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Ikeda, Shugo*; Kaneko, Koji; Tanaka, Yuki*; Kawasaki, Takuro; Hanashima, Takayasu*; Munakata, Koji*; Nakao, Akiko*; Kiyanagi, Ryoji; Ohara, Takashi; Mochizuki, Kensei*; et al.
Journal of the Physical Society of Japan, 89(1), p.014707_1 - 014707_7, 2020/01
Times Cited Count：1 Percentile：12.02(Physics, Multidisciplinary)
AsIkeda, Shugo*; Tsuchiya, Yu*; Zhang, X.-W.*; Kishimoto, Shunji*; Kikegawa, Takumi*; Yoda, Yoshitaka*; Nakamura, Hiroki; Machida, Masahiko; Glasbrenner, J.*; Kobayashi, Hisao*
Physical Review B, 98(10), p.100502_1 - 100502_6, 2018/09
Times Cited Count：6 Percentile：25.82(Materials Science, Multidisciplinary)The interplay between magnetism and superconductivity is one of important subjects to investigate the pairing mechanism in novel superconductors. We have found new coexistence between an antiferromagnetic order in the Fe sublattice and superconductivity of the FeAs-based EuFeAs superconductor in the pressure range from 2.4 to 3.0 GPa by 
Fe nuclear forward scattering (NFS) using a single crystal sample. The magnetic state in the Fe sublattice changes to a new antiferromagnetic one with superconductivity from a stripe-type antiferromagnetic one observed in normal conducting state at 2.7 GPa. Below the superconducting transition temperature, the temperature dependence of Fe NFS spectra reveals that the new antiferromagnetic order develops with the superconductivity. This non-trivial coupling of two ordered states in EuFeAs under pressure demonstrates a new and intriguing relationship between magnetism and superconductivity in Fe-based superconductors.
-US
Metoki, Naoto; Sakai, Hironori; Suzuki, Michito; Yamamoto, Etsuji; Haga, Yoshinori; Matsuda, Tatsuma; Ikeda, Shugo*
JPS Conference Proceedings (Internet), 3, p.013011_1 - 013011_6, 2014/06
Uranium chalcogenides are highly interesting because of the variety of the physical properties from 5
-based metal and Mott-Hibbard insulator. Especially dichalcogenide family exhibits a metal-insulator crossover depending on the chalcogenide elements, or under magnetic field and pressure. In this talk we present a recent result of our neutron scattering study on uranium dichalcogenide, which demonstrates that the metal-insulator crossover is related to the change of 5 character from localized to itinerant. The electronic structure related to this phenomenon is discussed based on the band calculation.
-USYamamoto, Etsuji; Tateiwa, Naoyuki; Haga, Yoshinori; Ikeda, Shugo*; Sakai, Hironori; Onuki, Yoshichika; Fisk, Z.
JPS Conference Proceedings (Internet), 3, p.011095_1 - 011095_4, 2014/06
electrons with localized and itinerant charancters in -USMetoki, Naoto; Sakai, Hironori; Yamamoto, Etsuji; Haga, Yoshinori; Matsuda, Tatsuma; Ikeda, Shugo*
Journal of the Korean Physical Society, 62(12), p.1782 - 1786, 2013/06
Times Cited Count：1 Percentile：11.42(Physics, Multidisciplinary)Magnetic excitation of a narrow gap semiconductor -USwas studied by means of neutron scattering. At low temperatures clear crystalline electric field (CEF) excitations were observed, indicating localized character of 5 electrons. The excitation energy 
7 meV of the pseudo doublet state is consistent with the level scheme based on 5
configuration of U
ions, which reproduces the magnetic susceptibility, magnetization, and specific heat. With elevating temperature the conductivity recovers with 5 carriers excited beyond the gap of 90 K. Simultaneously the CEF excitation becomes weak and broad, and is replaced by a quasi-elastic response of magnetic origin, indicating the crossover of 5 character from localized to itinerant.
Tateiwa, Naoyuki; Haga, Yoshinori; Matsuda, Tatsuma*; Fisk, Z.; Ikeda, Shugo*; Kobayashi, Hisao*
Review of Scientific Instruments, 84(4), p.046105_1 - 046105_3, 2013/04
Times Cited Count：13 Percentile：52.34(Instruments & Instrumentation)Two modifications have been made to a miniature ceramic anvil high pressure cell (mCAC) designed for magnetic measurements in a commercial superconducting quantum interference (SQUID) magnetometer. Replacing the Cu-Be piston in the former mCAC with a composite piston composed of the Cu-Be and ceramic cylinders reduces the background magnetization significantly smaller at low temperatures. A second modification to the mCAC is the utilization of a ceramic anvil with a hollow in the center of the culet surface.
AsKobayashi, Hisao*; Ikeda, Shugo*; Sakaguchi, Yui*; Yoda, Yoshitaka*; Nakamura, Hiroki; Machida, Masahiko
Journal of Physics; Condensed Matter, 25(2), p.022201_1 - 022201_6, 2013/01
Times Cited Count：9 Percentile：40.37(Physics, Condensed Matter)The electronic and vibrational properties of EuFeAs in the tetragonal phase between 0 and 5 GPa have been investigated using Fe M
ssbauer spectroscopy and Fe nuclear resonance inelastic scattering, respectively. We find a discontinuous increase of the center shift around 2.3 GPa, reflecting a change of the electronic state of Fe, and above 2.5 GPa a softening of the optical phonon modes associated with an increase of the relative volume of the FeAs tetrahedron in the unit cell. Our findings reveal that an effective As-As hybridization along the c axis appears at approximately 2.3 GPa in the tetragonal phase of EuFeAs, along with a change in the electronic state of Fe, causing bulk superconductivity to appear at a low temperature. Consequently, the change in the electronic state of the Fe atom and the effective As-As hybridization play key roles in the pressure-induced superconductivity in the tetragonal phase of AFeAs.
Haga, Yoshinori; Homma, Yoshiya*; Aoki, Dai*; Nakajima, Kunihisa; Arai, Yasuo; Matsuda, Tatsuma; Ikeda, Shugo*; Sakai, Hironori; Yamamoto, Etsuji; Nakamura, Akio; et al.
Journal of the Physical Society of Japan, 81(Suppl.B), p.SB007_1 - SB007_4, 2012/12
Times Cited Count：0 Percentile：0(Physics, Multidisciplinary)Fujimori, Shinichi; Okochi, Takuo*; Kawasaki, Ikuto; Yasui, Akira; Takeda, Yukiharu; Okane, Tetsuo; Saito, Yuji; Fujimori, Atsushi; Yamagami, Hiroshi; Haga, Yoshinori; et al.
Journal of the Physical Society of Japan, 81(1), p.014703_1 - 014703_9, 2012/01
Times Cited Count：37 Percentile：82.18(Physics, Multidisciplinary)High-energy-resolution core-level and valence-band photoelectron spectroscopic studies were performed for the heavy Fermion uranium compounds UGe, UCoGe, URhGe, URuSi, UNiAl
, UPdAl, and UPt as well as typical localized and itinerant uranium compounds to understand the relationship between the uranium valence state and their core-level spectral line shapes. The core-level spectra of UGe, UCoGe, URhGe, URuSi, and UNiAl are rather similar to those of itinerant compounds, suggesting that U 5 electrons in these compounds are well hybridized with ligand states. On the other hand, the core-level spectra of UPdAl and UPt show considerably different spectral line shapes from those of the itinerant compounds, suggesting that U 5 electrons in UPdAl and UPt are less hybridized with ligand states, leading to the correlated nature of U 5 electrons in these compounds.
Fe2As ( = Sr and Eu)Kobayashi, Hisao*; Ikeda, Shugo*; Yoda, Yoshitaka*; Nakamura, Hiroki; Machida, Masahiko
Physical Review B, 84(18), p.184304_1 - 184304_7, 2011/11
Times Cited Count：9 Percentile：39.35(Materials Science, Multidisciplinary)The structural and magnetic phase transition in FeAs ( = Sr and Eu) was investigated from the viewpoint of the Fe-specific phonon density of states, 
(
), utilizing Fe nuclear resonant inelastic-scattering and ab initio phonon calculations. The Fe optical phonon branches in () around 25 meV show no anomalies across the phase transition temperature, 
, with decreasing temperature. Meanwhile, the peak intensity around 32 meV decreases and increases at 1.25 and , respectively, which originates from phonon branches involving Fe and As atomic displacements in the tetragonal structure. This decrease is related to the phonon branches involving atomic displacements in the ab plane. Furthermore, the average sound velocity below 8 meV shows no temperature dependence down to , suggesting softening in some of the sound waves. Our results indicate the presence of orthorhombic fluctuations in tetragonal FeAs.
-USTateiwa, Naoyuki; Haga, Yoshinori; Sakai, Hironori; Ikeda, Shugo*; Matsuda, Tatsuma; Yamamoto, Etsuji; Onuki, Yoshichika*
Journal of the Physical Society of Japan, 80(Suppl.A), p.SA103_1 - SA103_3, 2011/07
Times Cited Count：2 Percentile：20.16(Physics, Multidisciplinary)We have measured the magnetization of an uranium chalcogenide compound -US under high pressure by piston cylinder and indenter type cells up to 2.42 GPa in a commercial SQUID magnetometer. The compound is a narrow-gap semiconductor that does not order magnetically down to 0.3 K at 1 bar. In this study, it was found that a ferromagnetic state is induced above 1 GPa. The pressure phase diagram were obtained. The values of the spontaneous magnetic moment at 0 K are less than 0.1 
, suggesting that the pressure-induced phase is a very weak ferromagnetic state. Considering the CEF singlet ground state in -US, it is suggest that the ferromagnetic state is an "induced-magnetic phase" often observed in some praseodymium compounds such as PrTl. The experimental results are discussed in terms of the mean field theory.
and NpGa
Haga, Yoshinori; Aoki, Dai*; Homma, Yoshiya*; Matsuda, Tatsuma; Ikeda, Shugo*; Sakai, Hironori; Yamamoto, Etsuji; Nakamura, Akio; Shiokawa, Yoshinobu*; Onuki, Yoshichika
Journal of the Physical Society of Japan, 80(Suppl.A), p.SA109_1 - SA109_3, 2011/07
Times Cited Count：0 Percentile：0(Physics, Multidisciplinary)-USSugiyama, Kiyohiro*; Hirose, Yusuke*; Enoki, Kentaro*; Ikeda, Shugo*; Yamamoto, Etsuji; Tateiwa, Naoyuki; Haga, Yoshinori; Kida, Takanori*; Hagiwara, Masayuki*; Kindo, Koichi*; et al.
Journal of the Physical Society of Japan, 80(Suppl.A), p.SA104_1 - SA104_3, 2011/07
Times Cited Count：3 Percentile：27.54(Physics, Multidisciplinary)Zn
Tateiwa, Naoyuki; Ikeda, Shugo*; Haga, Yoshinori; Matsuda, Tatsuma; Yamamoto, Etsuji; Sugiyama, Kiyohiro*; Hagiwara, Masayuki*; Kindo, Koichi*; Onuki, Yoshichika*
Journal of the Physical Society of Japan, 80(1), p.014706_1 - 014706_8, 2011/01
Times Cited Count：8 Percentile：50.77(Physics, Multidisciplinary)We have performed the magnetization measurements in high magnetic fields up to 53 T on single crystals of an uranium heavy fermion compound UZn. In the antiferromagnetic state below the N
el temperature 
= 9.7 K, a metamagnetic transition is found at 
32 T for the field along the [11
0] direction (
-axis). The magnetic phase diagram for the field along the [110] direction is given. We have also carried out high pressure resistivity measurement on UZn using a diamond anvil cell up to 8.7 GPa. The Nel temperature is almost pressure-independent up to 4.7 GPa and starts to increase at the higher pressure region. The pressure dependences of the coefficient of 
term in the electrical resistivity , an antiferromagnetic gap 
and a characteristic temperature 
are discussed.
SiMatsuda, Tatsuma; Ikeda, Shugo*; Yamamoto, Etsuji; Haga, Yoshinori; Shishido, Hiroaki*; Yamagami, Hiroshi; Settai, Rikio*; Onuki, Yoshichika
Journal of the Physical Society of Japan, 79(11), p.114712_1 - 114712_4, 2010/11
Times Cited Count：4 Percentile：32.94(Physics, Multidisciplinary)We have succeeded in growing a high-quality single crystal of UCuSi with the tetragonal structure by the Sn-flux method and measured the de Haas-van Alphen effect. UCuSi orders antiferromagnetically below = 106 K, and follows a successive ferromagnetic ordering at 
= 100 K. Several de Haas-van Alphen branches are observed in the ferromagnetic state, ranging from 5
10
to 710
Oe. The corresponding cyclotron effective masses are moderately large, ranging from 1.4 to 4.2 
, consistent with the electronic specific heat coefficient 
= 20 mJ/K
mol. These experimental results are approximately explained by the 5-itinerant spin- and orbital-polarized energy band model.
AlHaga, Yoshinori; Homma, Yoshiya*; Aoki, Dai*; Ikeda, Shugo*; Matsuda, Tatsuma; Tateiwa, Naoyuki; Yamamoto, Etsuji; Nakamura, Akio; Nakajima, Kenji; Arai, Yasuo; et al.
IOP Conference Series; Materials Science and Engineering, 9, p.012046_1 - 012046_7, 2010/05
Times Cited Count：0 Percentile：1.02(Chemistry, Inorganic & Nuclear)Metoki, Naoto; Kaneko, Koji; Ikeda, Shugo*; Sakai, Hironori; Yamamoto, Etsuji; Haga, Yoshinori; Homma, Yoshiya*; Shiokawa, Yoshinobu*
IOP Conference Series; Materials Science and Engineering, 9, p.012088_1 - 012088_8, 2010/05
Times Cited Count：1 Percentile：54.48(Chemistry, Inorganic & Nuclear)Inelastic neutron scattering study has been carried out on US. A clear crystalline electric field (CEF) peak has been observed at 7.4 meV, which is consistent with the level scheme obtained from susceptibility data assuming 5f
configuration. The CEF becomes very weak and broad, and a quasi-elastic response emerges above the characteristic temperature 100 K. It indicates the crossover of the 5f character between localized and itinerant. A quasi-elastic response can be concluded as the hybridization effect of the 5f electrons with ligand p-electrons. Our study suggests that the crossover of the 5f character is the origin of the metal-insulator transition in US.
SiMatsuda, Tatsuma; Dung, N. D.*; Haga, Yoshinori; Ikeda, Shugo*; Yamamoto, Etsuji; Ishikura, Tatsuro*; Endo, Toyoaki*; Takeuchi, Tetsuya*; Settai, Rikio*; Onuki, Yoshichika
Physica Status Solidi (B), 247(3), p.757 - 759, 2010/03
Times Cited Count：2 Percentile：12.12(Physics, Condensed Matter)We succeeded in growing high-quality single crystals of a heavy electron compound YbCuSi. The electronic specific heat coefficient is determined to be 150 mJ/Kmol, which is consistent with the value of the electrical resistivity from Kadowaki-Woods relation. We succeeded in observing the de Haas-van Alphen (dHvA) oscillations. The dHvA frequencies and large cyclotron effective masses in YbCuSi are highly different from those in a non-4 reference compound YCuSi. From these dHvA results, the heavy electron state of YbCuSi is characterized by the large contribution of 4-electrons to the volume of the Fermi surface and also to the strong enhancement of effective mass caused by the spin fluctuations.
-USIkeda, Shugo*; Sakai, Hironori; Tateiwa, Naoyuki; Matsuda, Tatsuma; Aoki, Dai*; Homma, Yoshiya*; Yamamoto, Etsuji; Nakamura, Akio; Shiokawa, Yoshinobu*; Ota, Yuki*; et al.
Journal of the Physical Society of Japan, 78(11), p.114704_1 - 114704_10, 2009/11
Times Cited Count：12 Percentile：59(Physics, Multidisciplinary)Kambe, Shinsaku; Sakai, Hironori; Tokunaga, Yo; Chudo, Hiroyuki; Yasuoka, Hiroshi; Matsuda, Tatsuma; Haga, Yoshinori; Ikeda, Shugo; Nakamura, Akio; Yamamoto, Etsuji; et al.
Journal of Nuclear Materials, 385(1), p.1 - 3, 2009/03
Times Cited Count：1 Percentile：10.22(Materials Science, Multidisciplinary)In 5f-electron systems, exotic magnetism and unconventional superconductivity appear at low temperatures owing to strong electron correlations and orbital degeneracy. For example, we have investigated the unconventional superconducting states of recently synthesized NpPd
Al (
=5K) and PuRhGa (=9K) in terms of both macroscopic (staticsusceptibility, specific heat, resistivity) and microscopic (NMR) probes. The relation between magnetic fluctuations and unconventional superconductivity in f-electron systems is also discussed. Another exciting topic concerns the series AnO (An; U, Np, Pu, Am). Besides the importance of AnO as a nuclear fuel, the magnetic properties of these compounds at low temperatures are particularly interesting. On the basis of 
O-NMR studies, multipolar ordered states have been identified in UO (dipolar + quadrupolar ordering) and NpO (quadrupolar + octupolar ordering), in contrast with the non-magnetic ground state of PuO.
