研究者詳細 - 武田　直也

論文 - 武田　直也

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Heavy fermion fluid in high magnetic fields: An infrared study of CeRu4Sb12 査読

SV Dordevic, KSD Beach, N Takeda, YJ Wang, MB Maple, DN Basov

PHYSICAL REVIEW LETTERS 96 ( 1 ) 017403(1) - 017403(4) 2006年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

We report a comprehensive infrared magnetospectroscopy study of a CeRu4Sb12 compound revealing quasiparticles with a heavy effective mass m(*), with a detailed analysis of optical constants in fields up to 17 T. We find that the applied magnetic field strongly affects the low-energy excitations in the system. In particular, the magnitude of m*similar or equal to 70 m(b) (m(b) is the quasiparticle band mass) at 10 K is suppressed by as much as 25% at 17 T. This effect is in quantitative agreement with the mean-field solution of the periodic Anderson model augmented with a Zeeman term.

DOI： 10.1103/PhysRevLett.96.017403

Probing the vortex state of PrRu4Sb12 through muon spin rotation and relaxation 査読

DT Adroja, AD Hillier, JG Park, EA Goremychkin, KA McEwen, N Takeda, R Osborn, BD Rainford, RM Ibberson

PHYSICAL REVIEW B 72 ( 18 ) 184503(1) - 184503(7) 2005年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

We have investigated the magnetic penetration depth lambda and the superconducting coherence length xi in the vortex lattice of the filled skutterudite compound PrRu4Sb12 (T-c similar to 0.97 K) using transverse-field muon-spin rotation measurements. Zero-field and longitudinal-field studies were also carried out to investigate the time-reversal symmetry of the superconducting state. We found lambda=3650(20) angstrom and xi=345(5) angstrom at 0.05 K and using these values of lambda and xi, together with the Sommerfeld constant we have calculated an effective mass of the quasiparticles m(*)approximate to 10m(e) and superfluid carrier density n(s)approximate to 4x10(27) carriers/m(3). The temperature dependence of the vortex state muon-spin relaxation rate sigma(s)(T) is consistent with the phenomenological two-fluid model. Further, the zero-field and longitudinal field measurements do not reveal any signature of a spontaneous internal magnetic field below the superconducting transition temperature, indicating the preservation of time-reversal symmetry in the superconducting state of PrRu4Sb12, unlike the broken time-reversal symmetry of the superconducting state of the heavy-fermion superconductor PrOs4Sb12.

DOI： 10.1103/PhysRevB.72.184503

Novel superconductivity coexisting with incipient electric-multipolar order in CePt3Si 査読

M Ishikawa, S Yamashita, Y Nakazawa, N Wada, N Takeda

JOURNAL OF PHYSICS-CONDENSED MATTER 17 ( 23 ) L231 - L234 2005年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：IOP PUBLISHING LTD

We measured the specific heat C-p of an annealed sample of CePt3Si in magnetic fields B up to 10 T and confirmed two conspicuous anomalies in the temperature range where no apparent magnetic anomalies were found in the susceptibility. The lower-temperature anomaly at 2.2 K was depressed with increasing B and seemed to get absorbed around 4 T into the other one around 2.8 K. The latter anomaly continues to grow into a solid peak with increasing B. These features imply that neither of these transitions is due to long-range antiferromagnetism (AF) as originally proposed, but they are reminiscent of quadrupolar transitions found in certain Ce-based Kondo compounds, in which two distinct anomalies were also observed in C-p and identified as a quadrupolar transition followed by an AF transition at a slightly lower T. In the case of CePt3Si, the lower-T anomaly is not due to AF but to a new phase transition of yet-unknown nature, and in this new phase the superconductivity emerges at 0.8 K. We thereby hypothesize that the superconductivity could be mediated by magnetic or other electronic excitations under the influence of the quadrupolar-like order induced around 2.8 K and that CePt3Si would be the first example of an unconventional superconductor coexisting with electric-multipolar order.

DOI： 10.1088/0953-8984/17/23/L02

NMR study of the new filled skutterudite superconductor YFe4P12 査読

K Magishi, T Saito, K Koyama, Shirotani, I, Y Shimaya, K Kihou, C Sekine, N Takeda, M Ishikawa, T Yagi

PHYSICA B-CONDENSED MATTER 359 883 - 885 2005年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE BV

We report the results of P-31 NMR measurements on YFe4P12, a new member of the family of the filled skutterudite compounds, which displays superconductivity below T-c = 7 K. In the normal state, the nuclear spin-lattice relaxation time T-1 obeys the Korringa relation T1T = constant below 40 K, but it changes its behavior at higher temperatures. The Korringa constant (T1T)(-1) for YFe4P12 is smaller than that for LaFe4P12, suggesting that the density of states at Fermi energy N(E-F) for YFe4P12 is lower than the case of LaFe4P12. In the superconducting state, T-1(-1)(T) shows a small coherence peak just below Tc, followed by an exponential decrease below 4 K revealing on energy gap 2 Delta(0)/k(B)T(c) = 3.3. Our results indicate that YFe4P12 is a conventional type s-wave superconductor. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2005.01.333

Influence of the crystal field potential on the superconducting properties of PrRU(4)Sb(12) 査読

DT Adroja, JG Park, EA Goremychkin, N Takeda, M Ishikawa, KA McEwen, R Osborn, AD Hillier, BD Rainford

PHYSICA B-CONDENSED MATTER 359 983 - 985 2005年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE BV

We have determined the crystal field potential of the superconductor PrRu(4)Sb(12) (T(c) similar to 0.9 K) using inelastic neutron scattering. The ground state of the Pr(+3) ions in T(h) point group symmetry is a Gamma(1) singlet with a Gamma(5) triplet at an energy of Delta similar to 5.6meV. Comparison with recent results on the PrOs(4)Sb(12) compound, where T(c) similar to 1.85 K and Delta similar to 0.6 meV, supports the conjecture that inelastic quadrupolar fluctuations (or aspherical Coulomb scattering) are responsible for the strong enhancement of T(c). In PrRu(4)Sb(12), the effect is reduced by the higher energy of the first excited level. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2005.01.286

Nonmagnetic heavy-fermion superconductor, CePt3Si 査読

M Ishikawa, N Takeda

SOLID STATE COMMUNICATIONS 133 ( 4 ) 249 - 251 2005年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：PERGAMON-ELSEVIER SCIENCE LTD

We have studied the effect of heat treatment and the Ge substitution in place of Si in the recently discovered heavy-fermion superconductor CePt3Si. The annealed CePt3Si exhibited nonmagnetic heavy-fermion behavior instead of the antiferromagnetism (AF) found in quenched samples. The AF state was destroyed by only about 1 at.% of Ge-substitution and may not be a stable phase. Specific-heat measurements on the annealed CePt3Si and the Ge-substituted samples revealed a large hump around 2.2 K, originally claimed as Neel temperature. Its true nature is not clarified yet but conjectured at present as a sort of quadrupolar transition rather than AF long-range order. The superconducting transition around 0.75 K was equally sharp with DeltaC(P)/gammaT(c) = 0.7 for clean quenched and annealed samples. The interplay between the 2.2 K-anomaly and the superconductivity is discussed. (C) 2004 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ssc.2004.10.032

24pTA-15 比熱から観たCePt_3Siの磁性と超伝導(重い電子超伝導(Ce115系,CePt_3Si),領域8(強相関系:高温超伝導,強相関f電子系など))

石川征靖, 山下智史, 中澤泰浩, 和田信雄, 武田直也

日本物理学会講演概要集 60 497 - 497 2005年

Anomalous magnetic properties and non-Fermi-liquid behaviour in single crystals of the Kondo lattice CeNiGe2-xSix 査読

DY Kim, DH Ryu, JB Hong, JG Park, YS Kwon, MA Jung, MH Jung, N Takeda, M Ishikawa, S Kimura

JOURNAL OF PHYSICS-CONDENSED MATTER 16 ( 46 ) 8323 - 8334 2004年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：IOP PUBLISHING LTD

We report on the magnetic susceptibility, specific heat and electrical resistivity of the heavy fermion compounds CeNiGe2-xSix (0 less than or equal to x less than or equal to 1). Compounds with x < 1 show anti ferro magnetic order, which with increasing x shifts toward lower temperature owing to increased exchange coupling between the localized 4f magnetic moments and conduction electrons. Eventually, the magnetic order almost becomes absent, for x = 1. An anomaly observed in the specific heat is well interpreted by the Kondo model for a degenerate impurity spin J = 1/2 in the Coqblin-Schrieffer limit. A coherence peak indicative of the formation of a Kondo lattice is found in the electrical resistivity, whose features are consistent with the results for the specific heat. Interestingly, there is a significant deviation from Fermi-liquid behaviour at the critical concentration x = 1. This deviation is attributed to a quantum phase transition in a model with two-dimensional antiferromagnetic fluctuations.

DOI： 10.1088/0953-8984/16/46/018

Effect of pressure on the electrical resistivity of CeCu2Si2 査読

Masahito Koeda, Masato Hedo, Yoshiya Uwatoko, Takehiko Matsumoto, Naoya Takeda, Masayasu Ishikawa

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 272 E175 - E176 2004年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE BV

We have performed electrical resistivity measurements at different pressures on two kinds of samples (low-T-c and high-T-c), which differ in physical properties of the heavy-fermion superconductor CeCu2Si2. It was found that one with lower T-c shows partial derivative T-c/partial derivative P of negative and the other with higher T-c shows partial derivative T-c/partial derivative P of positive. Therefore, the superconductivity for these two samples may have different origins. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2003.11.103

Competition between Kondo effect and ferromagnetic interaction in skutterudite La1-xSmxFe4P12 査読

N Takeda, M Ishikawa

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 272 64 - 65 2004年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE BV

We present the magnetic properties of a filled skutterudite La1-xSmxFe4P12. SmFe4P12 is a heavy ferimon compound with a novel ferromagnetic ground state. The magnetic susceptibility for x less than or equal to 0.4 shows a nearly temperature-independent Pauli paramagnetism, but samples for 0.6 less than or equal to x less than or equal to 0.85 reveals a broad maximum around 15 K. The origin of the maximum is discussed. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2003.11.058

Study of the hybridization gap in a heavy fermion compound: CeRu4Sb12 査読

D. T. Adroja, J. -G. Park, K. A. McEwen, N. Takeda, M. Ishikawa, J. -Y. So

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 272 E21 - E22 2004年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE BV

We have investigated the temperature and momentum dependence of the inelastic neutron scattering response of CeRu4Sb12. Our neutron scattering study shows the opening of a Q-independent spin gap of 30 meV in the excitation spectrum, which is smaller than the charge gap of 47 meV observed in optical studies. The spin gap is nearly temperature independent below 100 K, while above this temperature the gap starts to fill up and at higher temperatures the magnetic response becomes purely quasielastic. We discuss the origin of the spin gap and consider the difference between the magnitude of the spin and charge gaps on the basis of theoretical models. (C) 2004 Elsevier B. V. All rights reserved.

DOI： 10.1016/j.jmmm.2003.11.126

Spin gap formation in the heavy fermion skutterudite compound CeRu4Sb12 査読

D. T. Adroja, J.-G. Park, K. A. McEwen, N. Takeda, M. Ishikawa, J.-Y. So

Phys. Rev.B 68 094425(1) - 094425(6) 2003年12月

Ferromagnetic Heavy Fermion System SmFe4P12 査読

N. Takeda, M. Ishikawa

PhysicaB 329-333 460 - 461 2003年12月

Pairing Symmetry in two distinct superconducting states of CeCu2Si2 査読

M. Ishikawa, N. Takeda, M. Koeda, M. Hedo, Y. Uwatoko

Phys. Rev.B 68 024522(1) - 024522(5) 2003年12月

Structural, electrical, and magnetic properties of a series of molecular conductors based on BDT-TTP and lanthanoid nitrate complex anions (BDT-TTP=2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene) 査読

HB Cui, T Otsuka, A Kobayashi, N Takeda, M Ishikawa, Y Misaki, H Kobayashi

INORGANIC CHEMISTRY 42 ( 19 ) 6114 - 6122 2003年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER CHEMICAL SOC

The platelike crystals of a series of novel molecular conductors, which are based on the pi-donor molecules BDT-TTP (2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene) with a tetrathiapentalene skeleton and lanthanide nitrate complex anions [Ln(NO3)(x)](3-x) (Ln = La, Ce, (Pr), Tb, Dy, Ho, Er, Tm, Yb, Lu) with localized 4f magnetic moments, were synthesized. Except for the Ce complex, the salts were composed of (BDT-TTP)(5)[Ln(NO3)(5)] and were isostructural. Even though the Ce crystal had a different composition, (BDT-TTP)(6)[Ce(NO3)(6)](C2H5OH)(x) (x approximate to 3), the crystals all had the space group P (1) over bar. Although the X-ray examination of the Pr salts was insufficient, the existence of two modifications was suggested in these systems by preliminary X-ray examination. Previously, we reported the crystal structures and unique magnetic properties of (BDT-TTP)(5)[Ln(NO3)(5)] (Ln = Sm, Eu, Nd, Gd). Thus, by combining the results of this work with previous one, we for the first time succeeded in obtaining a complete set of organic conductors composed of the identical pi-donors (BDT-TTP in this case) and all the lanthanide nitrate complex anions (except the complex with Pm3+). The crystals were all metallic down to 2 K. Electronic band structure calculations resulted in two-dimensional Fermi surfaces, which was consistent with their stable,metallic states. Except for the Lu complex, which lacked paramagnetic moments, the magnetic susceptibilities were measured on the six heavy lanthanide ion complex salts by a SQUID magnetometer (Ln = Tb, Dy, Ho, Er, Tm, Yb). The large paramagnetic susceptibilities, which were caused by the paramagnetic moments of the rare-earth ions, were obtained. The Curie-Weiss law fairly accurately reproduced the temperature dependence of the magnetic susceptibilities of (BDT-TTP)(5)[Ho(NO3)(5)] in the experimental temperature range (2-300 K) and-a comparatively large Weiss temperature (\Theta\) was obtained (Theta(Ho) = -15 K). A Weiss temperature (Theta(Tm) = -8 K) was also obtained for Tm. The \Theta\ values of other (BDT-TTP)(5)[Ln(NO3)(5)] salts and (BDT-TTP)(6)[Ce(NO3)(6)](C2H5OH)(x)(x approximate to 3) were as follows: \Theta\/K = 4 (Er), less than or equal to2 (Ce, Tb, Dy, Yb). The comparatively strong intermolecular magnetic interaction between Ho3+ ions, which was suggested by the \Theta\ value, is inconsistent with the traditional image of strongly localized 4f orbitals shielded by the electrons in the outer 5s and 5p orbitals. The dipole interactions between Ln(3+) ions causing the Curie-Weiss behavior and the comparatively large Theta value of (BDT-TTP)(5)[Ho(NO3)(5)] is inconsistent with the data, since the complexes exhibit isostructural properties and there is not a clear relationship between the magnitudes of Theta values and those of magnetic moments. Therefore, it is possible that the 4f orbitals of Ho atom are sensitive to the ligand field, which will have an effect on the orbital moment of the Ho3+ ion and/or produce a small amount of mixing between 4f and ligand orbitals to give rise to "real" intermolecular antiferromagnetic interaction through intermolecular overlapping between pi (BDT-TTP) and ligand orbitals of lanthanide nitrate complex anions.

DOI： 10.1021/ic034459g

Superconductivity of new filled skutterudite YFe4P12 prepared at high pressure 査読

I. Shirotani, Y. Shimaya, K. Kihou, C. Sekine, N. Takeda, M. Ishikawa, T. Yagi

Journal of Physics Condensed Matter 15 ( 28 ) S2201 - S2205 2003年7月

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

A new filled skutterudite YFe4P12 has been prepared at high temperatures and high pressures. The electrical resistivity and dc magnetic susceptibility of this compound were measured at low temperatures. The superconductity in YFe4P12 is observed at around 7 K. This phosphide is a new superconductor. The specific heat of YFe4P12 has been studied between 2 and 20 K. The specific heat fitted to the expression C = γT + βT3, by a least squares analysis, yielded the values γ = 27.2 mJ mol-1 K-2 and β= 0.195 mJ mol-1 K-4. The Debye temperature (ΘD) is found to be 553 K. The electronic density of the states at the Fermi energy for YFe4P12 is 0.45 state eV-1/atom.

DOI： 10.1088/0953-8984/15/28/352

Non-Fermi-liquid behavior in CeNiGe2-xSix single crystals 査読

SO Hong, ED Mun, N Takeda, M Ishikawa, YS Kwon

PHYSICA B-CONDENSED MATTER 329 518 - 519 2003年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE BV

The specific heat in low temperature regions was measured for several pseudoternary compounds of the CeNiGe2-xSix System. Substitution of silicon for germanium enhances the coupling constant J and the Kondo interaction. The antiferromagnetism in CeNiGe2 is ultimately suppressed around x = 1 by the enhanced Kondo effect. A non-Fermi liquid behavior has been observed in x = 1 composition. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(02)02211-1

The ferromagnetic Kondo-lattice compound SmFe4P12 査読

Naoya Takeda, Masayasu Ishikawa

Journal of Physics Condensed Matter 15 ( 14 ) L229 - L233 2003年4月

記述言語：英語掲載種別：研究論文（学術雑誌）

We report on the magnetic properties of a filled skutterudite compound, SmFe4P12. Magnetic susceptibility and specific heat measurements revealed a ferromagnetic transition at 1.6 K. The temperature dependence of the electrical resistivity exhibits a Kondo-lattice behaviour and the electronic specific heat coefficient attains values as large as 370 mJ mol-1 K-2. This compound is thereby the first Sm-based heavy-fermion system found with a ferromagnetic ground state. The Kondo temperature is estimated to be about 30 K.

DOI： 10.1088/0953-8984/15/14/101

Electronic structure of the filled skutterudite compound CeRu4Sb12 査読

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

Phys.Rev.B. 65 41105(1) - 41105(4) 2002年12月

Ultrahigh resolution photoemission study of CeRu4Sb12 査読

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

Journal of Physics and Chemistry of Solids 63 ( 6-8 ) 1175 - 1178 2002年

記述言語：英語掲載種別：研究論文（学術雑誌）

Ultrahigh-resolution photoemission spectroscopy reveals the crossover behavior of CeRu4Sb12 into a low carrier state. The density of states (DOS) around the Fermi level (EF) decreases on lowering the temperature. The temperature dependence of the DOS at EF is consistent with the results of the transport measurements and explained in terms of the development of the coherence. © 2002 Published by Elsevier Science Ltd.

DOI： 10.1016/S0022-3697(02)00019-7

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