Faculty Profiles - YOKOYA Takayoshi

Papers - YOKOYA Takayoshi

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Enhanced thermoelectricity by controlled local structure in bismuth-chalcogenides Reviewed

Terashima K, Yano Y, Paris E, Goto Y, Mizuguchi Y, Kamihara Y, Wakita T, Muraoka Y, Saini N. L, Yokoya T

JOURNAL OF APPLIED PHYSICS 125 ( 14 ) 2019.4

Fermi level tuning of Ag-doped Bi2Se3 topological insulator Reviewed

Uesugi Eri, Uchiyama Takaki, Goto Hidenori, Ota Hiromi, Ueno Teppei, Fujiwara Hirokazu, Terashima Kensei, Yokoya Takayoshi, Matsui Fumihiko, Akimitsu Jun, Kobayashi Kaya, Kubozono Yoshihiro

SCIENTIFIC REPORTS 9 ( 1 ) 2019.3

Physics of heavily doped diamond: Electronic states and superconductivity

Takanori Wakita, Kensei Terashima, Takayoshi Yokoya

Physics and Chemistry of Carbon-Based Materials: Basics and Applications 65 - 96 2019.3

Language：English Publishing type：Part of collection (book) Publisher：Springer Singapore

Diamond is a popular material, not only as a gem but also for basic and applied research owing to its extraordinary properties. The discovery of superconductivity in heavily doped diamond has increased its importance. Although the highest superconducting critical temperature (Tc) is 10K, it is regarded as a superconductor with a potentially high Tc, owing to the strong covalent bonding between carbon atoms and the light atomic mass of carbon. In this chapter, after reviewing the fundamentals of the electronic states of pristine and doped diamond, we describe the electronic states, together with the vibrational properties, of borondoped superconductive diamond, which have been revealed by photoemission spectroscopy and other experimental techniques.

DOI： 10.1007/978-981-13-3417-7_3

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Origins of Thermal Spin Depolarization in Half-Metallic Ferromagnet CrO2 Reviewed

Fujiwara Hirokazu, Terashima Kensei, Sunagawa Masanori, Yano Yuko, Nagayama Takanobu, Fukura Tetsushi, Yoshii Fumiya, Matsuura Yuka, Ogata Makoto, Wakita Takanori, Yaji Koichiro, Harasawa Ayumi, Kuroda Kenta, Shin Shik, Horiba Koji, Kumigashira Hiroshi, Muraoka Yuji, Yokoya Takayoshi

PHYSICAL REVIEW LETTERS 121 ( 25 ) 2018.12

Authorship：Last author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

Using high-resolution spin-resolved photoemission spectroscopy, we observe a thermal spin depolarization to which all spin-polarized electrons contribute. Furthermore, we observe a distinct minority spin state near the Fermi level and a corresponding depolarization that seldom contributes to demagnetization. The origin of this depolarization has been identified as the many-body effect characteristic of half-metallic ferromagnets. Our investigation opens an experimental field of itinerant ferromagnetic physics focusing on phenomena with sub-meV energy scale.

DOI： 10.1103/PhysRevLett.121.257201

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Fermi surface topology in a metallic phase of VO2 thin films grown on TiO2(001) substrates Reviewed

Muraoka Yuji, Nagao Hiroki, Yao Yuichiro, Wakita Takanori, Terashima Kensei, Yokoya Takayoshi, Kumigashira Hiroshi, Oshima Masaharu

SCIENTIFIC REPORTS 8 2018.12

Multilayer formation via spinodal decomposition in TiO2-VO2 epitaxial films on sapphire substrates Reviewed

Matsuura Yuka, Yoshii Fumiya, Otsuka Tsubasa, Kadowaki Kenji, Ijiri Masataka, Takemoto Yoshito, Terashima Kensei, Wakita Takanori, Yokoya Takayoshi, Muraoka Yuji

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 38 ( 15 ) 5043 - 5050 2018.12

Suppression of structural instability in LaOBiS2-xSex by Se substitution Reviewed

Paris E, Mizuguchi Y, Wakita T, Terashima K, Yokoya T, Mizokawa T, Saini N. L

JOURNAL OF PHYSICS-CONDENSED MATTER 30 ( 45 ) 2018.11

Determination of the local structure of Sr2-xMxIrO4 (M = K, La) as a function of doping and temperature Reviewed

Terashima Kensei, Paris Eugenio, Salas-Colera Eduardo, Simonelli Laura, Joseph Boby, Wakita Takanori, Horigane Kazumasa, Fujii Masanori, Kobayashi Kaya, Horie Rie, Akimitsu Jun, Muraoka Yuji, Yokoya Takayoshi, Saini Naurang Lal

PHYSICAL CHEMISTRY CHEMICAL PHYSICS 20 ( 36 ) 23783 - 23788 2018.9

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

The local structure of correlated spin-orbit insulator Sr2-xMxIrO4 (M = K, La) has been investigated by Ir L-3-edge extended X-ray absorption fine structure measurements. The measurements were performed as a function of temperature for different dopings induced by substitution of Sr with La or K. It is found that Ir-O bonds have strong covalency and they hardly show any change across the Neel temperature. In the studied doping range, neither Ir-O bonds nor their dynamics, measured by their mean square relative displacements, show any appreciable change upon carrier doping, indicating the possibility of nanoscale phase separation in the doped system. On the other hand, there is a large increase of the static disorder in Ir-Sr correlation, larger for K doping than La doping. Similarities and differences with respect to the local lattice displacements in cuprates are briefly discussed.

DOI： 10.1039/c8cp03756f

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Temperature-dependent local structure and superconductivity of BaPd2As2 and SrPd2As2 Reviewed

Terashima K, Paris E, Simonelli L, Salas-Colera E, Puri A, Wakita T, Yamada Y, Nakano S, Idei H, Kudo K, Nohara M, Muraoka Y, Mizokawa T, Yokoya T, Saini N. L

PHYSICAL REVIEW B 98 ( 9 ) 2018.9

Change of bent band structure on ferroelectric BaTiO₃ by electric polarization reversal

Oshime Norihiro, Kano Jun, Ikenaga Eiji, Yasui Shintaro, Hinokuma Satoshi, Ikeda Naoshi, Hamasaki Yosuke, Yasuhara Sou, Yokoya Takayoshi, Itoh Mitsuru, Fujii Tatsuo, Yasui Akira, Osawa Hitoshi

JSAP Annual Meetings Extended Abstracts 2018.2 1334 - 1334 2018.9

Direct observation of bent band structure on ferroelectric BaTiO₃ II

Oshime Norihiro, Kano Jun, Ikenaga Eiji, Yasui Shintaro, Hinokuma Satoshi, Ikeda Naoshi, Hamasaki Yosuke, Yasuhara Sou, Yokoya Takayoshi, Itoh Mitsuru, Fujii Tatsuo, Yasui Akira

JSAP Annual Meetings Extended Abstracts 2018.1 1434 - 1434 2018.3

Metallic phase in stoichiometric CeOBiS2 revealed by space-resolved ARPES

T. Sugimoto, E. Paris, T. Wakita, K. Terashima, T. Yokoya, A. Barinov, J. Kajitani, R. Higashinaka, T. D. Matsuda, Y. Aoki, T. Mizokawa, N. L. Saini

Scientific Reports 8 ( 1 ) 2018.1

Direct observation of double valence-band extrema and anisotropic effective masses of the thermoelectric material SnSe Reviewed

Takanobu Nagayama, Kensei Terashima, Takanori Wakita, Hirokazu Fujiwara, Tetsushi Fukura, Yuko Yano, Kanta Ono, Hiroshi Kumigashira, Osamu Ogiso, Aichi Yamashita, Yoshihiko Takano, Hitoshi Mori, Hidetomo Usui, Masayuki Ochi, Kazuhiko Kuroki, Yuji Muraoka, Takayoshi Yokoya

JAPANESE JOURNAL OF APPLIED PHYSICS 57 ( 1 ) 2018.1

Authorship：Last author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

Synchrotron-based angle-resolved photoemission spectroscopy is used to determine the electronic structure of layered SnSe, which was recently turned out to be a potential thermoelectric material. We observe that the top of the valence band consists of two nearly independent hole bands, whose tops differ by similar to 20meV in energy, indicating the necessity of a multivalley model to describe the thermoelectric properties. The estimated effective masses are anisotropic, with in-plane values of 0.16-0.39 m(0) and an out-of-plane value of 0.71 m(0), where m(0) is the rest electron mass. Information of the electronic structure is essential to further enhance the thermoelectric performance of hole-doped SnSe. (C) 2018 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.57.010301

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Effect of molecular intercalation on the local structure of superconducting Na-x(NH3)(y)MoSe2 system Reviewed

L. Simonelli, E. Paris, T. Wakita, C. Marini, K. Terashima, X. Miao, W. Olszewski, N. Ramanan, D. Heinis, Y. Kubozono, T. Yokoya, N. L. Saini

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 111 70 - 74 2017.12

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We have studied the local structure of layered Na-x(NH3)(y)MoSe2 system by Mo K-edge extended X-ray absorption fine structure (EXAFS) measurements performed as a function of temperature. We find that molecular intercalation in MoSe2 largely affects the Mo-Se network while Mo-Mo seems to sustain small changes. The Einstein temperature (Theta(E)) of Mo-Mo distance hardly changes (similar to 264 K) indicating that bond strength of this distance remains unaffected by intercalation. On the other hand, Mo-Se distance suffers a softening, revealed by the decrease of (Theta(E)) from similar to 364 K to similar to 350 K. The results indicate that Na+ ion transported by NH3 molecules may enter between the two MoSe-layers resulting reduced Se-Se coupling. Therefore, increased hybridization between Se 4p and Mo 4d orbitals due to inter-layer disorder is the likely reason of metallicity in intercalated MoSe2 and superconductivity at low temperature.

DOI： 10.1016/j.jpcs.2017.07.011

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Distinct local structure of superconducting Ca10M4As8(Fe2As)(5) (M = Pt, Ir) Reviewed

E. Paris, T. Wakita, O. Proux, T. Yokoya, K. Kudo, D. Mitsuoka, T. Kimura, K. Fujimura, N. Nishimoto, S. Ioka, M. Nohara, T. Mizokawa, N. L. Saini

PHYSICAL REVIEW B 96 ( 22 ) 2017.12

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

We have studied the local structure of superconducting Ca10Pt4As8(Fe2As2)(5) (Pt10418) and Ca10Ir4As8(Fe2As2)(5) (Ir10418) iron arsenides, showing different transition temperatures (T-c = 38 and 16 K, respectively), by polarized Fe K-edge extended x-ray absorption fine-structure measurements. Despite the similar average crystal structures, the local structures of the FeAs4 tetrahedra in the two compounds are found to be very different. The FeAs4 in Pt10418 is close to a regular tetrahedron, while it deviates largely in Ir10418. The Fe-Fe correlations in the two compounds are characterized by similar bond-length characteristics; however, the static disorder in Pt10418 is significantly lower than that in Ir10418. The results suggest that the optimized local structure and reduced disorder are the reasons for higher Tc and well-defined electronic states in Pt10418 unlike Ir10418 showing the coexistence of glassy and normal electrons at the Fermi surface, and hence provide direct evidence of the local-structure-driven optimization of the electronic structure and superconductivity in iron arsenides.

DOI： 10.1103/PhysRevB.96.224507

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The electronic structure of Ag1-xSn1+xSe2 (x=0.0, 0.1, 0.2, 0.25 and 1.0) Reviewed

Takanori Wakita, Eugenio Paris, Kaya Kobayashi, Kensei Terashima, Muammer Yasin Hacisalihoglu, Teppei Ueno, Federica Bondino, Elena Magnano, Igor Pis, Luca Olivi, Jun Akimitsu, Yuji Muraoka, Takayoshi Yokoya, Naurang L. Saini

PHYSICAL CHEMISTRY CHEMICAL PHYSICS 19 ( 39 ) 26672 - 26678 2017.10

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

We have studied the valence electronic structure of Ag1-xSn1+xSe2 (x = 0.0, 0.1, 0.2, 0.25) and SnSe (x = 1.0) by a combined analysis of X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS) measurements. Both XAS and XPS reveal an increase in electron carriers in the system with x (i.e. excess Sn concentration) for 0 <= x <= 0.25. The core-level spectra (Sn 3d, Ag 3d and Se 3d) show that the charge state of Ag is almost 1+, while that of of Sn splits into Sn2+ and Sn4+ (providing clear evidence of valence skipping for the first time) with a concomitant splitting of Se into Se2- and Se2-delta states. The x dependence of the split components in Sn and Se together with the Se-K edge XAS reveals that the Se valence state may have an essential role in the transport properties of this system.

DOI： 10.1039/c7cp05369j

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Observation of intrinsic half-metallic behavior of CrO2 (100) epitaxial films by bulk-sensitive spin-resolved PES Reviewed

Hirokazu Fujiwara, Masanori Sunagawa, Kensei Terashima, Tomoko Kittaka, Takanori Wakita, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 220 46 - 49 2017.10

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

We have investigated the electronic states and spin polarization of half-metallic ferromagnet CrO2 (100) epitaxial films by bulk-sensitive spin-resolved photoemission spectroscopy with a focus on non-quasiparticle (NQP) states derived from electron-magnon interactions. We found that the averaged values of the spin polarization are approximately 100% and 40% at 40 K and 300 K, respectively. This is consistent with the previously reported result [H. Fujiwara et al., Appl. Phys. Lett. 106, 202404 (2015).]. At 100 K, peculiar spin depolarization was observed at the Fermi level (E-F), which is supported by theoretical calculations predicting NQP states. This suggests the possible appearance of NQP states in CrO2. We also compare the temperature dependence of our spin polarizations with that of the magnetization. (C) 2017 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2017.02.003

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Inhomogeneous electronic states associated with charge-orbital order/disorder in BaV10O15 probed by photoemission spectromicroscopy Reviewed

T. Yoshino, K. Wakita, E. Paris, A. Barinov, T. Kajita, T. Katsufuji, V. Kandyba, T. Sugimoto, T. Yokoya, N. L. Saini, T. Mizokawa

PHYSICAL REVIEW B 96 ( 11 ) 2017.9

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

We have performed scanning photoemission spectromicroscopy of BaV10O15 across the metal-insulator transition at 123 K, which is accompanied by V 3d charge/orbital order and V trimerization. Nucleation of metallic domains is observed at the cleaved surface of BaV10O15 single crystals, similar to Cr-doped V2O3 in which electronic configurations of Cr3+ and V3+ are the same as those of V2+ and V3+ in BaV10O15. Typical domain size is similar to 5- 10 mu m at 150 K, just above the transition temperature. The metallic domains continuously grow up to 240 K, well above the transition temperature. The temperature evolution of the metallic phase in BaV10O15 is different from that of Cr-doped V2O3, probably due to the charge degrees of freedom in BaV10O15.

DOI： 10.1103/PhysRevB.96.115161

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Direct observation of bent band structure on ferroelectric BaTiO₃

Oshime Norihiro, Kano Jun, Ikenaga Eiji, Yasui Shintaro, Hinokuma Satoshi, Ikeda Naoshi, Hamasaki Yosuke, Yasuhara Sou, Yokoya Takayoshi, Itoh Mitsuru, Fujii Tatsuo, Yasui Akira

JSAP Annual Meetings Extended Abstracts 2017.2 1242 - 1242 2017.8

Effect of aliovalent dopants on the kinetics of spinodal decomposition in rutile-type TiO2-VO2 Reviewed

Makoto Ogata, Kenji Kadowaki, Masataka Ijiri, Yoshito Takemoto, Kensei Terashima, Takanori Wakita, Takayoshi Yokoya, Yuji Muraoka

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 37 ( 9 ) 3177 - 3183 2017.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

The effects of aliovalent dopants on the kinetics of spinodal decomposition in a rutile-type TiO2-VO2 system were studied by means of x-ray diffraction and transmission electron microscopy. In this study, as prepared polycrystalline samples were annealed at 673 k inside the spinodal. For undoped samples, phase decomposition occurred along the [001] direction, and a lamellar structure formed on the nanometer length scale. The phase decomposition kinetics in this system were sensitive to the type of aliovalent dopant; the decomposition rate in a sample doped with Al2O3 was more than one order of magnitude higher than a sample doped with . These observations could be explained on the basis of the premise that cation interstitial mobility is greater than cation vacancy mobility. The obtained results are consistent with those for a TiO2-SnO2 system, indicating that phase separation in both rutile-type oxide systems can be interpreted through a common framework. (C) 2017 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jeurceramsoc.2017.03.039

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Papers - YOKOYA Takayoshi