American Chemical Society Acta Medica Okayama 2470-1343 9 19 2024 Superstructure of Fe5–xGeTe2 Determined by Te K-Edge Extended X-ray Absorption Fine Structure and Te Kα X-ray Fluorescence Holography 21287 21297 EN Ritsuko Eguchi Research Institute for Interdisciplinary Science, Okayama University Halubai Sekhar Department of Physical Science and Technology, Nagoya Institute of Technology Koji Kimura Department of Physical Science and Technology, Nagoya Institute of Technology Hirokazu Masai Department of Materials and Chemistry, National Institute of Advanced Industrial Science and Technology (AIST) Naohisa Happo Graduate School of Information Sciences, Hiroshima City University Mitsuki Ikeda Research Institute for Interdisciplinary Science, Okayama University Yuki Yamamoto Research Institute for Interdisciplinary Science, Okayama University Masaki Utsumi Research Institute for Interdisciplinary Science, Okayama University Hidenori Goto Research Institute for Interdisciplinary Science, Okayama University Yasuhiro Takabayashi Department of Physical Science and Technology, Nagoya Institute of Technology Hiroo Tajiri Japan Synchrotron Radiation Research Institute (JASRI) Koichi Hayashi Department of Physical Science and Technology, Nagoya Institute of Technology Yoshihiro Kubozono Research Institute for Interdisciplinary Science, Okayama University The local structure of the two-dimensional van der Waals material, Fe5–xGeTe2, which exhibits unique structural/magnetic phase transitions, was investigated by Te K-edge extended X-ray absorption fine structure (EXAFS) and Te Kα X-ray fluorescence holography (XFH) over a wide temperature range. The formation of a trimer of Te atoms at low temperatures has been fully explored using these methods. An increase in the Te–Fe distance at approximately 150 K was suggested by EXAFS and presumably indicates the formation of a Te trimer. Moreover, XFH displayed clear atomic images of Te atoms. Additionally, the distance between the Te atoms shortened, as confirmed from the atomic images reconstructed from XFH, indicating the formation of a trimer of Te atoms, i.e., a charge-ordered (3⎯⎯√×3⎯⎯√)𝑅30◦ superstructure. Furthermore, Te Kα XFH provided unambiguous atomic images of Fe atoms occupying the Fe1 site; the images were not clearly observed in the Ge Kα XFH that was previously reported because of the low occupancy of Fe and Ge atoms. In this study, EXAFS and XFH clearly showed the local structure around the Te atom; in particular, the formation of Te trimers caused by charge-ordered phase transitions was clearly confirmed. The charge-ordered phase transition is fully discussed based on the structural variation at low temperatures, as established from EXAFS and XFH. No potential conflict of interest relevant to this article was reported.

nature portfolio Acta Medica Okayama 2045-2322 13 1 2023 Semiconductor-metal transition in Bi2Se3 caused by impurity doping 537 EN Takaki Uchiyama Research Institute for Interdisciplinary Science, Okayama University Hidenori Goto Research Institute for Interdisciplinary Science, Okayama University Eri Uesugi Research Institute for Interdisciplinary Science, Okayama University Akihisa Takai Research Institute for Interdisciplinary Science, Okayama University Lei Zhi Research Institute for Interdisciplinary Science, Okayama University Akari Miura Research Institute for Interdisciplinary Science, Okayama University Shino Hamao Research Institute for Interdisciplinary Science, Okayama University Ritsuko Eguchi Research Institute for Interdisciplinary Science, Okayama University Hiromi Ota Advanced Science Research Center, Okayama University Kunihisa Sugimoto Faculty of Science and Engineering, Kindai University Akihiko Fujiwara Department of Nanotechnology for Sustainable Energy, Kwansei Gakuin University Fumihiko Matsui Institute for Molecular Science, UVSOR Synchrotron Facility Koji Kimura Department of Physical Science and Engineering, Nagoya Institute of Technology Kouichi Hayashi Department of Physical Science and Engineering, Nagoya Institute of Technology Teppei Ueno Research Institute for Interdisciplinary Science, Okayama University Kaya Kobayashi Research Institute for Interdisciplinary Science, Okayama University Jun Akimitsu Research Institute for Interdisciplinary Science, Okayama University Yoshihiro Kubozono Research Institute for Interdisciplinary Science, Okayama University Doping a typical topological insulator, Bi2Se3, with Ag impurity causes a semiconductor-metal (S-M) transition at 35 K. To deepen the understanding of this phenomenon, structural and transport properties of Ag-doped Bi2Se3 were studied. Single-crystal X-ray diffraction (SC-XRD) showed no structural transitions but slight shrinkage of the lattice, indicating no structural origin of the transition. To better understand electronic properties of Ag-doped Bi2Se3, extended analyses of Hall effect and electric-field effect were carried out. Hall effect measurements revealed that the reduction of resistance was accompanied by increases in not only carrier density but carrier mobility. The field-effect mobility is different for positive and negative gate voltages, indicating that the E-F is located at around the bottom of the bulk conduction band (BCB) and that the carrier mobility in the bulk is larger than that at the bottom surface at all temperatures. The pinning of the E-F at the BCB is found to be a key issue to induce the S-M transition, because the transition can be caused by depinning of the E-F or the crossover between the bulk and the top surface transport. No potential conflict of interest relevant to this article was reported.

American Chemical Society Acta Medica Okayama 2470-1343 7 6 2022 Evaluation of Effective Field-Effect Mobility in Thin-Film and Single-Crystal Transistors for Revisiting Various Phenacene-Type Molecules 5495 5501 EN Yanting Zhang Research Institute for Interdisciplinary Science, Okayama University Ritsuko Eguchi Research Institute for Interdisciplinary Science, Okayama University Shino Hamao Research Institute for Interdisciplinary Science, Okayama University Hideki Okamoto Department of Chemistry, Okayama University Hidenori Goto Research Institute for Interdisciplinary Science, Okayama University Yoshihiro Kubozono Research Institute for Interdisciplinary Science, Okayama University The magnitude of the field-effect mobility mu of organic thin-film and single-crystal field-effect transistors (FETs) has been over-estimated in certain recent studies. These reports set alarm bells ringing in the research field of organic electronics. Herein, we report a precise evaluation of the mu values using the effective field-effect mobility, mu(eff), a new indicator that is recently designed to prevent the FET performance of thin-film and single-crystal FETs based on various phenacene molecules from being overestimated. The transfer curves of a range of FETs based on phenacene are carefully categorized on the basis of a previous report. The exact evaluation of the value of mu(eff) depends on the exact classification of each transfer curve. The transfer curves of all our phenacene FETs could be successfully classified based on the method indicated in the aforementioned report, which made it possible to evaluate the exact value of mu(eff) for each FET. The FET performance based on the values of mu(eff) obtained in this study is discussed in detail. In particular, the mu(eff) values of single-crystal FETs are almost consistent with the mu values that were reported previously, but the mu(eff) values of thin-film FETs were much lower than those previously reported for mu, owing to a high absolute threshold voltage, vertical bar V-th vertical bar. The increase in the field-effect mobility as a function of the number of benzene rings, which was previously demonstrated based on the mu values of single-crystal FETs with phenacene molecules, is well reproduced from the mu(eff) values. The FET performance is discussed based on the newly evaluated mu(eff) values, and the future prospects of using phenacene molecules in FET devices are demonstrated. No potential conflict of interest relevant to this article was reported.

Royal Society of Chemistry Acta Medica Okayama 2046-2069 10 45 2020 A new protocol for the preparation of superconducting KBi2 26686 26692 EN Huan Li Research Institute for Interdisciplinary Science, Okayama University Yanan Wang Research Institute for Interdisciplinary Science, Okayama University Saki Nishiyama Research Institute for Interdisciplinary Science, Okayama University Xiaofan Yang Research Institute for Interdisciplinary Science, Okayama University Tomoya Taguchi Research Institute for Interdisciplinary Science, Okayama University Akari Miura Research Institute for Interdisciplinary Science, Okayama University Ai Suzuki Research Institute for Interdisciplinary Science, Okayama University Lei Zhi Research Institute for Interdisciplinary Science, Okayama University Hidenori Goto Research Institute for Interdisciplinary Science, Okayama University Ritsuko Eguchi Research Institute for Interdisciplinary Science, Okayama University Takashi Kambe Department of Physics, Okayama University Yen-Fa Liao National Synchrotron Radiation Research Center Hirofumi Ishii National Synchrotron Radiation Research Center Yoshihiro Kubozono Research Institute for Interdisciplinary Science, Okayama University A superconducting KBi2 sample was successfully prepared using a liquid ammonia (NH3) technique. The temperature dependence of the magnetic susceptibility (M/H) showed a superconducting transition temperature (Tc) as high as 3.6 K. In addition, the shielding fraction at 2.0 K was evaluated to be 87%, i.e., a bulk superconductor was realized using the above method. The Tc value was the same as that reported for the KBi2 sample prepared using a high-temperature annealing method. An X-ray diffraction pattern measured based on the synchrotron X-ray radiation was analyzed using the Rietveld method, with a lattice constant, a, of 9.5010(1) Å under the space group of Fd[3 with combining macron]m (face-centered cubic, no. 227). The lattice constant and space group found for the KBi2 sample using a liquid NH3 technique were the same as those reported for KBi2 through a high-temperature annealing method. Thus, the superconducting behavior and crystal structure of the KBi2 sample obtained in this study are almost the same as those for the KBi2 sample reported previously. Strictly speaking, the magnetic behavior of the superconductivity was different from that of a KBi2 sample reported previously, i.e., the KBi2 sample prepared using a liquid NH3 technique was a type-II like superconductor, contrary to that prepared using a high-temperature annealing method, the reason for which is fully discussed. These results indicate that the liquid NH3 technique is effective and simple for the preparation of a superconducting KBi2. In addition, the topological nature of the superconductivity for KBi2 was not confirmed. No potential conflict of interest relevant to this article was reported.

IOP Publishing Acta Medica Okayama 2053-1591 7 3 2020 Inhomogeneous superconductivity in thin crystals of FeSe1-xTex (x=1.0, 0.95, and 0.9) 036001 EN Ritsuko Eguchi Research Institute for Interdisciplinary Science, Okayama University Megumi Senda Graduate School of Natural Science and Technology, Okayama University Eri Uesugi Graduate School of Natural Science and Technology, Okayama University Hidenori Goto Research Institute for Interdisciplinary Science, Okayama University Akihiko Fujiwara Department of Nanotechnology for Sustainable Energy, Kwansei Gakuin University Yasuhiko Imai Japan Synchrotron Radiation Research Institute (JASRI) Shigeru Kimura Japan Synchrotron Radiation Research Institute (JASRI) Takashi Noji Department of Applied Physics, Tohoku University Yoji Koike Department of Applied Physics, Tohoku University Yoshihiro Kubozono Research Institute for Interdisciplinary Science, Okayama University We investigated the temperature dependence of resistivity in thin crystals of FeSe1-xTex (x = 1.0, 0.95, and 0.9), though bulk crystals with 1.0 x 0.9 are known to be non-superconducting. With decreasing thickness of the crystals, the resistivity of x = 0.95 and 0.9 decreases and reaches zero at a low temperature, which indicates a clear superconducting transition. The anomaly of resistivity related to the structural and magnetic transitions completely disappears in 55- to 155-nm-thick crystals of x = 0.9, resulting in metallic behavior in the normal state. Microbeam x-ray diffraction measurements were performed on bulk single crystals and thin crystals of FeSe1-xTex. A significant difference of the lattice constant, c, was observed in FeSe1-xTex, which varied with differing Te content (x), and even in crystals with the same x, which was mainly caused by inhomogeneity of the Se/Te distribution. It has been found that the characteristic temperatures causing the structural and magnetic transition (T-t), the superconducting transition (T-c), and the zero resistivity (T-c(zero)) are closely related to the value of c in thin crystals of FeSe1-xTex. No potential conflict of interest relevant to this article was reported.

Nature Publishing Group Acta Medica Okayama 2045-2322 9 2019 Synthesis of the extended phenacene molecules, [10]phenacene and [11]phenacene, and their performance in a field-effect transistor 4009 EN Hideki Okamoto Department of Chemistry, Okayama University Shino Hamao Research Institute for Interdisciplinary Science, Okayama University Ritsuko Eguchi Research Institute for Interdisciplinary Science, Okayama University Hidenori Goto Research Institute for Interdisciplinary Science, Okayama University Yasuhiro Takabayashi Department of Chemistry, Okayama University Paul Yu-Hsiang Yen Department of Physics, National Tsing Hua University Luo Uei Liang Department of Physics, National Tsing Hua University Chia-Wei Chou Department of Physics, National Tsing Hua University Germar Hoffmann Department of Physics, National Tsing Hua University Shin Gohda NARD Co Ltd Hisako Sugino NARD Co Ltd Yen-Fa Liaos National Synchrotron Radiation Center Hirofumi Ishii National Synchrotron Radiation Center Yoshihiro Kubozono Research Institute for Interdisciplinary Science, Okayama University The [10]phenacene and [11]phenacene molecules have been synthesized using a simple repetition of Wittig reactions followed by photocyclization. Sufficient amounts of [10]phenacene and [11]phenacene were obtained, and thin-film FETs using these molecules have been fabricated with SiO2 and ionic liquid gate dielectrics. These FETs operated in p-channel. The averaged measurements of field-effect mobility, <μ>, were 3.1(7) × 10-2 and 1.11(4) × 10-1 cm2 V-1 s-1, respectively, for [10]phenacene and [11]phenacene thin-film FETs with SiO2 gate dielectrics. Furthermore, [10]phenacene and [11]phenacene thin-film electric-double-layer (EDL) FETs with ionic liquid showed low-voltage p-channel FET properties, with <μ> values of 3(1) and 1(1) cm2 V-1 s-1, respectively. This study also discusses the future utility of the extremely extended π-network molecules [10]phenacene and [11]phenacene as the active layer of FET devices, based on the experimental results obtained. No potential conflict of interest relevant to this article was reported.

Nature Publishing Group Acta Medica Okayama 2045-2322 9 2019 Fermi level tuning of Ag-doped Bi2Se3 topological insulator 5376 EN Eri Uesugi Research Institute for Interdisciplinary Science, Okayama University Takaki Uchiyama Research Institute for Interdisciplinary Science, Okayama University Hidenori Goto Research Institute for Interdisciplinary Science, Okayama University Hiromi Ota Advanced Science Research Centre, Okayama University Teppei Ueno Research Institute for Interdisciplinary Science, Okayama University Hirokazu Fujiwara Research Institute for Interdisciplinary Science, Okayama University Kensei Terashima Research Institute for Interdisciplinary Science, Okayama University Takayoshi Yokoya Research Institute for Interdisciplinary Science, Okayama University Fumihiko Matsui Graduate School of Materials Science, Nara Institute of Science and Technology Jun Akimitsu Research Institute for Interdisciplinary Science, Okayama University Kaya Kobayashi Research Institute for Interdisciplinary Science, Okayama University Yoshihiro Kubozono Research Institute for Interdisciplinary Science, Okayama University The temperature dependence of the resistivity (rho) of Ag-doped Bi2Se3 (AgxBi2-xSe3) shows insulating behavior above 35 K, but below 35 K, rho suddenly decreases with decreasing temperature, in contrast to the metallic behavior for non-doped Bi2Se3 at 1.5-300 K. This significant change in transport properties from metallic behavior clearly shows that the Ag doping of Bi2Se3 can effectively tune the Fermi level downward. The Hall effect measurement shows that carrier is still electron in AgxBi2-xSe3 and the electron density changes with temperature to reasonably explain the transport properties. Furthermore, the positive gating of AgxBi2-xSe3 provides metallic behavior that is similar to that of non-doped Bi2Se3, indicating a successful upward tuning of the Fermi level. No potential conflict of interest relevant to this article was reported.

American Physical Society Acta Medica Okayama 01631829 97 9 2018 Pressure dependence of superconductivity in low- and high-T-c phases of (NH3)(y)NaxFeSe 094505 EN Takahiro Terao Research Institute for Interdisciplinary Science, Okayama University Xiaofan Yang Research Institute for Interdisciplinary Science, Okayama University Xiao Miao Research Institute for Interdisciplinary Science, Okayama University Lu Zheng Research Institute for Interdisciplinary Science, Okayama University Hidenori Goto Research Institute for Interdisciplinary Science, Okayama University Takafumi Miyazaki Research Laboratory for Surface science, Okayama University Hitoshi Yamaoka RIKEN SPring-8 Center Hirofumi Ishii National Synchrotron Radiation Research Center Yen-Fa Liao National Synchrotron Radiation Research Center Yoshihiro Kubozono Research Institute for Interdisciplinary Science, Okayama University We prepared two superconducting phases, which are called “low-Tc phase” and “high-Tc phase” of (NH3)yNaxFeSe showing Tc’s of 35 and 44 K, respectively, at ambient pressure, and studied the superconducting behavior and structure of each phase under pressure. The Tc of the 35 K at ambient pressure rapidly decreases with increasing pressure up to 10 GPa, and it remains unchanged up to 22 GPa. Finally, superconductivity was not observed down to 1.4 K at 29 GPa, i.e., Tc < 1.4K. The Tc of the 44 K phase also shows a monotonic decrease up to 15 GPa and it weakly decreases up to 25 GPa. These behaviors suggest no pressure-driven high-Tc phase (called “SC-II”) between 0 and 25 GPa for the low-Tc and high-Tc phases of (NH3)yNaxFeSe, differing from the behavior of (NH3)yCsxFeSe,which has a pressure-driven high-Tc phase (SC-II) in addition to the superconducting phase (SC-I) observed at ambient and low pressures. The Tc-c phase diagram for both low-Tc and high-Tc phases shows that the Tc can be linearly scaled with c (or FeSe plane spacing), where c is a lattice constant. The reason why a pressure-driven high-Tc phase (SC-II) was found for neither low-Tc nor high-Tc phases of (NH3)yNaxFeSe is fully discussed, suggesting a critical c value as the key to forming the pressure-driven high-Tc phase (SC-II). Finally, the precise Tc-c phase diagram is depicted using the data obtained thus far from FeSe codoped with a metal and NH3 or amine, indicating two distinct Tc-c lines below c = 17.5A° . No potential conflict of interest relevant to this article was reported.

Acta Medica Okayama 2045-2322 3 2013 Electric double-layer capacitance between an ionic liquid and few-layer graphene EN Eri Uesugi Hidenori Goto Ritsuko Eguchi Akihiko Fujiwara Yoshihiro Kubozono Ionic-liquid gates have a high carrier density due to their atomically thin electric double layer (EDL) and extremely large geometrical capacitance C-g. However, a high carrier density in graphene has not been achieved even with ionic-liquid gates because the EDL capacitance C-EDL between the ionic liquid and graphene involves the series connection of C-g and the quantum capacitance C-q, which is proportional to the density of states. We investigated the variables that determine C-EDL at the molecular level by varying the number of graphene layers n and thereby optimising C-q. The C-EDL value is governed by C-q at n, 4, and by C-g at n > 4. This transition with n indicates a composite nature for C-EDL. Our finding clarifies a universal principle that determines capacitance on a microscopic scale, and provides nanotechnological perspectives on charge accumulation and energy storage using an ultimately thin capacitor. No potential conflict of interest relevant to this article was reported.

The American Physical Society Acta Medica Okayama 1098-0121 88 9 2012 Superconductivity in (NH3)(y)Cs0.4FeSe EN Lu Zheng Masanari Izumi Yusuke Sakai Ritsuko Eguchi Hidenori Goto Yasuhiro Takabayashi Takashi Kambe Taiki Onji Shingo Araki Tatsuo C. Kobayashi Jungeun Kim Akihiko Fujiwara Yoshihiro Kubozono Alkali-metal-intercalated FeSe materials, (NH3)(y)M0.4FeSe (M: K, Rb, and Cs), have been synthesized using the liquid NH3 technique. (NH3)(y)Cs0.4FeSe shows a superconducting transition temperature (T-c) as high as 31.2 K, which is higher by 3.8 K than the T-c of nonammoniated Cs0.4FeSe. The T(c)s of (NH3)(y)K0.4FeSe and (NH3)(y)Rb0.4FeSe are almost the same as those of nonammoniated K0.4FeSe and Rb0.4FeSe. The T-c of (NH3)(y)Cs0.4FeSe shows a negative pressure dependence. A clear correlation between T-c and lattice constant c is found for ammoniated metal-intercalated FeSe materials, suggesting a correlation between Fermi-surface nesting and superconductivity. No potential conflict of interest relevant to this article was reported.