Faculty Profiles - YOKOYA Takayoshi

Papers - YOKOYA Takayoshi

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High-resolution photoemission spectroscopy for the layered antiferromagnetic (La1-zNdz)(0.46)Sr0.54MnO3 Reviewed

T Takeuchi, H Nozaki, K Soda, T Kondo, U Mizutani, T Yokoya, T Sato, T Takahashi, S Shin, T Muro, Y Saitoh, Y Moritomo

JOURNAL OF SYNCHROTRON RADIATION 9 237 - 241 2002.7

Language：English Publishing type：Research paper (scientific journal) Publisher：BLACKWELL MUNKSGAARD

High-resolution HeI photoemission spectroscopy (UPS), Mn 2p-3d resonant photoemission spectroscopy (RPES) and Mn 2p X-ray absorption spectroscopy (XAS) have been performed to investigate the electronic structure and its effect on the electrical resistivity in (La1-zNdz)(0.46)Sr0.54MnO3 (z = 0, 0,2, 0.6 and 1.0). It was found that in the UPS and RPES spectra the Fermi edge persisted over the temperature range 15 less than or equal to T less than or equal to 340 K regardless of the magnetic structure or the composition of the samples. The density of states at the Fermi level [N(E-F)] in the samples where 0 less than or equal to z less than or equal to 0.6 was increased drastically at the Curie temperature (T-C) with decreasing temperature, but essentially kept unchanged across the Neel temperature (T-N). A fairly large reduction at T-C and a small increase at T-N in the electrical resistivity with decreasing temperature are found to be well accounted for in terms of the temperature dependence of N(E-F). The presence of a finite N(E-F) in the insulating Nd0.46Sr0.54MnO3 was also found. Thus the origin of the insulating behavior in this sample can be regarded as the Anderson localization associated with the small density of states and the chemical disorder between Nd and Sr.

DOI： 10.1107/S0909049502008889

High-resolution photoelectron spectroscopy of Heusler-type Fe2VAl alloy Reviewed

K Soda, T Mizutani, O Yoshimoto, S Yagi, U Mizutani, H Sumi, Y Nishino, Y Yamada, T Yokoya, S Shin, A Sekiyama, S Suga

JOURNAL OF SYNCHROTRON RADIATION 9 233 - 236 2002.7

Language：English Publishing type：Research paper (scientific journal) Publisher：BLACKWELL MUNKSGAARD

The electronic structure of Heusler-type Fe2VAl has been studied by high-resolution photoelectron spectroscopy with the excitation photon energy hnu ranging from 21.2 eV (the He I laboratory light source) to 904 eV (the soft X-ray synchrotron light source) for clean surfaces prepared by scraping or fracturing polycrystalline and single crystalline specimens. Photoelectron spectra recorded for the fractured surfaces show a 10 eV-wide valence band with fine structures and a clear decrease in the intensity towards the Fermi level E-F, while a high intensity at EF and no fine structures are observed for the scraped surface. Comparison with the theoretical density of states (DOS) indicates that the vacuum ultraviolet photoelectron spectra emphasize the transition-metal 3d bands but the soft X-ray photoelectron spectra agree remarkably well with the DOS including the fine structures and the pseudogap at EF. The present results suggest that the electronic structure of Fe2VAl is highly sensitive to possible strain and defects induced by scraping. Bulk electronic structures of Fe2VAl are discussed in relation to the reported fascinating transport properties.

DOI： 10.1107/S0909049502009214

Comparative study of the binary icosahedral quasicrystal Cd5.7Yb and its crystalline approximant Cd6Yb by low-temperature ultrahigh-resolution photoemission spectroscopy Reviewed

Ryuji Tamura, Yohsuke Murao, Shin Takeuchi, Takayuki Kiss, Takayoshi Yokoya, Shik Shin

Physical Review B - Condensed Matter and Materials Physics 65 ( 22 ) 2242071 - 2242075 2002.6

Language：English Publishing type：Research paper (scientific journal)

Low-temperature ultrahigh-resolution photoemission (PE) spectroscopy of the recently discovered binary icosahedral (i) Cd5.7Yb and its 1/1-cubic approximant Cd6Yb has commonly revealed a sharp Fermi edge with a marked spectral intensity decrease towards the Fermi level (EF), indicating the presence of a dip near EF which is consistent with the recent result of electronic structure calculations [Y. Ishii and T. Fujiwara, Phys. Rev. Lett. 87, 206408 (2001)], and the dip at EF is found to be much deeper in i-Cd5.7Yb than in 1/1-Cd6Yb. Moreover, surface core-level shifts (SCS's) are clearly observed in the quasicrystal and the approximant, and the SCS's are found to be substantially different between these two compounds of different long-range orders.

DOI： 10.1103/PhysRevB.65.224207

Comparative study of the binary icosahedral quasicrystal Cd5.7Yb and its crystalline approximant Cd6Yb by low-temperature ultrahigh-resolution photoemission spectroscopy Reviewed

R Tamura, Y Murao, S Takeuchi, T Kiss, T Yokoya, S Shin

PHYSICAL REVIEW B 65 ( 22 ) 2002.6

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

Low-temperature ultrahigh-resolution photoemission (PE) spectroscopy of the recently discovered binary icosahedral (i) Cd5.7Yb and its 1/1-cubic approximant Cd6Yb has commonly revealed a sharp Fermi edge with a marked spectral intensity decrease towards the Fermi level (E-F), indicating the presence of a dip near E-F which is consistent with the recent result of electronic structure calculations [Y. Ishii and T. Fujiwara, Phys. Rev. Lett. 87, 206408 (2001)], and the dip at E-F is found to be much deeper in i-Cd5.7Yb than in 1/1-Cd6Yb. Moreover, surface core-level shifts (SCS's) are clearly observed in the quasicrystal and the approximant, and the SCS's are found to be substantially different between these two compounds of different long-range orders.

DOI： 10.1103/PhysRevB.65.224207

Angle-resolved photoemission study of the mixed valence oxide V6O13: Quasi-one-dimensional electronic structure and its change across the metal-insulator transition

R. Eguchi, T. Yokoya, T. Kiss, Y. Ueda, S. Shin

Physical Review B - Condensed Matter and Materials Physics 65 ( 20 ) 2051241 - 2051244 2002.5

Language：English Publishing type：Research paper (scientific journal)

We have performed angle-resolved photoemission spectroscopy of mixed valence oxide V6O13 that shows a metal-insulator transition (MIT) at ≃ 150 K. In the metallic phase, we observe two bands near the Fermi level (EF). One is a prominent band located around 0.8 eV and the other is a weak structure around 0.2 eV that shows dispersion toward EF only along the b axis. Furthermore, though the momentum distribution curve at EF shows a peak at kb/π=0.29±0.01 indicative of a EF crossing, the intensity of the band near EF is strongly suppressed in the region of EF. These observations indicate quasi-one-dimensional electronic states of V6O13, consistent with a highly anisotropic behavior observed from resistivity and optical conductivity measurements. Across the MIT, the band near EF shifts to higher binding energy and becomes less dispersive, resulting in opening of an energy gap of 0.2 eV. We discuss some implications of the experimental results.

Angle-resolved photoemission study of the mixed valence oxide V6O13: Quasi-one-dimensional electronic structure and its change across the metal-insulator transition Reviewed

R Eguchi, T Yokoya, T Kiss, Y Ueda, S Shin

PHYSICAL REVIEW B 65 ( 20 ) 2002.5

Language：English Publishing type：Research paper (scientific journal) Publisher：AMERICAN PHYSICAL SOC

We have performed angle-resolved photoemission spectroscopy of mixed valence oxide V6O13 that shows a metal-insulator transition (MIT) at similar or equal to150 K. In the metallic phase, we observe two bands near the Fermi level (E-F). One is a prominent band located around 0.8 eV and the other is a weak structure around 0.2 eV that shows dispersion toward E-F only along the b axis. Furthermore, though the momentum distribution curve at E-F shows a peak at kb/pi=0.29+/-0.01 indicative of a E-F crossing, the intensity of the band near E-F is strongly suppressed in the region of E-F. These observations indicate quasi-one-dimensional electronic states of V6O13, consistent with a highly anisotropic behavior observed from resistivity and optical conductivity measurements. Across the MIT, the band near E-F shifts to higher binding energy and becomes less dispersive, resulting in opening of an energy gap of 0.2 eV. We discuss some implications of the experimental results.

DOI： 10.1103/PhysRevB.65.205124

Electronic structure of black SmS. I. 4d-4f resonance and angle-integrated valence-band photoemission spectroscopy Reviewed

A. Chainani, H. Kumigashira, T. Ito, T. Sato, T. Takahashi, T. Yokoya, T. Higuchi, T. Takeuchi, S. Shin, N. K. Sato

Physical Review B - Condensed Matter and Materials Physics 65 ( 15 ) 1552011 - 1552017 2002.4

Language：English Publishing type：Research paper (scientific journal)

We study the electronic structure of single-crystal "black" semiconducting SmS using x-ray absorption and resonance photoemission spectroscopy across the 4d-4f threshold and high-resolution temperature-dependent valence-band photoemission spectroscopy. The 4d-4f on-resonance spectra show mixed valency of Sm2+ and Sm3+ states in semiconducting SmS at low temperature (T = 30 K). The high-resolution spectra show a pseudogap within 20 meV of the Fermi level at low temperatures. This pseudogap is gradually filled up accompanied by a redistribution of spectral weight over a larger energy scale of ∼200 meV on increasing temperature, resulting in an incipient metallic phase at room temperature. The two energy scales can be associated with the coherence temperature Tc and the Kondo temperature TK, respectively. The changes in the single-particle density of states at and about the Fermi level are compatible with recent theoretical results on "exhaustion physics" in the periodic Anderson model. The present study indicates a Kondo lattice picture for SmS and suggests a relation between the mixed valency and the temperature dependence of the pseudogap for "black" semiconducting SmS.

DOI： 10.1103/PhysRevB.65.155201

Electronic structure of black SmS. I. 4d-4f resonance and angle-integrated valence-band photoemission spectroscopy Reviewed

A Chainani, H Kumigashira, T Ito, T Sato, T Takahashi, T Yokoya, T Higuchi, T Takeuchi, S Shin, NK Sato

PHYSICAL REVIEW B 65 ( 15 ) 2002.4

Language：English Publishing type：Research paper (scientific journal) Publisher：AMERICAN PHYSICAL SOC

We study the electronic structure of single-crystal "black" semiconducting SmS using x-ray absorption and resonance photoemission spectroscopy across the 4d-4f threshold and high-resolution temperature-dependent valence-band photoemission spectroscopy. The 4d-4f on-resonance spectra show mixed valency of Sm2+ and Sm3+ states in semiconducting SmS at low temperature (T=30 K). The high-resolution spectra show a pseudogap within 20 meV of the Fermi level at low temperatures. This pseudogap is gradually filled up accompanied by a redistribution of spectral weight over a larger energy scale of similar to200 meV on increasing temperature, resulting in an incipient metallic phase at room temperature. The two energy scales can be associated with the coherence temperature T-c and the Kondo temperature T-K, respectively. The changes in the single-particle density of states at and about the Fermi level are compatible with recent theoretical results on "exhaustion physics" in the periodic Anderson model. The present study indicates a Kondo lattice picture for SmS and suggests a relation between the mixed valency and the temperature dependence of the pseudogap for "black" semiconducting SmS.

DOI： 10.1103/PhysRevB.65.155201

Superconducting gap of MgB2 observed using ultrahigh-resolution photoemission spectroscopy Reviewed

S Tsuda, T Yokoya, T Kiss, A Chainani, Y Takano, K Togano, H Kito, H Ihara, S Shin

PHYSICA B-CONDENSED MATTER 312 150 - + 2002.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We study the superconducting gap of MgB2 using high-resolution photoemission. In the spectrum measured at 5.4 K, a coherent peak with a shoulder structure is observed. We find that a simulation using two Dynes functions with the gap sizes of 1.7 and 5.6 meV reproduces the superconducting-state spectrum better than that using a single Dynes function and anisotropic Dynes functions. We also find that both of the smaller and larger gap close at the bulk transition temperature. These results indicate a multiple superconducting gap of MgB2. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(01)01282-0

Fermi surface and superconducting gap of 2H-NbSe2 using low-temperature ultrahigh-resolution angle-resolved photoemission spectroscopy Reviewed

T Kiss, T Yokoya, A Chainani, S Shin, M Nohara, H Takagi

PHYSICA B-CONDENSED MATTER 312 666 - 667 2002.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have studied the Fermi surface (FS) and superconducting gap of 2H-NbSe2, using angle-resolved photoemission spectroscopy with an energy resolution of 2.2 meV and an angle resolution of +/-0.13degrees. We succeeded in distinguishing all FS sheets predicted from band calculations and found that the observed hexagonal Fermi surface nesting vector centered at the Gamma point is larger than the CDW nesting vector reported from neutron diffraction measurements. Furthermore, a superconducting gap at one FS sheet is successfully observed. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(01)01293-5

Angle-resolved photoemission spectroscopy, optical conductivity, and soft X-ray Raman spectroscopy of quasi-one-dimensional V6O13 Reviewed

R Eguchi, T Yokoya, T Kiss, Y Ueda, H Tajima, J Yamazaki, S Shin

PHYSICA B-CONDENSED MATTER 312 600 - 602 2002.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We study the electronic properties of quasi-one-dimensional V6O13 which exhibits a metal-insulator transition using angle-resolved photoemission spectroscopy (ARPES), optical conductivity, and soft X-ray Raman spectroscopy (SXRS). From ARPES, we observe an energy gap of 0.2eV across the metal-insulator transition with change in the lower Hubbard band. Band dispersion is found only along the b-axis, consistent with highly anisotropic behavior observed in optical conductivity and SXRS measured on the same samples. These results indicate the electronic properties of V6O13 are dominated by b-axis electronic structure. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(01)01186-3

Electronic structure of the filled skutterudite compound CeRu<inf>4</inf>Sb<inf>12</inf>

K. Kanai, N. Takeda, S. Nozawa, T. Yokoya, M. Ishikawa, S. Shin, S. Shin

Physical Review B - Condensed Matter and Materials Physics 65 ( 4 ) 411051 - 411054 2002.1

Publishing type：Research paper (scientific journal)

Ultrahigh-resolution photoemission spectra of CeRu4Sb12 reveal crossover behavior into a low carrier state. The density of states (DOS) around the Fermi level (EF) decreases on lowering temperature and the semimetalliclike DOS is observed at 5.9 K. The temperature dependence of DOS at EF is explained in terms of the development of the coherence. The smaller hybridization strength between the Ru-d state and the Ce-4f state prevents the hybridization gap from opening in CeRu4Sb12, which is realized in CeRu4P12.

Electronic structure of the filled skutterudite compound CeRu4Sb12 Reviewed

K Kanai, N Takeda, S Nozawa, T Yokoya, M Ishikawa, S Shin

PHYSICAL REVIEW B 65 ( 4 ) 2002.1

Language：English Publishing type：Research paper (scientific journal) Publisher：AMERICAN PHYSICAL SOC

Ultrahigh-resolution photoemission spectra of CeRu4Sb12 reveal crossover behavior into a low carrier state. The density of states (DOS) around the Fermi level (E-F) decreases on lowering temperature and the semimetalliclike DOS is observed at 5.9 K. The temperature dependence of DOS at E-F is explained in terms of the development of the coherence. The smaller hybridization strength between the Ru-d state and the Ce-4f state prevents the hybridization gap from opening in CeRu4Sb12, which is realized in CeRu4P12.

DOI： 10.1103/PhysRevB.65.041105

Photoemission spectroscopy of the strong-coupling superconducting transitions in Lead and Niobium

T. Yokoya, T. Kiss, A. Chainani, S. Shin

Journal of Electron Spectroscopy and Related Phenomena 124 99 2002

Electronic structure of the filled skutterudite compound (formula presented) Reviewed

K. Kanai, N. Takeda, S. Nozawa, T. Yokoya, M. Ishikawa, S. Shin

Physical Review B - Condensed Matter and Materials Physics 65 ( 4 ) 1 - 4 2002

Language：English Publishing type：Research paper (scientific journal)

Ultrahigh-resolution photoemission spectra of (formula presented) reveal crossover behavior into a low carrier state. The density of states (DOS) around the Fermi level (formula presented) decreases on lowering temperature and the semimetalliclike DOS is observed at 5.9 K. The temperature dependence of DOS at (formula presented) is explained in terms of the development of the coherence. The smaller hybridization strength between the Ru- (formula presented) state and the (formula presented) state prevents the hybridization gap from opening in (formula presented) which is realized in (formula presented). © 2002 The American Physical Society.

DOI： 10.1103/PhysRevB.65.041105

Fermi surface sheet-dependent superconductivity in 2H-NbSe2 Reviewed

T Yokoya, T Kiss, A Chainani, S Shin, M Nohara, H Takagi

SCIENCE 294 ( 5551 ) 2518 - 2520 2001.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER ASSOC ADVANCEMENT SCIENCE

High-resolution angle-resolved photoemission spectroscopy was used to study the superconducting energy gap and changes in the spectra[ function across the superconducting transition in the quasi-two-dimensional superconductor 2H-NbSe2. The momentum dependence of the superconducting gap was determined on different Fermi surface sheets. The results indicate Fermi surface sheet-dependent superconductivity in this Low-transition temperature multiband system and provide a description consistent with thermodynamic measurements and the anomalous de Haas-van Alphen oscillations observed in the superconducting phase. The present data suggest the importance of Fermi surface sheet-dependent superconductivity in explaining exotic superconductivity in other multiband systems with complex Fermi surface topology, such as the borides and f-electron superconductors.

DOI： 10.1126/science.1065068

Electron-phonon coupling induced pseudogap and the superconducting transition in Ba0.67K0.33BiO3

A. Chainani, T. Yokoya, T. Kiss, S. Shin, T. Nishio, H. Uwe

Physical Review B - Condensed Matter and Materials Physics 64 ( 18 ) 1805091 - 1805094 2001.11

Language：English Publishing type：Research paper (scientific journal)

We study the single-particle density of states (DOS) across the superconducting transition (Tc=31 K) in single-crystal Ba0.67K0.33BiO3 using ultrahigh resolution angle-integrated photoemission spectroscopy. The superconducting gap opens with a pileup in the DOS, Δ(5.3 K) = 5.2 meV and 2Δ(0)/kBTc = 3.9. In addition, we observe a pseudogap below and above Tc, occurring as a suppression in intensity over an energy scale up to the breathing mode phonon (∼70 meV). The results indicate electron-phonon coupling induces a pseudogap in Ba0.67K0.33BiO3.

Electronic structure and superconducting gap of silicon clathrate Ba8Si46 studied with ultrahigh-resolution photoemission spectroscopy

T. Yokoya, A. Fukushima, T. Kiss, K. Kobayashi, S. Shin, K. Moriguchi, A. Shintani, H. Fukuoka, S. Yamanaka

Physical Review B - Condensed Matter and Materials Physics 64 ( 17 ) 1725041 - 1725044 2001.11

Language：English Publishing type：Research paper (scientific journal) Publisher：American Physical Society

We study the electronic structure and superconducting transition of silicon clathrate Ba8Si46 (Tc=8 K) using photoemission spectroscopy. We observe a narrow band at the Fermi level (EF), whose width (∼0.3 eV) is substantially smaller than that of band structure calculations (∼1.5 eV). Ultrahigh-resolution measurements show a superconducting gap at 5.4 K [2Δ(0)/kBTc=3.51]. Fine structures associated with phonons are observed within 70 meV of EF. These results characterize Ba8Si46 as a weak-coupling superconductor most probably driven by phonon.

Electron-phonon coupling induced pseudogap and the superconducting transition in Ba0.67K0.33BiO3 Reviewed

A Chainani, T Yokoya, T Kiss, S Shin, T Nishio, H Uwe

PHYSICAL REVIEW B 64 ( 18 ) 2001.11

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We study the single-particle density of states (DOS) across the superconducting transition (T-c = 31 K) in single-crystal Ba0.67K0.33BiO3 using ultrahigh resolution angle-integrated photoemission spectroscopy. The superconducting gap opens with a pileup in the DOS, Delta (5.3 K) = 5.2 meV and 2 Delta (0)/k(B)T(c) = 3.9. In addition, we observe a pseudogap below and above T-c occurring as a suppression in intensity over an energy scale up to the breathing mode phonon (similar to 70 meV). The results indicate electron-phonon coupling induces a pseudogap in Ba0.67K0.33BiO3.

DOI： 10.1103/PhysRevB.64.180509

Electronic structure and superconducting gap of silicon clathrate Ba8Si46 studied with ultrahigh-resolution photoemission spectroscopy Reviewed

T Yokoya, A Fukushima, T Kiss, K Kobayashi, S Shin, K Moriguchi, A Shintani, H Fukuoka, S Yamanaka

PHYSICAL REVIEW B 64 ( 17 ) 2001.11

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We study the electronic structure and superconducting transition of silicon clathrate Ba8Si46 (T-c = 8 K) using photoemission spectroscopy. We observe a narrow band at the Fermi level (E-F), whose width (similar to0.3 eV) is substantially smaller than that of band structure calculations (similar to1.5 eV). Ultrahigh-resolution measurements show a superconducting gap at 5.4 K [2 Delta (0)/k(B)T(c)=3.51]. Fine structures associated with phonons are observed within 70 meV of E-F. These results characterize Ba8Si46 as a weak-coupling superconductor most probably driven by phonon.

DOI： 10.1103/PhysRevB.64.172504

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