied field.
244 研究論文(学術雑誌)
共著
Metal-insulator transition with oxygen content in CaVO3-y  
F. Iga, Y. Nishihara
Journal of the Physical Society of Japan

61/ 6, 1867-1870
1992/06

10.1143/JPSJ.62.3575

To date, CaVO3-y has been reported and believed to be one of the typical metallic oxides. From our detailed study, however, it has been found that this system shows insulating properties with stoichiometric oxygen content. This insulator phase was obtained by oxidation of as-cast single crystals. The magnetic susceptibility of this stoichiometric sample shows an antiferromagnetic like kink around 170 K.
245 研究論文(国際会議プロシーディングス)
共著
Calorimetric Observation of a structural phase-transition at elevated-temperatures in single-crystal C60
N. A. Fortune, K. Murata, F. Iga, Y. Nishihara, K. Kikuchi, S. Suzu,I, I. Ikemoto, Y. Achiba
Physica C

185-189/ 3-4, 425-426
1991



We present the first observation of multiple structural phase transitions in single crystal undopod C60. Crystals annealed in He gas at 500 K (225 °C) exhibit transitions at 250 K and 425 K (150 oC). The 250 K transition has been attributed to an orientational ordering of individual C60 molecules in powdered crystal C60. A third (metastable) transition at 185 K in as-grown crystals is eliminated by annealing at temperatures above 425 K. These results have direct relevance to vapor phase doping of alkali metals at elevated temperatures to induce bulk superconductivity in C60.
246 研究論文(国際会議プロシーディングス)
共著
Hall-effect and magnetic-properties in Pr2-xCexCuO4
N. A. Fortune,K. Murata, Y. Hidaka, M. Ishibashi, Y. Yokoyama, F. Iga, Y. Nishihara
Physica C

185-189/ 3-4, 1277-1278
1991



In magnetization and transport measurements on single crystals of as-grown Pr2-xCexCuO4, we have found that the temperature dependence of the Hall coefficient and Hall mobility is strongly related to the onset of antiferromagnetic order at the Cu sites in the Cu-O2 planes (below TNeel) and to the appearance of a weak induced moment on the Pr sites. We present here our results for Pr1.92Ce0.05CuO4 (TNeel = 170 K).
247 研究論文(国際会議プロシーディングス)
共著
Negative muon spin rotation studies on single crystal La2CuO4+d above TN
H. Kitazawa, E. Torikai, K. Nishiyama, K. Nagamine, F. Iga, Y. Nishihara
Physica C

185-189/ 3-4, 1087-1088
1991



The negative muon spin rotation (μ-SR) experiments have been performed on the La2Cu04+d single crystals (TN= 255K)
in the paramagnetic region. The temperature dependence of the two components of paramagnetic shift was obtained from
the electron spectrum emitted from μ-O. The hyperfine coupling constants, Ahf I and Ahf 2 ( Ahf 1 > Ahf 2 ) obtained from the K - χ plot are proposed to be ascribed to the O(1) and 0(2) sites, respectively.
248 研究論文(国際会議プロシーディングス)
共著
Fermi surfaces of UB12
H. Harima, A. Yanase, Y. Onuki, I. Umehara, Y. Kurosawa, N. Nagai, K. Satoh, M. Kasaya, F. Iga
Physica B

165&166, 343-344
1990



We have measured the de Haas-van Alphen(dHvA) oscillation, and calculated the Fermi surfaces from the band structure of UB12. The angular dependence of the dHvA frequencies are consistent to the calculated extremal areas. Fermi surfaces consist of electron and hole ones, being a compensated metal. The measured cyclotron masses in the range of 0.6m0 to 6m0 are also agreement with the calculations.
249 研究論文(国際会議プロシーディングス)
共著
Magnetic and transport properties of uranium borides, UB12, UIr3B2 and UOs3B2
M. Kasaya, F. Iga, K. Katoh, K. Takegahara, T. Kasuya
Journal of Magnetism and Magnetic Materials

90&91, 521-522
1990



From the magnetic susceptibility and specific heat measurements, it is found that UB12, UIr3B2 and UOs3B2 have no magnetic order down to 1.7 K. The observed specific heat coefficient γ is 20, 39 and 80 mJ/mol K2 for UB12, UOs3B2 and UIr3B2, respectively. Mass enhancement factor for UIr3B2 is about 5.5.
250 研究論文(国際会議プロシーディングス)
共著
Acoustic de Haas-van Alphen effect in f-electron systems
R. Settai, H. Matsui, Y. Ohe, T. Goto, T. Fujimura, S. Sakatsume, Y. Onuki, F. Iga, M. Kasaya
Journal of Magnetism and Magnetic Materials

90&91, 446-448
1990



The acoustic de Haas-van Alphen effects of CeCu2, YCu2 and UB12 have been observed by the sound velocity measurements. The angular variation of the frequencies and the mass enhancement effect are determined.
251 研究論文(国際会議プロシーディングス)
共著
Forced volume-magnetostriction in Ce(Fe1-xAlx)2
G. Kido, Y. Nakagawa, Y. Nishihara, F. Iga
Journal of Magnetism and Magnetic Materials

90&91, 75-76
1990



Magnetostriction processes in Ce(Fe1-xAlx)2 systems have been measured by means of a capacitance method in pulsed high magnetic fields up to 300 kOe. A discontinuous increase in the volume was found at the rnetamagnetic transition from an antiferromagnetic state to a ferromagnetic one, in the low temperature regime. This transition was attributed to be a latt ice distortion.
252 研究論文(学術雑誌)
共著
Magnetoresistance and de Haas-van Alphen effects in UB12
Y. Onuki, I. Umehara, Y. Kurosawa, N. Nagai, K. Satoh, M. Kasaya, F. Iga
Journal of the Physical Society of Japan

59/ 7, 2320-2323
1990/07



We have measured the magnetoresistance and de Haas-van Alphen (dHvA) effect in UB12. Shown by the resultof magnetoresitance, this material is a compensated metal, and possesses an open orbit along the <100> direction. The detected dHvA branches are roughly explained by the recent band calculation performed by Harima et al. The cyclotron mass is in the range of 0.6 m0 to 6 m0.
253 研究論文(国際会議プロシーディングス)
共著
Electrical resitivity, Hall constant and magnetic susceptibility in the Kondo states Ce(Ni1-xPdx)Sn and YbNiSn
M. Kasaya, T. Tani, F. Iga, T. Kasuya
Journal of Magnetism and Magnetic Materials

76&77, 278-280
1988



The compound CePdSn is found to be an antiferromagnet with a Neel point of 7 K. Experimental results on the alloy system
Ce(Ni1-xPdx)Sn revealed that the transport properties change from a semiconducting-like Kondo state to a metallic one with
increase in x. YbNiSn is a new Kondo compound with a small ferromagnetic moment below 5.5 K.
254 研究論文(国際会議プロシーディングス)
共著
Specific heat measurements of YbB12 and YbxLu1-xB12
F. Iga, M. Kasaya, T. Kasuya
Journal of Magnetism and Magnetic Materials

76&77, 156-158
1988



Specific heat measurements of YbB12 have revealed that the band gap is formed on the Kondo lattice originated from the
Γ8 ground state. Increase of C/T of YbxLu1-xB12 in the low temperature region takes place in the concentration range larger than x = 1/2 and its origin also is from 4f-band gap formation.
255 研究論文(学術雑誌)
共著
Field-induced metallic state in YbB12 under high magnetic field
K. Sugiyama, F. Iga, M. Kasaya, T. Kasuya, M. Date
Journal of the Physical Society of Japan

57/ 11, 3946-3953
1988/11



High field magnetization and magnetoresistance of Ybb12 are investigated up to 550 kOe at low temperatures with special inr¥terest in the semiconductive band gap energy under the field. Very large negative magnetoresistance is found and the system is perfectly metallic around 500 kOe with anomalous increase in the magnetization. The experimental results are explained by introducing model that the band gap produced by the mixing of f- and conduction electrons is destroyed by applying the field and the released f-electron gives an anomalous magnetization. The gJ-value is estimated as 1.9 for the f-level and this means that Γ8 may be most favourable for the responsible f-electron at the Fermi energy.
256 研究論文(国際会議プロシーディングス)
共著
Systematics of valence fluctuating states in f-electron systems
T. Kasuya, M. Kasaya, K. Takegahara, F. Iga, B. Liu, N. Kobayashi
Journal of the Less-Common Metals

127, 337-347
1987



Results of our theoretical and experimental studies on the anomalous Kondo ferromagnet CeRh3B2 based on the f0 (lz = 0) band and the d, screening of the 4f hole are shown. Gap formation in the unique compounds YbB12 and Ce(Pd1-xCux)3 is described by comparing with SmB6. Gap creation on the Kondo lattice is thereby established. Finally, a dense Kondo state in samarium compounds is shown in our recent experiments compared with that in cerium compounds.
257 研究論文(国際会議プロシーディングス)
共著
Kondo effect in Sm1-yYbyB6 and Tm1/2Yb1/2B6
B. Liu, M. Kasaya, F. Iga, T. Kasuya
Journal of Magnetism and Magnetic Materials

47&48, 472-474
1985



The transport and specific heat measurements have indicated that (1) Sm1-yYbyB6, shows Kondo effect in the low
concentration range of Sm, (2) the ground state of the crystal field split J = 5/2 level of the Sm ion is a quartet Γ8. (3) Tm1/2Yb1/2B6 also shows Kondo effect.
258 研究論文(国際会議プロシーディングス)
共著
Mixed valence properties of YbB12
M. Kasaya, F. Iga, M. Takigawa, T. Kasuya
Journal of Magnetism and Magnetic Materials

47&48, 429-435
1985



The magnetic susceptibility, electrical resistivity, Hall effect, NMR, XPS and specific heat measurements indicate that (1) YbB12 is a mixed valence compound with the valence of + 2.9, (2) as the temperature decreases χ goes through a pronounced peak at 75 K and then decreases rapidly to a constant value after subtraction of impurity effect, (3) the transport properties are very similar to those of Sm6, (4) 1/T1 obtained from 11B NMR shows a temperature dependence of activation type, (5) the magnetic specific heat is of Schottky type. These results suggest that YbB12 is a new mixed valence compound accompanied with the formation of an energy gap. Some of the experimental results are analysed by the model used in SmB6.
259 研究論文(国際会議プロシーディングス)
共著
Magnetic and electric properties of YbB12 under high magnetic field
K. Sugiyama, A. Ohya, M. Date, F. Iga, M. Kasaya, T. Kasuya
Journal of Magnetism and Magnetic Materials

52, 283-285
1985



High magnetic field studies up to 550 kOe are done on the magnetic and electric properties of YbB12 at low temperatures with special interest in the band gap energy under the field. The gap is quenched around 200 kOe and a large magnetization appears above 450 kOe.
260 研究論文(国際会議プロシーディングス)
共著
Kondo state in the alloy system Lu1-xYbxB12
F. Iga, M. Kasaya, T. Kasuya
Journal of Magnetism and Magnetic Materials

52, 279-282
1985



Magnetic susceptibility and lattice parameter measurements indicate that Yb valence in Lu1-xYbxB12 is independent of x and about 2.9-3.0. Resistivity and Hall constant measurements reveal that this alloy system changes from a Kondo-like state to the SmB6-type mixed valence state with increasing x.
261 研究論文(国際会議プロシーディングス)
共著
Preparation and physical properties of rare earth hexaborides and dodecaborides
M. Kasaya, F. Iga
AIP Conference Proceedings, Boron Rich Solids

11-18
1985




262 研究論文(学術雑誌)
共著
XPS study of rare earth dosecaborides: TmB12, YbB12 and LuB12
F. Iga, Y. Takakuwa, T. Takahashi, M. Kasaya, T. Kasuya
Solid State Communications

50/ 10, 903-905
1984



Single phase TmB12, YbB12 and LuB12 were successfully obtained by the arc-melting and floating zone methods. The XPS measurements of these compounds have indicated that (1) only YbB12 is a valence fluctuating compound with the valency of +2.9 at room temperature, (2) the correlation energy between Yb2+ and Yb3+ ions is about 6.0 eV, (3) Tm and Lu ions in the dodecaborides are in a trivalent state, and (4) the binding energy of the 4f level in TmB12 and LuB12 is 5.0 and 7.7 eV, respectively.
263 研究論文(国際会議プロシーディングス)
共著
A new and typical valence fluctuating system YbB12
M. Kasaya, F. Iga, K. Negishi, S. Nakai, T. Kasuya
Journal of Magnetism and Magnetic Materials

31-34, 437-438
1983



Magnetic susceptibility, electrical resistivity, Hall effect and LIII absorption measurements indicate that YbBl2 exhibits unique behavior uncharacteristic of hitherto studied ytterbium compounds. We suggest that YbB12 is a valence fluctuating system accompanied with the formation of an energy gap.
264 研究論文(国際会議プロシーディングス)
共著
Susceptibility, LIII absorption and specific heat across the mixed valence range in R1-xLaxB6 and R1-yYbyB6 (R=Tm, Sm)
M. Kasaya, F. Iga, H. Yashima, T. Satoh, M. Ohashi, S. Nakai, T. Kasuya
Journal of Magnetism and Magnetic Materials

31-34, 389-390
1983



Magnetic susceptibility, L 3 absorption and lattice parameter measurements indicate that Tm valence changes from 3+ to mixed valence with decreasing y in Tm1-yYbyB6, whereas mixed valence seems to persist even to the single impurity limit in Tm1-xLaxB6. These results are compared with experimental one on Sm in the isomorphous alloys.