Faculty Profiles - KONTANI, Hiroshi

Papers - KONTANI, Hiroshi

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Optical conductivity of bad metals: Shift of the Drude peak

Mutou Tetsuya, Kontani Hiroshi

PHYSICAL REVIEW B Vol. 74 ( 11 ) 2006.9

Effect of a nonmagnetic impurity in a nearly antiferromagnetic Fermi liquid: Magnetic correlations and transport phenomena

Kontani Hiroshi, Ohno Masanori

PHYSICAL REVIEW B Vol. 74 ( 1 ) 2006.7

Transport phenomena in a three-dimensional system close to the magnetic quantum critical point: The conserving approximation with current vertex corrections, Reviewed

S. Onari, H. Kontani, and Y. Tanaka,

Physical Review B Vol. 74 ( 22 ) page： 224434 (10p) 2006.6

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

It is known that various transport coefficients strongly deviate from conventional Fermi-liquid behaviors in many electron systems which are close to antiferromagnetic (AF) quantum critical points (QCP). For example, Hall coefficients and Nernst coefficients in three-dimensional heavy fermion CeCoIn5 and CeCu6–xAux increase strikingly at low temperatures, whose overall behaviors are similar to those in high-Tc cuprates. These temperature dependencies are too strong to explain in terms of the relaxation time approximation. To elucidate the origin of these anomalous transport phenomena in three-dimensional systems, we study the current vertex corrections (CVC) based on the fluctuation exchange approximation, and find out the decisive role of the CVC. The main finding of the present paper is that the Hall coefficient and the Nernst coefficient strongly increase thanks to the CVC in the vicinity of the AF QCP, irrespective of dimensionality. We also study the relaxation time of quasiparticles, and find that "hot points" and "cold lines" are formed in general three-dimensional systems due to strong AF fluctuations.

Effect of a nonmagnetic impurity in a nearly antiferromagnetic Fermi liquid: Magnetic correlations and transport phenomena Reviewed

H. Kontani and M. Ohno

Physical Review B Vol. 74 ( 01 ) page： 014406 (21p) 2006.6

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

In nearly antiferromagnetic (AF) metals such as high-Tc superconductors (HTSCs), a single nonmagnetic impurity frequently causes nontrivial widespread change of the electronic states. To elucidate this long-standing issue, we study a Hubbard model with a strong onsite impurity potential based on an improved fluctuation-exchange (FLEX) approximation, which we call the GVI-FLEX method. This model corresponds to the HTSC with dilute nonmagnetic impurity concentration. We find that (i) both local and staggered susceptibilities are strongly enhanced around the impurity. By this reason, (ii) the quasiparticle lifetime as well as the local density of states are strongly suppressed in a wide area around the impurity (like a Swiss cheese hole), which causes the "huge residual resistivity" beyond the s-wave unitary scattering limit. We stress that the excess quasiparticle damping rate caused by impurities has strong k-dependence due to non-s-wave scatterings induced by many-body effects, so the structure of the "hot spot/cold spot" in the host system persists against impurity doping. This result could be examined by the ARPES measurements. In addition, (iii) only a few percent of impurities can cause a "Kondo-like" upturn of resistivity (d/dT<0) at low T when the system is very close to the AF quantum critical point. The results (i)–(iii) obtained in the present study, which cannot be derived by the simple FLEX approximation, naturally explain the main impurity effects in HTSCs. We also discuss the impurity effect in heavy fermion systems and organic superconductors.

Transport phenomena in a three-dimensional system close to the magnetic quantum critical point: The conserving approximation with current vertex corrections

Onari Seiichiro, Kontani Hiroshi, Tanaka Yukio

PHYSICAL REVIEW B Vol. 73 ( 22 ) 2006.6

Grand Kadowaki-Woods relation of heavy-fermion systems with degeneracy

Tsujii N., Kontani H., Yoshimura K.

PHYSICA B-CONDENSED MATTER Vol. 378-80 page： 730 - 731 2006.5

Electron-Phonon Mechanism for Superconductivity in Na0.35CoO2: Valence-Band Suhl-Kondo Effect Driven by Shear Phonons Reviewed

K. Yada and H. Kontai

Journal of the Physical Society of Japan Vol. 75 ( 3 ) page： 033705 (4p) 2006.3

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

To study the possible mechanism of superconductivity in Na0.35CoO2, we examine the interaction between all the relevant optical phonons (breathing and shear phonons) and t2g (a1g+eg') electrons of Co ions, and study the transition temperature for s-wave superconductivity. The obtained Tc is very low when the eg' valence bands are far below the Fermi level. However, Tc is strongly enhanced when the top of the eg' valence bands is close to the Fermi level (e.g., -50 meV), thanks to the interband hopping of Cooper pairs caused by shear phonons. This “valence-band Suhl–Kondo mechanism" due to shear phonons is important in understanding the superconductivity in Na0.35CoO2. By the same mechanism, the kink structure of the band dispersion observed by angle-resolved photoemission spectroscopy (ARPES) measurements, which indicates the strong mass enhancement (m*/m∼3) due to optical phonons, is also explained.

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

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