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Journal Articles

New antiferromagnetic order with pressure-induced superconductivity in EuFe$$_2$$As$$_2$$

Ikeda, 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

 Percentile:100(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 EuFe$$_2$$As$$_2$$ superconductor in the pressure range from 2.4 to 3.0 GPa by $$^{57}$$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 $$^{57}$$Fe NFS spectra reveals that the new antiferromagnetic order develops with the superconductivity. This non-trivial coupling of two ordered states in EuFe$$_2$$As$$_2$$ under pressure demonstrates a new and intriguing relationship between magnetism and superconductivity in Fe-based superconductors.

Journal Articles

Spin polarized neutron scattering study on metal-insulator crossover in uranium dichalcogenide $$beta$$-US$$_{2}$$

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$$f$$-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$$f$$ character from localized to itinerant. The electronic structure related to this phenomenon is discussed based on the band calculation.

Journal Articles

Analysis of magnetization behavior in magnetic semiconductor $$beta$$-US$$_{2}$$

Yamamoto, 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

Journal Articles

Metal-insulator crossover accompanied by the dual nature of 5$$f$$ electrons with localized and itinerant charancters in $$beta$$-US$$_2$$

Metoki, 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:84.55(Physics, Multidisciplinary)

Magnetic excitation of a narrow gap semiconductor $$beta$$-US$$_2$$was studied by means of neutron scattering. At low temperatures clear crystalline electric field (CEF) excitations were observed, indicating localized character of 5$$f$$ electrons. The excitation energy $$sim$$7 meV of the pseudo doublet state is consistent with the level scheme based on 5$$f^2$$ configuration of U$$^ {4+}$$ ions, which reproduces the magnetic susceptibility, magnetization, and specific heat. With elevating temperature the conductivity recovers with 5$$f$$ 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$$f$$ character from localized to itinerant.

Journal Articles

Improved sensitivity of magnetic measurements under high pressure in miniature ceramic anvil cell for a commercial SQUID magnetometer

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:11 Percentile:35.57(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.

Journal Articles

Observation of a pressure-induced As-As hybridization associated with a change in the electronic state of Fe in the tetragonal phase of EuFe$$_2$$As$$_2$$

Kobayashi, 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:7 Percentile:54.93(Physics, Condensed Matter)

The electronic and vibrational properties of EuFe$$_2$$As$$_2$$ in the tetragonal phase between 0 and 5 GPa have been investigated using $$^{57}$$Fe M$"o$ssbauer spectroscopy and $$^{57}$$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$$_4$$ 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 EuFe$$_2$$As$$_2$$, 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 AFe$$_2$$As$$_2$$.

Journal Articles

Single crystal growth and magnetic anisotropy of hexagonal PuGa$$_{3}$$

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

 Percentile:100(Physics, Multidisciplinary)

Journal Articles

Electronic structure of heavy fermion uranium compounds studied by core-level photoelectron spectroscopy

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:27 Percentile:16.55(Physics, Multidisciplinary)

High-energy-resolution core-level and valence-band photoelectron spectroscopic studies were performed for the heavy Fermion uranium compounds UGe$$_2$$, UCoGe, URhGe, URu$$_2$$Si$$_2$$, UNi$$_2$$Al$$_3$$, UPd$$_2$$Al$$_3$$, and UPt$$_3$$ 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$$_2$$, UCoGe, URhGe, URu$$_2$$Si$$_2$$, and UNi$$_2$$Al$$_3$$ are rather similar to those of itinerant compounds, suggesting that U 5$$f$$ electrons in these compounds are well hybridized with ligand states. On the other hand, the core-level spectra of UPd$$_2$$Al$$_3$$ and UPt$$_3$$ show considerably different spectral line shapes from those of the itinerant compounds, suggesting that U 5$$f$$ electrons in UPd$$_2$$Al$$_3$$ and UPt$$_3$$ are less hybridized with ligand states, leading to the correlated nature of U 5$$f$$ electrons in these compounds.

Journal Articles

Orthorhombic fluctuations in tetragonal $$A$$Fe$$_2$$2As$$_2$$ ($$A$$ = 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:7 Percentile:59.49(Materials Science, Multidisciplinary)

The structural and magnetic phase transition in $$A$$Fe$$_2$$As$$_2$$ ($$A$$ = Sr and Eu) was investigated from the viewpoint of the Fe-specific phonon density of states, $$g$$$$^{e}$$$$_{rm Fe}$$($$E$$), utilizing $$^{57}$$Fe nuclear resonant inelastic-scattering and ab initio phonon calculations. The Fe optical phonon branches in $$g$$$$^{e}$$$$_{rm Fe}$$($$E$$) around 25 meV show no anomalies across the phase transition temperature, $$T_{rm SDW}$$, with decreasing temperature. Meanwhile, the peak intensity around 32 meV decreases and increases at 1.25$$T_{rm SDW}$$ and $$T_{rm SDW}$$, 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 $$T_{rm SDW}$$, suggesting softening in some of the sound waves. Our results indicate the presence of orthorhombic fluctuations in tetragonal $$A$$Fe$$_2$$As$$_2$$.

Journal Articles

Non-magnetic to magnetic transition under high pressure in narrow-gap semiconductor $$beta$$-US$$_2$$

Tateiwa, 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:75.59(Physics, Multidisciplinary)

We have measured the magnetization of an uranium chalcogenide compound $$beta$$-US$$_2$$ 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 $${mu}_{rm B}/{rm U}$$, suggesting that the pressure-induced phase is a very weak ferromagnetic state. Considering the CEF singlet ground state in $$beta$$-US$$_2$$, it is suggest that the ferromagnetic state is an "induced-magnetic phase" often observed in some praseodymium compounds such as Pr$$_3$$Tl. The experimental results are discussed in terms of the mean field theory.

Journal Articles

Crystal structure and physical properties of trigonal NpGa$$_3$$ and Np$$_{3}$$Ga$$_{11}$$

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

 Percentile:100(Physics, Multidisciplinary)

Journal Articles

Magnetic-field-induced metallic state in $$beta$$-US$$_2$$

Sugiyama, 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:1 Percentile:86.08(Physics, Multidisciplinary)

Journal Articles

Magnetic field and pressure phase diagrams of uranium heavy-fermion compound U$$_2$$Zn$$_{17}$$

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:5 Percentile:54.24(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 U$$_2$$Zn$$_{17}$$. In the antiferromagnetic state below the N$'e$el temperature $$T_{rm N}$$ = 9.7 K, a metamagnetic transition is found at $$H_c$$ $$simeq$$ 32 T for the field along the [11$$bar{2}$$0] direction ($$a$$-axis). The magnetic phase diagram for the field along the [11$$bar{2}$$0] direction is given. We have also carried out high pressure resistivity measurement on U$$_2$$Zn$$_{17}$$ using a diamond anvil cell up to 8.7 GPa. The N$'e$el temperature $$T_{rm N}$$ 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 $$T^2$$ term in the electrical resistivity $$A$$, an antiferromagnetic gap $$Delta$$ and a characteristic temperature $$T_{{rho}{rm max}}$$ are discussed.

Journal Articles

Fermi surface properties of ferromagnet UCu$$_2$$Si$$_2$$

Matsuda, 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:61.41(Physics, Multidisciplinary)

We have succeeded in growing a high-quality single crystal of UCu$$_2$$Si$$_2$$ with the tetragonal structure by the Sn-flux method and measured the de Haas-van Alphen effect. UCu$$_2$$Si$$_2$$ orders antiferromagnetically below $$T_{rm N}$$ = 106 K, and follows a successive ferromagnetic ordering at $$T_{rm C}$$ = 100 K. Several de Haas-van Alphen branches are observed in the ferromagnetic state, ranging from 5$$times$$10$$^6$$ to 7$$times$$10$$^7$$ Oe. The corresponding cyclotron effective masses are moderately large, ranging from 1.4 to 4.2 $$m_0$$, consistent with the electronic specific heat coefficient $$gamma$$ = 20 mJ/K$$^2cdot$$mol. These experimental results are approximately explained by the 5$$f$$-itinerant spin- and orbital-polarized energy band model.

Journal Articles

Magnetism and superconductivity in the new family of actinide compounds; AnPd$$_{5}$$Al$$_{2}$$

Haga, 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

 Percentile:100

Journal Articles

Neutron scattering study on U-dichalcogenides

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

 Percentile:100

Inelastic neutron scattering study has been carried out on US$$_{2}$$. 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$$^{2}$$ configuration. The CEF becomes very weak and broad, and a quasi-elastic response emerges above the characteristic temperature $$sim$$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$$_{2}$$.

Journal Articles

Fermi surface properties of YbCu$$_2$$Si$$_2$$

Matsuda, 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:84.88(Physics, Condensed Matter)

We succeeded in growing high-quality single crystals of a heavy electron compound YbCu$$_2$$Si$$_2$$. The electronic specific heat coefficient $$gamma$$ is determined to be 150 mJ/K$$^2cdot$$mol, which is consistent with the $$A$$ 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 YbCu$$_2$$Si$$_2$$ are highly different from those in a non-4$$f$$ reference compound YCu$$_2$$Si$$_2$$. From these dHvA results, the heavy electron state of YbCu$$_2$$Si$$_2$$ is characterized by the large contribution of 4$$f$$-electrons to the volume of the Fermi surface and also to the strong enhancement of effective mass caused by the spin fluctuations.

Journal Articles

Possible existence of magnetic polaron in nearly ferromagnetic semiconductor $$beta$$-US$$_2$$

Ikeda, 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:9 Percentile:42.24(Physics, Multidisciplinary)

Journal Articles

Hyperfine interactions in the itinerant system UFeGa$$_{5}$$

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:87.33(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$$_5$$Al$$_2$$ ($$T_c$$=5K) and PuRhGa$$_5$$ ($$T_c$$=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$$_2$$ (An; U, Np, Pu, Am). Besides the importance of AnO$$_2$$ as a nuclear fuel, the magnetic properties of these compounds at low temperatures are particularly interesting. On the basis of $$^{17}$$O-NMR studies, multipolar ordered states have been identified in UO$$_2$$ (dipolar + quadrupolar ordering) and NpO$$_2$$ (quadrupolar + octupolar ordering), in contrast with the non-magnetic ground state of PuO$$_2$$.

Journal Articles

High pressure study on uranium heavy fermion compounds with antiferromagnetic ground state

Tateiwa, Naoyuki; Ikeda, Shugo*; Haga, Yoshinori; Matsuda, Tatsuma; Nakashima, Miho*; Aoki, Dai*; Settai, Rikio*; Onuki, Yoshichika

Journal of Physics; Conference Series, 150, p.042206_1 - 042206_4, 2009/03

 Times Cited Count:3 Percentile:13.48

We have studied the high pressure effect on the antiferromagnetic ground states of uranium heavy fermion compounds. In this paper we show our high pressure studies on U$$_2$$Zn$$_{17}$$ by the electrical resistivity measurements. In U$$_2$$Zn$$_{17}$$, the N$'e$el temperature $$T_{rm N}$$ in U$$_2$$Zn$$_{17}$$ decreases slightly from 9.66 K at 1 bar to 9.33 K at 2.5 GPa, becomes almost pressure-independent up to 4.7 GPa, and starts to increase at the higher pressure region. We discuss the pressure dependences of the coefficient of $$T^2$$ term in the electrical resistivity $$A$$, an antiferromagnetic gap $$Delta$$ and a characteristic temperature $$T_{{rho}{rm max}}$$. It is found that the effect of pressure on the electronic states in U$$_2$$Zn$$_{17}$$ is weak compared with the other heavy fermion compounds.

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