研究者詳細 - 横谷　尚睦

論文 - 横谷　尚睦

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Electronic excitations in Bi2Sr2 CaCu2O8: Fermi surface, dispersion, and absence of bilayer splitting

H. Ding, A. F. Bellman, J. C. Campuzano, M. Randeria, M. R. Norman, T. Yokoya, T. Takahashi, H. Katayama-Yoshida, T. Mochiku, K. Kadowaki, G. Jennings, G. P. Brivio

Physical Review Letters 76 ( 9 ) 1533 - 1536 1996年

記述言語：英語掲載種別：研究論文（学術雑誌）

From a detailed study, including polarization dependence, of the normal state angle-resolved photoemission spectra for Bi2Sr2CaCu2O8, we find only one CuO2 band related feature. All other spectral features can be ascribed either to umklapps from the superlattice or to “shadow bands.” Even though the dispersion of the peaks looks like band theory, the line shape is anomalously broad and no evidence is found for bilayer splitting. We argue that the “dip feature” in the spectrum below Tc arises not from bilayer splitting, but rather from many-body effects. © 1996 The American Physical Society.

DOI： 10.1103/PhysRevLett.76.1533

High-Resolution Photoemission Study of UNi2Al3 and URu2Si2

See-Hun Yang, Hiroshi Kumigashira, Takayoshi Yokoya, Ashishi Chainani, Takashi Takahashi, Se-Jung Oh, Noriaki Sato, Takemi Komatsubara

Journal of the Physical Society of Japan 65 ( 8 ) 2685 - 2689 1996年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Society of Japan

High-resolution low-temperature photoemission spectroscopy has been performed for UNi2Al3 and URu2Si2 to study the nature of 5f electrons in U-based heavy fermion materials. Photoemission spectra of both compounds have a sharp peak at EF and a large broad peak around 2 eV, while only UNi2Al3 exhibits an additional small broad feature at 0.6 eV. The two features in the vicinity of EF (the EF-peak and the small structure at 0.6 eV) are found to have a dominant U 5f character while the prominent peak around 2 eV is due to the Ni 3d or Ru 4d states. Comparison with the band structure calculations and experimental results on UPd2Al3 suggests that the EF-peak is ascribed to itinerant U 5f electrons while the following broad feature at 0.6 eV represents localized f electrons.

DOI： 10.1143/JPSJ.65.2685

MOMENTUM DEPENDENCE OF THE SUPERCONDUCTING GAP IN BI2SR2CACU2O8 (VOL 74, PG 2784, 1995) 査読

H DING, JC CAMPUZANO, AF BELLMAN, T YOKOYA, MR NORMAN, M RANDERIA, T TAKAHASHI, H KATAYAMAYOSHIDA, T MOCHIKU, K KADOWAKI, G JENNINGS

PHYSICAL REVIEW LETTERS 75 ( 7 ) 1425 - 1425 1995年8月

Angle-resolved and resonant photoemission study of Sr2RuO4

T. Yokoya, A. Chainani, T. Takahashi, H. Katayama-Yoshida, M. Kasai, Y. Tokura

Journal of Physics and Chemistry of Solids 56 ( 12 ) 1885 - 1886 1995年

記述言語：英語掲載種別：研究論文（学術雑誌）

Resonant and angle-resolved photoemission measurements were performed for the non-cuprate layered superconductor Sr2RuO4 (Tc = 0.93 K). Resonant valence band photoemission spectroscopy shows the existence of a correlation induced satellite to the main d band across the Ru 4p-4d threshold. Angle-resolved measurements show a surprisingly narrow band of about 50 meV width and positioned at 26 meV below the Fermi level, along the ΓX and ΓZ directions. This band, arising fom Ru 4d-O 2p antibonding states, is nearly dispersionless over a wide range of k points about the zone boundary along the Γ-Z direction (Ru-O direction), reminiscent of the extended Van-Hove singularity observed in the cuprates. The results emphasize the presence of strong correlations, substantial charge-transfer character to states at the Fermi level and the existence of a singular flat band near the Fermi-level in this low Tc layered superconductor, just like the cuprates. © 1995.

DOI： 10.1016/0022-3697(95)00193-X

High-resolution photoemission spectroscopy of the Verwey transition in Fe3O4

A. Chainani, T. Yokoya, T. Morimoto, T. Takahashi, S. Todo

Physical Review B 51 ( 24 ) 17976 - 17979 1995年

記述言語：英語掲載種別：研究論文（学術雑誌）

We study the first-order Verwey transition in single-crystal Fe3O4 using high-resolution temperature-dependent (100-300 K) photoemission spectroscopy. The near-Fermi-level (EF) spectrum exhibits a clear gap of ∼70 meV in the occupied part of the density of states (DOS) in the low-temperature semiconducting phase. The gap is closed above the transition temperature TV=122 K, establishing a metal-semiconductor transition. The Fe 3d derived features arising from Fe3+ (eg2) majority- and Fe2+ (t2g1) minority-spin states responsible for the transition retain their character just above the transition but merge into a single feature by 300 K. Simultaneously, the DOS at EF increases systematically in the metallic phase. The results indicate the existence of, and a gradual change in, the short-range order for T&gt
TV, which causes the nonmetalliclike electrical conductivity observed just above TV. © 1995 The American Physical Society.

DOI： 10.1103/PhysRevB.51.17976

Crystal-field splitting in CeB6 as observed by high-resolution photoemission

T. Takahashi, T. Morimoto, T. Yokoya, S. Kunii, T. Komatsubara, O. Sakai

Physical Review B 52 ( 13 ) 9140 - 9142 1995年

記述言語：英語掲載種別：研究論文（学術雑誌）

Comparative high-resolution photoemission measurement was performed for CeB6 and CeCu6 to study the electronic structure near EF. The photoemission spectra near EF of both compounds show a characteristic satellite at 270 meV from EF due to the spin-orbit splitting of a Ce 4f electron (4f7/2-4f5/2), while the spectrum of CeB6 exhibits an additional small feature around 70 meV. Model calculations based on the noncrossing approximation show that this satellite is due to the crystal-field splitting (7-8) of the Ce 4f5/2 state in CeB6. © 1995 The American Physical Society.

DOI： 10.1103/PhysRevB.52.9140

Electronic structure of the quasi-one-dimensional organic conductors DCNQI (N,N′-dicyanoquinonediimine)-Cu salts

Akinori Tanaka, Ashish Chainani, Takayoshi Yokoya, Takashi Takahashi, Takahumi Miyazaki, Shinji Hasegawa, Takehiko Mori

Physical Review B 52 ( 11 ) 7951 - 7959 1995年

記述言語：英語掲載種別：研究論文（学術雑誌）

A comparative study of the electronic structure of in situ synthesized quasi-one-dimensional organic conductors (DMe-DCNQI)2Cu and (MeBr-DCNQI)2Cu has been carried out using various techniques of electron spectroscopy, where DMe-DCNQI and MeBr-DCNQI are 2,5-dimethyl-N,N′-dicyanoquinonediimine and 2,5-methylbromine-N,N′-dicyanoquinonediimine, respectively. From the photon-energy dependence of the valence-band photoemission spectra obtained using synchrotron radiation, the origins of each observed feature are unambiguously characterized. While the feature at the Fermi level is primarily derived from π-bonded C and N 2p states, the contribution of Cu 3d states at the Fermi level is larger in the (MeBr-DCNQI)2Cu compared to the (DMe-DCNQI)2Cu. X-ray photoemission spectra of the valence band imply extensive hybridization of the Cu 3d states with C and N 2p states near the Fermi level. Line-shape analyses of the Cu 2p core-level spectra show that the ratio of Cu2+ to Cu+ is higher in (MeBr-DCNQI)2Cu compared to (DMe-DCNQI)2Cu, with the ratio being closer to 1:2 for (MeBr-DCNQI)2Cu. From a comparison of C KVV and Cu LVV Auger spectra with the self-convolution of the valence-band spectra, it is found that the effective on-site Coulomb correlation energies between the valence electrons are high on C sites as well as Cu sites in both salts, with U(pp)=6.5 eV and U(dd)=8.0 eV, respectively. In conjunction with core-level spectra, the spectra indicate that the on-site Coulomb correlation, the hybridization strength, and the charge-transfer energy between the Cu 3d and N 2p ligands are very similar in the two salts. The metal-insulator transition in (MeBr-DCNQI)2Cu at 160 K is then facilitated by the proximity of the Cu2+-to-Cu+ ratio to 1:2 supporting charge disproportion, while deviation from it stabilizes the metallic phase in (DMe-DCNQI)2Cu down to very low temperatures. © 1995 The American Physical Society.

DOI： 10.1103/PhysRevB.52.7951

Momentum distribution sum rule for angle-resolved photoemission

Mohit Randeria, Hong Ding, J. C. Campuzano, A. Bellman, G. Jennings, T. Yokoya, T. Takahashi, H. Katayama-Yoshida, T. Mochiku, K. Kadowaki

Physical Review Letters 74 ( 24 ) 4951 - 4954 1995年

記述言語：英語掲載種別：研究論文（学術雑誌）

Within a spectral function A(k,ω) interpretation of angle-resolved photoemission (ARPES), the intensity obeys the sum rule dω fωA(k,ω)=nk, where f is the Fermi function and nk the momentum distribution. We show the usefulness of this sum rule for analyzing ARPES data. The integrated intensity in Bi2Sr2CaCu2O8 at kF is independent of temperature even though the spectrum is strongly T dependent. We also demonstrate the possibility of measuring nk using ARPES for YBa2Cu4O8. © 1995 The American Physical Society.

DOI： 10.1103/PhysRevLett.74.4951

Anisotropy of the electronic structure and superconducting gap in Bi2Sr2CaCu2O8

T. Yokoya, T. Takahashi, T. Mochiku, K. Kadowaki

Physical Review B 51 ( 6 ) 3945 - 3948 1995年

記述言語：英語掲載種別：研究論文（学術雑誌）

Existence of a large anisotropy of the superconducting energy gap in Bi2Sr2CaCu2O8 was confirmed by angle-resolved photoemission. The gap size, Δ (the full superconducting energy gap being 2Δ), is 0-4 meV for the ΓX and ΓY directions while that of the ΓM̄ direction, 45°away from the former two, is 17-19 meV. Comparison with previous reports was made with regard to the symmetry of the superconducting order parameter. It was also found that the electronic structure of the normal state in the ΓY direction is considerably modified by the incommensurate superstructure in the BiO plane. In contrast, the size of superconducting gap is identical within the present accuracy for both ΓX and ΓY directions, suggesting that the superstructure has a negligible effect on the size of the superconducting energy gap. © 1995 The American Physical Society.

DOI： 10.1103/PhysRevB.51.3945

Arpes studies in the normal and superconducting state of Bi2Sr2CaCu2O8

J. C. Campuzano, H. Ding, A. Bellman, M. R. Norman, M. Randeria, G. Jennings, T. Yokoya, T. Takahashi, H. Katayama-Yoshida, T. Mochiku, K. Kadowaki

Journal of Physics and Chemistry of Solids 56 ( 12 ) 1863 - 1864 1995年

High-energy resolution photoemission study of CsxC60. Comparative study with KxC60

T. Takahashi, T. Morimoto, T. Yokoya

Physica C: Superconductivity and its applications 232 ( 3-4 ) 227 - 232 1994年11月

記述言語：英語掲載種別：研究論文（学術雑誌）

Comparative high-energy resolution (ΔE ∼ 30 meV) photoemission spectroscopy has been performed for CsxC60 and KxC60 to study the difference and similarity in the electronic structure near the Fermi level (EF) between the superconducting and the non-superconducting alkali-doped C60. It was found that CsxC60 (0≤x≤6) is a semiconductor or an insulator in the whole composition range except for 0&lt
x≤1 where the Cs doped C60 appears to be metallic with a finite density of states at EF. This shows a sharp contrast with the case of KxC60 which exhibits a relatively large density of states at EF in the composition of 0&lt
x&lt
4 with the largest intensity around x = 3. © 1994.

DOI： 10.1016/0921-4534(94)90780-3

HIGH-ENERGY-RESOLUTION PHOTOEMISSION-STUDY OF SINGLE-CRYSTAL CESIX (X=1.86 AND 1.7) - A TEST OF THE KONDO MODEL 査読

T TAKAHASHI, T MORIMOTO, T YOKOYA, M KOHGI, T SATOH

PHYSICAL REVIEW B 49 ( 22 ) 15688 - 15691 1994年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMERICAN PHYSICAL SOC

Photoemission spectra of single crystal CeSi(x) (x = 1.86; nonmagnetic, T(K) approximately 20 K, and x = 1.7; ferromagnetic, T(c) = 13.7 K) were measured at very high-energy resolution and low temperature. The Fermi-edge band in the photoemission spectrum of CeSi1.86 is narrower and closer to E(F) Compared with the previous reports on polycrystals, showing closer resemblance to the calculation based on the Kondo model. The intensity of the Fermi-level peak is drastically reduced in going from CeSi1.86 to CeSi1.7, which is in good qualitative agreement with the Kondo model.

Photoemission study of phase transition of solid C60

T. Takahashi, T. Morimoto, A. Ito, T. Yokoya

Solid State Communications 92 ( 4 ) 331 - 335 1994年

記述言語：英語掲載種別：研究論文（学術雑誌）

The change of electronic structure due to the phase transition in solid C60 from face-centered-cubic to simple-cubic was observed by photoemission spectroscopy. The effect of disorder in solid C60 to the electronic structure was also studied by comparing the photoemission spectra of a highly disordered C60 film prepared by vapor deposition on a cold (≈20K) substrate and a single crystal C60 film. © 1994.

DOI： 10.1016/0038-1098(94)90711-0

dx2-y2-like superconducting gap in Bi2Sr2CaCu2O6 observed by angle-resolved photoemission

T. Yokoya, T. Takahashi, T. Mochiku, K. Kadowaki

Physica C: Superconductivity and its applications 235-240 ( 2 ) 1013 - 1014 1994年

記述言語：英語掲載種別：研究論文（学術雑誌）

A large anisotropy of superconducting energy gap in Bi2Sr2CaCu2O8 was directly observed by high-energy resolution angle-resolved photoemission spectroscopy with a high-quality twin-free single crystal. The superconducting gap (Δ) observed in the ΓM direction is 17-19 meV while that in the ΓX (or ΓY) direction is 0-2 meV, suggesting the dx2-y2 superconducting order parameter. © 1994.

DOI： 10.1016/0921-4534(94)91731-0

Anomalous band folding due to the BiO superstructure in Bi2Sr2CaCu2O8 studied by angle-resolved photoemission

T. Yokoya, T. Takahashi, T. Mochiku, K. Kadowaki

Physical Review B 50 ( 14 ) 10225 - 10229 1994年

記述言語：英語掲載種別：研究論文（学術雑誌）

Angle-resolved photoemission measurement has been performed on a twin-free Bi2Sr2CaCu2O8 single crystal to study the band-folding effect due to the BiO superstructure. It was found that the superstructure running along the b axis considerably modulates the electronic structure near the Fermi level (EF) in that direction, almost smearing out the dispersive feature of a band which crosses the Fermi level. In contrast with the energy region near EF, the main body of the valence band (1 to 7 eV in binding energy) shows a comparable but different energy dispersion between the a- and b-axis directions. This indicates that the band-folding effect may be different between the near-EF region and the whole valence band in Bi2Sr2CaCu2O8. © 1994 The American Physical Society.

DOI： 10.1103/PhysRevB.50.10225

High-resolution temperature-dependent photoemission spectroscopy of FeSi: Evidence for localized states

A. Chainani, T. Yokoya, T. Morimoto, T. Takahashi, S. Yoshii, M. Kasaya

Physical Review B 50 ( 12 ) 8915 - 8917 1994年

記述言語：英語掲載種別：研究論文（学術雑誌）

We report the high-resolution temperature-dependent photoemission spectra of FeSi. We measure an extremely small gap of less than 5 meV in contrast to electrical-resistivity, magnetic-susceptibility, and optical-conductivity measurements, which show an activation energy of about 60 meV. The difference in the gap value is attributed to localized states near the Fermi level, which effectively increase the activation energy associated with a charge or spin excitation as observed in conductivity and susceptibility studies. The density of states at the Fermi level increase systematically on increasing temperature and result in a transition to a metallic phase at 300 K. The data suggest that localization effects play an important role in the ground-state properties of FeSi. © 1994 The American Physical Society.

DOI： 10.1103/PhysRevB.50.8915

SUPERCONDUCTING ENERGY-GAP IN BI2SR2CACU2O8 OBSERVED BY HIGH-RESOLUTION PHOTOEMISSION SPECTROSCOPY 査読

T YOKOYA, T TAKAHASHI, A FUJIWARA, Y KOIKE

SOLID STATE COMMUNICATIONS 87 ( 6 ) 553 - 556 1993年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：PERGAMON-ELSEVIER SCIENCE LTD

A high-energy resolved low-temperature photoemission spectrometer of angle-integrated mode was constructed to study the detailed electronic structure near the Fermi level of novel materials, in particular, cuprate high-Tc superconductors. Photoemission measurement with this spectrometer on a Bi2Sr2CaCu2O8 polycrystal clearly showed a transfer of spectral. weight from the vicinity of the Fermi level to the higher binding-energy region below the superconducting transition temperature, indicating opening of the superconducting gap. Numerical simulation for both normal and superconducting states showed that the superconducting gap parameter (DELTA) of 18 meV with dx2-y2 symmetry gives a most reasonable fitting.

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