Faculty Profiles - KONTANI, Hiroshi

Papers - KONTANI, Hiroshi

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Theoretical Study of the Phase Diagram of β'-(BEDT-TTF)2AuCl2 at Hydrostatic Pressure Reviewed

Journal of the Physical Society of Japan Vol. 75 ( 10 ) page： 104702 (4) 2006.3

Language：English Publishing type：Research paper (scientific journal)

We study the electronic structure of β'-(BEDT-TTF)2AuCl2 under hydrostatic pressures of up to 20 GPa. We employ the effective single-band Hubbard model, the band structure of which is based on the first-principles result, and apply the FLEX approximation to take into account of the effective on-site Coulomb interaction. We examine magnetic and superconducting instabilities to find that the resulting theoretical phase diagram shows that the Néel temperature is high under hydrostatic pressure. In contrast to β'-(BEDT-TTF)2ICl2, β'-(BEDT-TTF)2AuCl2 is expected to show no superconducting state.

Electron-phonon mechanism for superconductivity in Na0.35CoO2: Valence-band Suhl-Kondo effect driven by shear phonons

Yada K, Kontani H

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN Vol. 75 ( 3 ) 2006.3

Evolution of Hall coefficient in two-dimensional heavy fermion CeCoIn5

Nakajima Y, Izawa K, Matsuda Y, Behnia K, Kontani H, Hedo M, Uwatoko Y, Matsumoto T, Shishido H, Settai R, Onuki Y

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN Vol. 75 ( 2 ) 2006.2

Evolution of Hall Coefficient in Two-Dimensional Heavy Fermion CeCoIn5 Reviewed

Y. Nakajima, K. Izawa, Y. Matsuda, K. Behnia, H. Kontani, M. Hedo, Y. Uwatoko, T. Matsumoto, H. Shishido, R. Settai and Y. Onuki

Journal of the Physical Society of Japan Vol. 2006 ( 1 ) page： 023705(4p) 2006.1

Language：English Publishing type：Research paper (scientific journal)

We report on the pressure dependence of the Hall coefficient RH in quasi-2D heavy fermion CeCoIn5. At ambient pressure, below a temperature associated with the emergence of non-Fermi liquid properties, RH is anomalously enhanced. We found that the restoration of the Fermi liquid state with applied pressure leads to a gradual suppression of this dramatic enhancement. Moreover, the enhancement in RH was found to be confined to an intermediate temperature window, where inelastic electron-electron scattering is dominant. Our results strongly support the presence of cold and hot spots on the Fermi surface probably due to anisotropic scattering by antiferromagnetic fluctuations, which may also prove relevant for the debate on the anomalous normal-state properties of high-Tc cuprates.

Optical Conductivity and Hall Coefficient in High-Tc Superconductors: Significant Role of Current Vertex Corrections Reviewed

Hiroshi Kontani

Journal of the Physical Society of Japan Vol. 75 ( 1 ) page： 013703(4p) 2006.1

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

We study AC conductivities in high-Tc cuprates, which offer us significant information to determine the true ground states. We take the current vertex correction (CVC) into account correctly to satisfy the conservation laws in terms of the fluctuation-exchange (FLEX) approximation. The significant role of the CVC on the optical Hall conductivity [σxy(ω)] is confirmed in the presence of strong antiferromagnetic (AF) fluctuations. This fact leads to the failure of the relaxation time approximation (RTA). As a result, experimental highly unusual behaviors, (i) prominent ω and temperature dependences of the optical Hall conductivity and the optical Hall coefficient [RH(ω)], and (ii) a simple Drude form of the optical Hall angle [θH(ω)] for a wide range of ω, are satisfactorily reproduced. In conclusion, both DC and AC transport phenomena in (slightly under-doped) high-Tc cuprates can be explained comprehensively in terms of a nearly AF Fermi liquid, without assuming an exotic ground state.

Optical conductivity and hall coefficient in high-T-c superconductors: Significant role of current vertex corrections

Kontani H

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN Vol. 75 ( 1 ) 2006.1

Physical meaning of the current vertex corrections: DC and AC transport phenomena in high-T-c superconductors

Kontani Hiroshi

Low Temperature Physics, Pts A and B Vol. 850 page： 565-566 2006

Spin fluctuations and weak pseudogap behaviors in Na0.35CoO2: Renormalization of band structure

Yada Keiji, Kontani Hiroshi

LOW TEMPERATURE PHYSICS, PTS A AND B Vol. 850 page： 707-+ 2006

軌道縮退を有する重い電子系のGrand Kadowaki-Woods則 Invited Reviewed

辻井直人、紺谷浩、吉村一良

日本物理学会誌 Vol. 60 ( 11 ) page： 872-875 2005.11

Origin of Weak Pseudogap Behaviors in Na0.35CoO2: Absence of Small Hole Pockets Reviewed

Keiji Yada and Hiroshi Kontani

Journal of the Physical Society of Japan Vol. 74 ( 8 ) page： 2161-2164 2005.8

Language：English Publishing type：Research paper (scientific journal)

We analyze the normal electronic states of Na0.35CoO2 based on the effective d- p model with a full d-orbital freedom using the fluctuation-exchange (FLEX) approximation. The electronic states sensitively depend on the topology of Fermi surfaces, which changes with the crystalline electric splitting (CES) due to the trigonal deformation. We succeed in reproducing the weak pseudogap behaviors in the density of states (DOS) and in the uniform magnetic susceptibility below 300 K, assuming that six small hole pockets predicted by LDA band calculations are absent. If these pockets exist, on the contrary, “anti-pseudogap behaviors" should inevitably appear. Thus, the present study strongly supports the absence of the small hole pockets in Na0.35CoO2, as reported by recent ARPES measurements. A large Fermi surface around the Γ-point would account for the superconductivity in water-intercalated samples.

Origin of weak pseudogap behaviors in Na0.35CoO2: Absence of small hole pockets

Yada K, Kontani H

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN Vol. 74 ( 8 ) page： 2161 - 2164 2005.8

Generalized Kadowaki-Woods relation in Er- and Sm-based heavy fermion systems

Kontani H

PHYSICA B-CONDENSED MATTER Vol. 359 page： 202-204 2005.4

Universality in heavy fermion systems with general degeneracy

Tsujii N, Kontani H, Yoshimura K

PHYSICAL REVIEW LETTERS Vol. 94 ( 5 ) 2005.2

*Universality in Heavy Fermion Systems with General Degeneracy Reviewed

N. Tsujii, H. Kontani, and K. Yoshimura

Physical Review Letter Vol. 94 ( 11 ) page： 057201(4p) 2005.2

Language：English Publishing type：Research paper (scientific journal)

We discuss the relation between the T2 coefficient of electrical resistivity A and the T-linear specific-heat coefficient γ for heavy-fermion systems with general N, where N is the degeneracy of quasiparticles. A set of experimental data reveals that the Kadowaki-Woods relation, A/γ2=1×10-5  μΩ cm(K mol/mJ)2, collapses remarkably for large-N systems, although this relation has been regarded to be commonly applicable to the Fermi liquids. Instead, based on the Fermi-liquid theory we propose a new relation, A˜/γ˜2=1×10-5 with A˜=A/1/2N(N-1) and γ˜=γ/1/2N(N-1). This new relation exhibits an excellent agreement with the data for the whole range of degenerate heavy fermions.

From Kondo Effect to Fermi Liquid Invited Reviewed

Hiroshi Kontani and Kosaku Yamada

Journal of the Physical Society of Japan Vol. 74 ( 1 ) page： 155-166 2005.1

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

The Kondo effect has been playing an important role in strongly correlated electon systems. The important point is that the magnetic impurity in metals is a typical example of the Fermi liquid. In the system the local spin is conserved in the ground state and continuity with respect to Coulomb repulsion U is satisfied. This nature is satisfied also in the periodic systems as far as the systems remain as the Fermi liquid. This property of the Fermi liquid is essential to understand the cuprate high-Tc superconductors (HTSC). On the basis of the Fermi liquid theory we develop the transport theory such as the resistivity and the Hall coefficient in strongly correlated electron systems, such as HTSC, organic metals and heavy Fermion systems. The significant role of the vertex corrections for total charge- and heat-currents on the transport phenomena is explained. By taking the effect of the current vertex corrections into account, various typical non-Fermi-liquid-like transport phenomena in systems with strong magnetic and/or superconducting flucutations are explained within the Fermi liquid theory.

Magnetic-Field-induced Antiferromagnetism in Two-dimensional Hubbard Model: Analysis of CeRhIn5 Reviewed

Keitaro Sakurazawa, Hiroshi Kontani and Tetsuro Saso

Journal of the Physical Society of Japan Vol. 74 ( 1 ) page： 271-274 2005.1

Language：English Publishing type：Research paper (scientific journal)

We propose a mechanism for the magnetic-field-induced antiferromagnetic (AF) state in a two-dimensional Hubbard model in the vicinity of the AF quantum critical point (QCP), using the fluctuation-exchange (FLEX) approximation by taking the Zeeman energy due to the magnetic field B into account. In the vicinity of the QCP, we find that the AF correlation perpendicular to B is enhanced, whereas that parallel to B is reduced. This fact means that the finite magnetic field increases TN, with the AF order perpendicular to B. The increment of TN can be understood in terms of the reduction of both quantum and thermal fluctuations due to the magnetic field, which is caused by the self-energy effect within the FLEX approximation. The present study naturally explains the increment of TN in CeRhIn5 under the magnetic field found recently. We will also discuss a relation to some organic metals.

From Kondo effect to Fermi liquid

Kontani H, Yamada K

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN Vol. 74 ( 1 ) page： 155 - 166 2005.1

Magnetic-field-induced antiferromagnetism in two-dimensional Hubbard model: Analysis of CeRhIn5

Sakurazawa K, Kontani H, Saso T

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN Vol. 74 ( 1 ) page： 271 - 274 2005.1

Generalized Kadowaki-Woods relation in Er- and Sm-based Heavy Fermion Systems, Invited Reviewed

Hiroshi Kontani

Physica B. Vol. 359-361 page： 202-204 2005

Theory of anisotropic s-wave superconductivity with point-node-like gap minima: Analysis of (Y,Lu)Ni2B2C Reviewed

H. Kontani

Physical Review B Vol. 70 ( 5 ) page： 054507(13p) 2004.8

Language：English Publishing type：Research paper (scientific journal)

Recent intensive experimental studies revealed that (Y,Lu)Ni2B2C is an extremely anisotropic s-wave superconductor. In addition, its gap function possesses deep point minima, whose ratio of the gap anisotropy is more than 10. On the theoretical side, however, it is nontrivial to understand the origin of such a peculiar superconductivity. In the present paper, we propose a mechanism of the s-wave superconductivity with deep gap minima, based on the theoretical model where strong electron-phonon coupling as well as the moderate magnetic fluctuations coexist. By analyzing the strong coupling Eliashberg equation, we find that s-wave superconducting gap function owing to the electron-phonon coupling becomes highly anisotropic as the magnetic fluctuations increase. The set of model parameters for realizing the strong gap anisotropy in the present model will be appropriate for (Y,Lu)Ni2B2C. According to the present mechanism, (groups of) pairs of gap minima appear at points on the Fermi surface which are connected by the nesting vector Q, in both cases of s-wave superconductors and non-s-wave ones. We briefly discuss other superconductors with highly anisotropic gap function, e.g., PrOs4Sb12 and Na0.33CoO2.

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Papers - KONTANI, Hiroshi