Publishing type：Research paper (international conference proceedings)  

The superconductivity in tungsten bronze Ax WO3 (A═alkali metal) is studied based on first-principles calculations and orbital fluctuation theory. We discuss the effects of the electron-Jahn-Teller phonon interaction and the on-site Coulomb interaction in the random phase approximation, and obtain a phase diagram on the U - g plane in the low-doped regime at x∼0.05, where high-temperature superconductivity has been experimentally observed.

DOI： 10.1088/1742-6596/2164/1/012012

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Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

The superconductivity in the one-band Hubbard model on a Bethe lattice is investigated on the basis of the dynamical mean-field theory (DMFT) in which the irreducible vertex function has no k-dependence and then only the s-wave superconductivities with the spin-singlet even-frequency and the spin-triplet odd-frequency pairings are possibly realized. We calculate the pair susceptibility by solving the Bethe-Salpeter equation with the use of the linearized DMFT and find that the spin-singlet pair susceptibility is suppressed by the on-site Coulomb interaction U especially for the region near the Mott transition. On the other hand, the spin-triplet pair susceptibility is apparently enhanced by U in the strong correlation regime at very low temperatures where the effective interaction for the triplet pairing is considered to be attractive and then the triplet superconductivity is expected to be realized.

DOI： 10.1088/1742-6596/2164/1/012017

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Other Link： https://iopscience.iop.org/article/10.1088/1742-6596/2164/1/012017/pdf

Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/jpsj.89.113704

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Publishing type：Research paper (scientific journal)   Publisher：Wiley  

DOI： 10.1002/pssb.202000108

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/pssb.202000108

Publishing type：Research paper (international conference proceedings)   Publisher：Journal of the Physical Society of Japan  

DOI： 10.7566/jpscp.30.011043

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Publishing type：Research paper (international conference proceedings)   Publisher：Journal of the Physical Society of Japan  

DOI： 10.7566/jpscp.30.011152

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Publishing type：Research paper (international conference proceedings)   Publisher：Journal of the Physical Society of Japan  

DOI： 10.7566/jpscp.30.011070

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Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/jpsj.88.064701

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

The pressure dependence of the structural (Ts), antiferromagnetic (Tm), and superconducting (Tc) transition temperatures in FeSe is investigated on the basis of the 16-band d-p model. At ambient pressure, a shallow hole pocket disappears due to the correlation effect, as observed in the angular-resolved photoemission spectroscopy (ARPES) and quantum oscillation (QO) experiments, resulting in the suppression of the antiferromagnetic order, in contrast to the other iron pnictides. The orbital-polarization interaction between the Fe d orbital and Se p orbital is found to drive the ferro-orbital order responsible for the structural transition without accompanying the antiferromagnetic order. The pressure dependence of the Fermi surfaces is derived from the first-principles calculation and is found to well account for the opposite pressure dependences of Ts and Tm, around which the enhanced orbital and magnetic fluctuations cause the double-dome structure of the eigenvalue λ in the Eliashberg equation, as consistent with that of Tc in FeSe.

DOI： 10.7566/JPSJ.87.014705

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

Transition metal chalcogenide Ta2NiSe5, a promising material for the excitonic insulator, is investigated on the basis of the three-chain Hubbard model with two conduction (c) bands and one valence (f) band. In the semimetallic case where only one of two c bands and the f band cross the Fermi level, the transition from the c–f compensated semimetal to the uniform excitonic order, the so-called excitonic insulator, takes place at low temperature as the same as in the semiconducting case. On the other hand, when another c band also crosses the Fermi level, the system shows three types of Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) excitonic orders characterized by the condensation of excitons with finite center-of-mass momentum q corresponding to the three types of nesting vectors between the imbalanced two c and one f Fermi surfaces. The obtained FFLO excitonic states are metallic in contrast to the excitonic insulator and are expected to be observed in the semimetallic Ta2NiSe5 under high pressure. The effect of the electron–lattice coupling is also discussed briefly and is found to induce the monoclinic distortion not only in the uniform excitonic state but also in the FFLO one resulting in the orthorhombic-monoclinic structural phase transition for both cases as observed in Ta2NiSe5 for both low-pressure semiconducting and high-pressure semimetallic regimes.

DOI： 10.7566/JPSJ.87.054701

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We investigate anomalous oscillations due to the Aharonov-Bohm (AB) and Aharonov-Casher (AC) effects of the one-dimensional Hubbard ring with flux in the strong coupling limit. By using the exact diagonalization method and the Shiba transformation, we examine the energies of the ground-state and a few excited states in the presence of the flux producing the AB or AC effect, where the transformation not only reverses the sign of the interaction U but also exchanges the role between the AB and AC effects in the model Hamiltonian. We systematically classify the AB and AC oscillations by using the number of minima N-min of the ground-state energy as a function of a normalized phase shift phi for 0 <= phi < 1, and clarify the close relationship between the AB and AC effects. For example, it is shown that N-min is given by N-L - N-e (N-L - N-up arrow + N-down arrow) for the AB (AC) effect in the very strong attraction, where N-L, Ne, N-up arrow, and N-down arrow are the system size, the total number of electrons, the number of electrons with up-spin, and the number of electrons with down-spin, respectively, under the condition of NL > Ne > N-up arrow > N-down arrow. In more special cases, such as for a half-filled band and the spin-balanced case (NL = Ne and N-up arrow = N-down arrow), we find Nmin to be 0 (2) for the AB (AC) effect in the case of very strong repulsion. These results show us the nature of interesting phenomena originating from the interplay between the strong correlation and the quantum interference effect in a mesoscopic ring.

DOI： 10.7566/JPSJ.85.124716

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

To examine the electronic phase diagram of superconducting CuO double chains, we report the effect of external pressure on the magneto-transport properties in superconducting and non-superconducting polycrystalline samples of Pr2Ba4Cu7O15-delta at low temperatures (1.8-40 K) under various magnetic fields (up to 14 T). In the as-sintered non-superconducting sample, the magneto-resistance (MR) follows a power law of H-3/2 at low temperatures, which is in no agreement with the H-2 dependence of MR in the PrBa2Cu4O8 system. The negative pressure dependence of the superconducting phase is qualitatively consistent with a theoretical prediction on the basis of the Tomonaga-Luttinger liquid theory. The 48-h-reduced superconducting sample at ambient pressure exhibits no clear increase in MR for T > T-c,(on) = 26.5 K. In contrast, with the application of pressure to the superconducting sample, the MR effects reappear and are also well fitted by H-3/2. The model of slightly warped Fermi surfaces explains not only the MR effect of the non-superconducting sample, but is also related to the reasons for the pressure-induced MR phenomena of the superconducting sample.

DOI： 10.7566/JPSJ.85.124704

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

The five-orbital Hubbard model for iron- based superconductors is investigated using the dynamical mean-field theory combined with the Eliashberg equation to clarify the local correlation effects on the electronic states and the superconductivity. In the specific case where the antiferromagnetic (AFM) and antiferro-orbital (AFO) fluctuations are comparably enhanced, the orbital dependence of the vertex function is significantly large, while that of the self-energy is small, in contrast to the AFM fluctuation-dominated case where the vertex function (the self-energy) shows a small (large) orbital dependence. The orbital-dependent vertex function together with the nesting between the inner and outer hole Fermi surfaces results in the enhancement of the inter-orbital ferromagnetic (FM) fluctuation in addition to the AFM and AFO fluctuations. In this case, the hole-s(+/-)-wave pairing with the sign change of the two hole Fermi surfaces is mediated by the coexisting three fluctuations as expected to be observed in the specific compound LiFeAs.

DOI： 10.7566/JPSJ.85.114709

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We examine the free energy and the thermodynamic properties in the three-chain Hubbard model for Ta2NiSe5 to clarify the phase transitions between the uniform and the FFLO excitonic states which are expected to be observed in Ta2NiSe5 under high pressure.

DOI： 10.7566/JPSJ.85.065005

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

The three-chain Hubbard model for Ta2NiSe5, known as a candidate material for an excitonic insulator, is investigated over the wide range of the energy gap D between the twofold degenerate conduction bands and the nondegenerate valence band including both semiconducting (D > 0) and semimetallic (D < 0) cases. In the semimetallic case, the difference in the band degeneracy inevitably causes the imbalance of each Fermi wavenumber, resulting in a remarkable excitonic state characterized by the condensation of excitons with finite center-of-mass momentum q, the so-called Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) excitonic state. With decreasing D corresponding to increasing pressure, the obtained excitonic phase diagram shows a crossover from BEC (D greater than or similar to 0) to BCS (D less than or similar to 0) regime, and then shows a distinct phase transition at a certain critical value D-c (< 0) from the uniform (q = 0) to the FFLO (q not equal 0) excitonic state, as expected to be observed in Ta2NiSe5 under high pressure.

DOI： 10.7566/JPSJ.85.053703

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

The five-orbital Hubbard model for iron pnictides is investigated by using the dynamical mean-field theory (DMFT) in conjunction with the Eliashberg equation in the strong correlation regime where the ferromagnetic (FM) and antiferromagnetic (AFM) fluctuations are simultaneously enhanced. When the intraorbital Coulomb interaction is a little smaller than the interorbital one, the FM fluctuation is largely enhanced due to the significant orbital dependence of the vertex correction and becomes comparable to the AFM fluctuation, in contrast to the result from the random phase approximation where the AFM fluctuation is exclusively enhanced due to the nesting between hole and electron Fermi surfaces (FSs). The enhanced FM fluctuation induces the large repulsive pairing interaction between inner and outer hole FSs while the AFM fluctuation does between the hole and electron FSs. The latter is partially counteracted by the attractive pairing interaction mediated by the antiferro-orbital fluctuation. The resulting pairing state is found to be a novel s-wave in which the gap function of the inner hole FS has the opposite sign to that of the outer hole and electron FSs.

DOI： 10.1088/1742-6596/592/1/012077

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

The electronic states and superconductivity in iron pnictides are studied on the basis of the 16 band d-p model which includes both the onsite Coulomb interaction between Fe d electrons and the intersite one between Fe d and pnictogen p electrons. The model well accounts for experimentally observed two fluctuations: the d-d interaction-enhanced antiferromagnetic (AFM) fluctuation and the d-p interaction-enhanced ferro-orbital (FO) fluctuation responsible for the C-66 elastic softening. The AFM fluctuation induces the repulsive pairing interaction for q similar to Q(AF) while the FO does the attractive one for q similar to 0 resulting in the s(+/-)-wave superconductivity where the two fluctuations cooperatively enhance the superconducting transition temperature T-c without any competition by virtue of the q-space segregation.

DOI： 10.7566/JPSJ.83.043704

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We investigate a two-orbital model for iron-based superconductors to elucidate the effect of interplay between electron correlation and Jahn-Teller electron-phonon coupling by using the dynamical mean-field theory combined with the exact diagonalization method. When the intra-and inter-orbital Coulomb interactions, U and U', increase with U = U', both the local spin and orbital susceptibilities, chi(s) and chi(o), increase with chi(s) = chi(o) in the absence of the Hund's rule coupling J and the electron-phonon coupling g. In the presence of J and g, there are distinct two regimes: for J greater than or similar to 2g(2)/omega(0) with the phonon frequency omega(0), chi s is enhanced relative to chi(o) and shows a divergence at J = J(c) above which the system becomes Mott insulator, while for J less than or similar to 2g(2)/omega(0), chi(o) is enhanced relative to chi(s) and shows a divergence at g = g(c) above which the system becomes bipolaronic insulator. In the former regime, the superconductivity is mediated by antiferromagnetic fluctuations enhanced due to Fermi-surface nesting and is found to be largely dependent on carrier doping. On the other hand, in the latter regime, the superconductivity is mediated by ferro-orbital fluctuations and is observed for wide doping region including heavily doped case without the Fermi-surface nesting.

DOI： 10.7566/JPSJ.83.044711

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：The Physical Society of Japan  

DOI： 10.7566/JPSCP.3.015041

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：The Physical Society of Japan  

DOI： 10.7566/JPSCP.3.015020

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We investigate the electronic state and the superconductivity in the five-orbital Hubbard model for iron pnictides by using the dynamical mean-field theory in conjunction with the Eliashberg equation. The renormalization factor exhibits significant orbital dependence resulting in the large change in the band dispersion as observed in recent ARPES experiments. The critical interactions towards the magnetic, orbital and superconducting instabilities are suppressed as compared with those from the random phase approximation (RPA) due to local correlation effects. Remarkably, the s(++)-pairing phase due to the orbital fluctuation is largely expanded relative to the RPA result, while the s(+/-)-pairing phase due to the magnetic fluctuation is reduced.

DOI： 10.7566/JPSJ.82.123712

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Physical Society of Japan  

DOI： 10.7566/JPSJ.82.105003

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Physical Society of Japan  

We report the magnetotransport and dc magnetic susceptibility of the polycrystalline samples of Pr2Ba4Cu7O 15-δ, to examine the effect of magnetic field on the superconducting phase of the metallic CuO double-chain. The resistive critical magnetic field is estimated to be about 21 T at low temperatures from the resistive transition data. On the other hand, the corresponding critical field determined from the magnetization measurements gives rise to a very low value of ̃0.3 T at 2 K. These discrepancies in the magnetic response between the resistivity and magnetization data are caused by disappearance of the magnetically shielding effect even in relatively lower fields. In spite of the observation of the resistive drop associated with the superconducting transport currents, the suppression of the diamagnetic signal is probably related to the superconductivity of quasi one-dimensional CuO double-chain. The behavior of Seebeck coefficient in the superconducting Pr2Ba4Cu 7O15-δ is discussed on the basis of the double chain model from the density functional band calculation. © 2013 The Physical Society of Japan.

DOI： 10.7566/JPSJ.82.074706

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We investigate the one-dimensional Hubbard ring with attractive interaction in the presence of imbalanced spin populations by using the exact diagonalization method. The singlet pairing correlation function is found to show spatial oscillations with power-law decay as expected in the Fulde-Ferrell-Larkin-Ovchinnikov state of a Tomonaga-Luttinger liquid. In the strong coupling regime, the system shows an anomalous flux quantization of period h/4e, half of the superconducting flux quantum of h/2e, as recently predicted by mean-field analysis, together with various flux quanta smaller than h/4e. Notably, the observed flux quanta are determined by the difference between the system size N-L and electron number N-e as h/(N-L - N-e)e.

DOI： 10.7566/JPSJ.82.054707

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We investigate the electronic states and the superconductivity in the two-dimensional 16-band d-p model extracted from a tight-binding fit to the band structure of iron pnictides, in the presence of both the Coulomb interaction between Fe d-electrons and the electron-lattice coupling g with the orthorhombic mode which is crucial for reproducing the recently observed ultrasonic softening of the elastic constant C-66. Due to the cooperative effects of these interactions, the ferro-orbital order with different occupations of d(yz) and d(zx) orbitals occurs and induces the tetragonal-orthorhombic structural transition at T-s, together with the stripe-type antiferromagnetic (AFM) order below T-N. For a large g case, we obtain the phase diagram consistent with the doped iron pnictides with T-s > T-N for x > 0, where the s(++)-wave superconductivity is mediated by the ferro-orbital fluctuation which is largely enhanced near the ferro-orbital QCP at x(c) with T-s -> 0. On the other hand, for a small g case, the simultaneous phase transition occurs at T-s = T-N even for x > 0, where the s(+/-)-wave superconductivity is mediated by the AFM fluctuation. Both the s-wave states with full superconducting gaps are consistent with most of the experiments but only the former is considered to account for the small T-c-suppression against nonmagnetic impurities. (C) 2012 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ssc.2012.01.017

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigate the periodic Anderson model with k-dependent c-f mixing reproducing the point nodes of the hybridization gap by using the dynamical mean-field theory combined with the exact diagonalization method. At low temperature below a coherence temperature T-0, the imaginary part of the self-energy is found to be proportional to T-2 and the pseudogap with two characteristic energies (Delta) over tilde (1) and (Delta) over tilde (2) is clearly observed for T << T-0, while the pseudogap is smeared with increasing T and then disappears at high temperature T greater than or similar to T-0 due to the evolution of the imaginary self-energy. When the Coulomb interaction between f electrons U increases, (Delta) over tilde (1), (Delta) over tilde (2), and T-0 together with T-max at which the magnetic susceptibility is maximum decrease in proportion to the renormalization factor Z resulting in a heavy-fermion semiconductor with a large mass enhancement m*/m = Z(-1) for large U. We also examine the effect of the external magnetic field H and find that the magnetization M shows two metamagnetic anomalies H-1 and H-2 corresponding to (Delta) over tilde (1) and (Delta) over tilde (2) which are reduced due to the effect of H together with Z. Remarkably, Z(-1) is found to be largely enhanced due to H especially for H-1 less than or similar to H less than or similar to H-2, where the field induced heavy fermion state is realized. The obtained results seem to be consistent with the experimental results observed in the anisotropic Kondo semiconductors such as CeNiSn.

DOI： 10.1103/PhysRevB.85.165114

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Physical Society of Japan  

DOI： 10.11+RC43/JPSJ.81.044701

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We investigate the metamagnetism in the periodic Anderson model with the k-dependent c - f mixing by using the dynamical mean-field theory combined with the exact diagonalization method. It is found that both effects of the k-dependent c - f mixing and strong correlation due to the Coulomb interaction between f electrons are significant for determining both the magnetization and the mass enhancement factor. For the case away from the half-filling, the results is consistent with the metamagnetic behavior observed in CeRu2Si2

DOI： 10.1143/JPSJS.80SA.SA131

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Using the dynamical mean-field theory, we investigate the two-orbital periodic Anderson model including both the Coulomb interaction U between f-electrons and the electron-phonon interaction g which couple the local orbital fluctuations off-electrons with Jahn-Teller phonons. It is found that the heavy fermion state caused by U is largely enhanced due to the effect of g. In the heavy fermion state for large U and g, both the orbital and lattice fluctuations are enhanced, while the charge (valence) and spin fluctuations are suppressed; this is a nonmagnetic origin of the heavy fermions. The effect of the mass enhancement for the case with twofold-degenerate phonon mode is larger than that for one phonon mode. The obtained results show a possible nonmagnetic origin of the heavy fermion state recently observed in SmOs4Sb12.

DOI： 10.1143/JPSJS.80SA.SA138

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We investigate the heavy fermion states in the half-filled anharmonic Holstein model by using the dynamical mean-field theory combined with the exact diagonalization method at low temperatures down to 1/10(4) of the conduction bandwidth. For the weak anharmonic cases with single-well or shallow double-well potentials, the bipolaronic first order phase transition takes place at a critical value of the electron-phonon coupling below a critical temperature as recently observed in the harmonic Holstein model. On the other hand, for the strong anharmonic case with deep double-well potential, the bipolaronic transition is suppressed and changes to a crossover where the heavy fermion state with a large effective mass is realized due to the effect of largely anharmonic local oscillations called rattling.

DOI： 10.1143/JPSJS.80SA.SA136

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

An NQR experiment revealed superconductivity of Pr2Ba4Cu7O15-delta (Pr247) to be realized on CuO double chain layers and suggests possibility of novel one-dimensional (1D) superconductivity. To clarify the nature of the 1D superconductivity, we calculate the band dispersions of Pr247 by using the generalized gradient approximation (GGA). It indicates that Fermi surface of CuO double chains is well described to the electronic structure of a quasi-1D system. Assuming the zigzag Hubbard chain model to be an effective model of the system, we derive tight binding parameters of the model from a fit to the result of GGA. Based on the Tomonaga-Luttinger liquid theory, we estimate transition temperature (T-c) of the quasi-1D zigzag Hubbard model from the calculated value of the Luttinger liquid parameter K-rho. The result of T-c is consistent with that of experiments in Pr247 and it suggests that the mechanism of the superconductivity is well understood within the concept of the Tomonaga-Luttinger liquid.

DOI： 10.1143/JPSJ.80.024708

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We investigate the Hubbard-Holstein model by using the dynamical mean-field theory combined with the exact diagonalization method. Below a critical temperature T(cr), a coexistence of two solutions, small lattice fluctuating solution (polaronic state) and large one (bipolaronic state), is found for the same value of the electron-phonon coupling g in the range g(c1) < g < g(c2) for a fixed value of the Coulomb interaction U. As the result, the system shows a first-order phase transition at T(p)(<T(cr)) from the bipolaronic to the polaronic states as T decreases. The effective potential for oscillating atoms, which is renormalized due to the effect of g, is double-well type for T > T(p), while it is single-well type for T < T(p). The obtained transition for small U case with small T(p) and discontinuities in physical quantities seems to be account for the rattling transition observed in the beta-pyrochlore oxide KOs(2)O(6). (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2010.01.022

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The electronic state of the Fe(2)As(2) plane in iron-based superconductors is investigated on the basis of the two-dimensional 16-band d-p model. Using the random phase approximation for the on-site Coulomb interaction between Fe d electrons, we obtain the phase diagram including the magnetic ordered states and the superconductivity. It is found that the s(+/-)-wave superconductivity, where the gap functions have different signs between the electron pockets and the hole pockets, is realized near the incommensurate magnetic ordered phase with q similar to (pi, pi). The absolute values of the gap functions on the Fermi surfaces are almost isotropic but largely depend on the energy bands. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.10.109

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We investigate the 16-band d-p model for iron pnictide superconductors in the presence of the electron-phonon coupling g with the orthorhombic mode which is crucial for reproducing the recently observed ultrasonic softening. Within the RPA, we obtain the ferro-orbital order below T-Q which induces the tetragonal-orthorhombic structural transition at T-s = T-Q, together with the stripe-type antiferro-magnetic order below T-N. Near the phase transitions, the system shows the s(++)-wave superconductivity due to the orbital fluctuation for a large g case with T-Q > T-N, while the s(+/-)-wave due to the magnetic fluctuation for a small g case with T-Q <= T-N. The former case is consistent with the phase diagram of doped iron pnictides with T-s > T-N.

DOI： 10.1143/JPSJ.79.123707

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We investigate the Holstein model by using the dynamical mean-field theory combined with the exact diagonalization method. Below a critical temperature T-cr, a coexistence of the polaronic and the bipolaronic solutions is found for the same value of the electron-phonon coupling g in the range g(c1)(T) < g < g(c2)(T). In the coexistence region, the system shows a first order phase transition from the bipolaronic to the polaronic states as T decreases at T = T-p (< T-cr), where the double occupancy and the lattice fluctuation together with the anharmonicity of the effective ion potential change discontinuously without any symmetry breaking. The obtained bipolaronic transition seems to be consistent with the rattling transition in the beta-pyrochlore oxide KOs2O6.

DOI： 10.1143/JPSJ.79.093702

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the electronic states and the superconductivity in the two-dimensional 16-band d-p model coupled with A(1g), B-1g, and E-g local phonons and obtain the rich phase diagram including the magnetic-, charge-, and orbital-ordered phases on the parameter plane of the Coulomb and electron-phonon interactions. When the electron-phonon interaction is dominant, the charge fluctuations induce the s(++)-wave superconductivity, while when the Coulomb interaction is dominant, the magnetic fluctuations induce the s(+/-)-wave superconductivity. Remarkably, the orbital fluctuations are enhanced due to the cooperative effects of the Coulomb and electronphonon interactions and induce the s(++)-wave and the nodal s(+/-)-wave superconductivities.

DOI： 10.1103/PhysRevB.82.064518

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We investigate the two-orbital periodic Anderson model, where the local orbital fluctuations of f-electrons couple with a two-fold degenerate Jahn-Teller phonon, by using the dynamical mean-field theory. It is found that the heavy fermion state caused by the Coulomb interaction between f-electrons U is largely enhanced due to the electron-phonon coupling g, in contrast to the case with the single-orbital periodic Anderson model where the effects of U and g compete to each other. In the heavy fermion state for large U and g, both the orbital and lattice fluctuations are enhanced, while the charge (valence) and spin fluctuations are suppressed. In the strong coupling regime, a sharp soft phonon mode with a large spectral weight is observed for small U, while a broad soft phonon mode with a small spectral weight is observed for large U. The cooperative effect of U and g for half-filling with two f-electrons per atom n(f) = 2 is more pronounced than that for quarter-filling with n(f) = 1.

DOI： 10.1143/JPSJ.79.054707

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study superconductivity in the two-dimensional 16-band d-p model extracted from a tight-binding fit to the band structure of LaFeAsO, using the random phase approximation. When the intraorbital repulsion U is larger than the interorbital one U', an extended s-wave (s(+/-)-wave) pairing with sign reversal of order parameter is mediated by antiferromagnetic spin fluctuations, while when U < U' another kind of s-wave (s(++)-wave) pairing without sign reversal is mediated by ferro-orbital fluctuations. The s(++)-wave pairing is enhanced due to the electron-phonon coupling and then can be expanded over the realistic parameter region with U > U'.

DOI： 10.1103/PhysRevB.81.054518

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

The electronic state of the CoO2 plane in the layered cobalt oxide Na0.5CoO2 is investigated on the basis of the 11 band d-p model on a two dimensional triangular lattice, where the tight-binding parameters are determined so as to fit the LDA band structure and an effective one dimensional potential is added so as to include the effect of the Na order. We treat the Coulomb interaction on a Co site: the intra- and inter-orbital direct terms U and U', the exchange coupling J and the pair-transfer, within the Hartree-Fock approximation. It is found that the metallic antiferromagnetic order takes place for T-c2 < T < T-c1 and the coexistent state of the antiferromagnetic, orbital and charge orders takes place for T < T-c2 where the system becomes insulator. The obtained results are consistent with the successive phase transitions observed in Na0.5CoO2

DOI： 10.1088/1742-6596/200/1/012233

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

The electronic state around a single vacancy in silicon crystal is investigated by using the Green's function approach. The triply degenerate charge states are found to be widely extended and account for extremely large elastic softening at low temperature as observed in recent ultrasonic experiments. When we include the LS coupling As, on each Si atom, the 6-fold spin-orbital degeneracy for the V(+) state with the valence +1 and spin 1/2 splits into Gamma(7) doublet groundstates and Gamma(8) quartet excited states with a reduced excited energy of O(lambda(Si)/10). We also consider the effect of couplings between electrons and Jahn-Teller phonons in the dangling bonds within the second order perturbation and find that the groundstate becomes Gamma(8) quartet which is responsible for the magnetic-field suppression of the softening in B-doped silicon.

DOI： 10.1088/1742-6596/200/1/012228

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We investigate the electronic states and the transport properties of the CoO2 plane in NaxCoO2 on the basis of the two-dimensional triangular lattice 11-band d-p model by using the fluctuation exchange approximation, where we consider the Coulomb interaction between the t(2g) electrons on a Co site. It is found that all of the effective mass of quasiparticles, the resistivity, the thermoelectric power and the uniform spin susceptibility increase with increasing x for x > 0.6. This implies that the ferromagnetic fluctuations play significant roles on determining the electronic states. These results are qualitatively consistent with the experiments

DOI： 10.1088/1742-6596/200/1/012235

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We investigate the electronic states of a one-dimensional two-orbital Hubbard model with band splitting by the exact diagonalization method. The Luttinger liquid parameter K-rho is calculated to obtain superconducting (SC) phase diagram as a function of on-site interactions: the intra- and inter-orbital Coulomb U and U', the Hund coupling J, and the pair transfer J'. In this model, electron and hole Fermi pockets are produced when the Fermi level crosses both the upper and lower orbital bands. We find that the system shows two types of SC phases, the SC I for U > U' and the SC II for U < U', in the wide parameter region including both weak and strong correlation regimes. Pairing correlation functions indicate that the most dominant pairing for the SC I (SC II) is the intersite (on-site) intraorbital spin-singlet with (without) sign reversal of the order parameters between two Fermi pockets. The result of the SC I is consistent with the sign-reversing s-wave pairing that has recently been proposed for iron oxypnictide superconductors.

DOI： 10.1143/JPSJ.78.124706

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A pseudo honeycomb system Li2RuO3 exhibits a second-order-like transition at temperature T = T-c similar to 540 K to a low-T nonmagnetic phase with a significant lattice distortion forming Ru-Ru pairs. For this system, we have calculated the band structure, using the generalized gradient approximation (GGA) in both the high- and low-T phases, and found that the results of the calculation can naturally explain the insulating behavior observed in the low-T phase. The detailed characters of the Ru 4d t(2g) bands obtained by the tight-binding fit to the calculated dispersion Curves show clear evidence that the structural transition is driven by the formation of the Ru-Ru molecular orbit, as proposed in our previous experimental studies.

DOI： 10.1143/JPSJ.78.094706

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We investigate the electronic states around a single vacancy in silicon crystal by using the Green's function approach. The triply degenerate vacancy states within the band gap are found to be extended over a large distance similar to 20 angstrom from the vacancy site and contribute to the reciprocal temperature dependence of the quadrupole susceptibility resulting in the elastic softening at low temperture. The Curie constant of the quadrupole susceptibility for the trigonal mode (O-yz, O-zx, O-xy) is largely enhanced as compared to that for the tetragonal mode (O-2(0), O-2(2)). The obtained results are consistent with the recent ultrasonic experiments in silicon crystal down to 20 mK. We also calculate the dipole and octupole susceptibilities and find that the octupole susceptibilities are extremely enhannced for a specific mode.

DOI： 10.1143/JPSJ.78.054702

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To investigate the charge states of a silicon vacancy, we introduce a cluster model which includes both the Coulomb interaction U between electrons in the dangling bond and the coupling g between the electrons and Jahn-Teller phonons, and solve the model by using the numerical diagonalization method. It is found that, for U > 0 and g = 0, the ground state of the neutral charge state V-0 is spin singlet (S = 0) and orbital doublet. When g is varied for a finite U, the ground state changes to the orbital triplet state with S = 1 at an intermediate coupling g(c1), and finally changes to the orbital triplet state with S = 0 at a strong coupling g(c2). The obtained result for g > g(c2) is consistent with the low temperature elastic softening observed in non-doped crystalline silicon.

DOI： 10.1088/1742-6596/150/4/042233

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We investigate the electronic state of a CoO2 plane in the layered cobalt oxides NaxCoC2. using the 11 band d-p model on a two-dimensional triangular lattice, where the tight-binding parameters are determined so as to fit the LIDA band structure. Effects of the Coulomb interaction at a Co site: the intraand inter-orbital direct terms U and U', the exchange coupling J and the pair-transfer J'. are treated within the Hartree-Fock approximation. We also consider the effect of the Na order at x = 0.5, where Na ions form one-dimensional chains, by taking into account of an effective one-dimensional potential Delta epsilon(d). It is found that the one-dimensional Na order enhances the Fermi surface nesting and antiferromagnetism is caused which is suppressed due to the frustration effect in the case without the Na, order. Furthermore, we consider the effect of the Coulomb interaction between the nearest-neighbor Co sites V and find that a coexistence of the magnetic, charge and orbital ordered states takes place for V>V-c, where the system becomes insulator. (C) 2008 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2008.06.128

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Electronic states of the CoO2 plane in NaxCoO2 are investigated on the basis of the triangular lattice 11-band d-p model by using the second order perturbation (SOP) and the random phase approximation (RPA). Due to the effect of the in-plane ferromagnetic fluctuations enhanced for 0.6 less than or similar to x less than or similar to 0.75, the quasiparticle dispersion near the Gamma-point is strongly reduced to yield an almost flat dispersion just above the fermi level. With increasing x, the electronic specific heat coefficient gamma increases towards the critical doping x(c)similar to 0.75 above which the in-plane ferromagnetism takes place. We also find that the six small hole pockets near the K-point are stabilized due to the electron correlation effect within both the SOP and the RPA. (C) 2008 Published by Elsevier Ltd

DOI： 10.1016/j.jpcs.2008.06.078

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The electronic states of the Fe2As2 plane in iron-based superconductors are investigated oil the basis of the two-dimensional 16-band d-p model Which includes file Coulomb interaction oil a Fe site: the intra-and inter-orbital direct terms U and U', the Hand's coupling J and the pair-transfer J'. Using the random phase approximation (RPA), we obtain the magnetic phase diagram including the stripe and the incommensurate order on the U'-J plane. We also solve the superconducting gap equation within the RPA and find that, for large J, the most favorable pairing symmetry is extended s-wave whose order parameter changes its sign between the hole pockets and the electron pockets, while it is d(xy)-wave for small J.

DOI： 10.1143/JPSJ.77.123701

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We investigate the two-orbital Anderson lattice model which includes both the Coulomb interaction between f-electrons and the electron-phonon coupling between f-electrons and Jahn-Teller phonons, The model is solved by using the dynamical mean-field theory in conjunction with the exact diagonalization method. It is found that the heavy fermion state is realized due to the effect of cooperation between the Coulomb interaction and the electron-phonon coupling. Remarkably, the enhancement of lattice fluctuation and the phonon softening observed in the strong coupling regime are largely enhanced due to the Coulomb interaction in contrast to the case with the single-orbital Anderson lattice model. (c) 2007 Published by Elsevier B.V.

DOI： 10.1016/j.physb.2007.10.047

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Recently, Goto et al. have discovered an anomalous elastic softening due to the vacancy in the crystalline silicon at low temperature by using the ultrasonic measurements. To investigate the quantum state of the silicon vacancy, we introduce a cluster model which includes both the Coulomb interaction between electrons in the dangling bond and the coupling between the electrons and Jahn-Teller phonons. We solve the model by using the numerical diagonalization method and find that the V-0 state with neutral charge shows a transition from S=0 to S=1 in the strong coupling regime. Remarkably, the V+ state with S = 1/2 becomes stable due to the trigonal phonon while unstable due to the tetragonal phonon. The obtained result with the trigonal phonon is consistent with the elastic softening in the B-doped silicon.

DOI： 10.1143/JPSJS.77SA.266

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Physical Society of Japan  

The electronic states of the FeAs plane in iron-based superconductors are investigated on the basis of the two-dimensional 16-band d-p model, where the tight-binding parameters are determined so as to fit the band structure obtained by the density functional calculation for LaFeAsO. The model includes the Coulomb interaction on a Fe site: the intra- and inter-orbital direct terms U and U′, the exchange coupling J and the pair-transfer J′. Within the random phase approximation (RPA), we discuss the pairing symmetry of possible superconducting states including s-wave and d-wave pairing on the U′-J plane. ©2008 The Physical Society of Japan.

DOI： 10.1143/JPSJS.77SC.149

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Using the numerical diagonalization method, we examine the one-dimensional t(1)-t(2)-J(1)-J(2) model (zigzag-chain t-J model) which is an effective model for metallic CuO double chains in the superconductor Pr2Ba4Cu7O15-delta. Based on the Tomonaga-Luttinger liquid theory, we calculate the Luttinger liquid parameter K, as a function of the electron density n. It is found that superconductivity is realized in the parameter region, which is in accordance with experimental results. We show the phase diagram of a spin gap in the t(2)/vertical bar t(1)vertical bar-n plane by analyzing the expectation value of the twist operator Z(sigma) in the spin sector. The spin gap appears in the region with a large t(2)/vertical bar t(1)vertical bar, where the phase boundary at half filling is consistent with that of the known frustrated quantum spin system. The analysis also suggests that the estimated value of the spin gap reaches similar to 100K in the realistic parameter region of Pr2Ba4Cu7O15-delta.

DOI： 10.1143/JPSJ.76.113701

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We investigate the electronic state of the I I band d-p model on the two-dimensional triangular lattice simulating a CoO2 plane in the layered cobalt oxides such as NaxCoO2 and NaxCoO2 center dot yH(2)O. The tight-binding parameters are determined so as to fit the LDA band structure. Using the slave boson approach together with the 1/N expansion method, we obtain the renormalized quasiparticle bands, where the a(1g) band with a large hole Fermi surface is largely renormalized due to the strong correlation effect, while the e'(g) bands with six hole pockets are almost unchanged. We also discuss the metal-insulator transition (MIT) with varying the d-p charge transfer energy Delta, and find that the MIT takes place at a critical value Delta(c) = 4.01 eV. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.03.212

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The electronic state in a CoO2 plane of the layered cobalt oxides NaxCoO(2) is investigated by using the 11- band d - p model on a two-dimensional triangular lattice, where the tight- binding parameters are determined so as to fit the LDA band structure. The effects of the Coulomb interaction at a Co site ( the intra- and inter- orbital direct terms U and U-', the exchange coupling J and the pair- transfer J(')) are treated within the Hartree - Fock approximation. We also consider the effect of the Na order at x = 0.5, where Na ions form one- dimensional chains, by taking into account an effective one- dimensional potential Delta epsilon(d) on the CoO2 plane. It is found that the Na order enhances the Fermi surface nesting, resulting in the antiferromagnetism ( AFM) which is suppressed due to the frustration effect in the case without the Na order. When U and Delta epsilon d are varied, we observe three types of AFM: ( 1) metallic AFM with a large density of states N-F at the Fermi level for small values of U and Delta e(d), ( 2) semimetallic AFM with tiny N-F for large U with small Delta e(d), and ( 3) insulating AFM with a finite energy gap for large values of U and Delta e(d).

DOI： 10.1088/0953-8984/19/14/145289

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Using the numerical diagonalization method, we investigate superconductivity and related ferromagnetism in the one- dimensional two- orbital Hubbard model with a finite band splitting at less than half filling. We obtain a superconducting ( SC) region with the Luttinger liquid parameter K-rho > 1 and confirm anomalous flux quantization in the SC state. It is found that the SC phase appears near the partially polarized ferromagnetic phase. We also calculate various pairing correlation functions to clarify the nature of the SC phase. Detailed analysis of these functions indicates that the triplet pairing between the nearest neighbour sites is relevant to the superconductivity. This suggests that ferromagnetic fluctuation plays an important role for the superconductivity.

DOI： 10.1088/0953-8984/19/14/145284

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Using the numerical diagonalization method, we investigate the ferromagnetism and the superconductivity in the multi-orbital Hubbard model in one-dimension. To examine the superconductivity, we calculate the critical exponent K-p, based on the Luttinger liquid theory and the anomalous flux quantization in the superconducting region. Analyzing the value of K-p, we obtain the phase diagram on the U '-J parameter plane including the superconducting phase and the fully polarized ferromagnetic phase. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.267

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We propose a different combined approach, which consists of the exact diagonalization, renormalization group, and Bethe ansatz methods, for precise estimates of the charge gap Delta in the one-dimensional extended Hubbard model, with on-site and nearest-neighbor interactions U and V at quarter filling. This approach enables us to obtain the absolute value of Delta, including the prefactor, without ambiguity even in the critical regime of the metal-insulator transition (MIT) where Delta is exponentially small, beyond usual renormalization group methods and/or finite-size scaling approaches. The detailed results of Delta, down to the order of 10(-10) near the MIT, are shown as contour lines on the U-V plane.

DOI： 10.1103/PhysRevB.75.113103

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Recently, Goto et al. have discovered anomalous elastic softening due to vacancy in crystalline silicon at very low temperature. To investigate the quantum state of the silicon vacancy, we introduce a cluster model which includes the electron-lattice coupling between electrons and Jahn-Teller distortions together with the Coulomb interaction between electrons. By using the numerical diagonalization method, we take into account of both the nonadiabatic and the correlation effects. We find that the 3-fold orbital degeneracy in the ground state of a silicon vacancy is stable against the Jahn-Teller distortions because of the strong quantum fluctuation due to the nonadiabatic coupling between electrons and Jahn-Teller distortions. The obtained result is a striking contrast to the previous theoretical results within the adiabatic approximation and is consistent with the newly observed experiment. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1007/j.jmmm.2006.10.414

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We investigate the electronic state of the periodic Anderson-Holstein model, which includes both the electron-electron Coulomb interaction and the electron-phonon coupling, away from half filling. The model is solved by using the dynamical mean-field theory combined with the exact diagonalization method. The heavy fermion state with large effective mass due to the electron-phonon coupling is realized in the wide range of the f-electron number n(f), while that due to the Coulomb interaction is realized in the narrow range of n(f)similar to 1. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.257

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We investigate a heavy fermion behaviour and a deformation of an effective potential of the ions due to electron-phonon interaction for the periodic Anderson-Holstein model by using the dynamical mean-field theory combined with the exact diagonalization method. For the strong electron-phonon coupling, the system shows an anomalous heavy fermion behaviour with a large lattice fluctuation and an extreme phonon softening, and then an effective potential for the ions changes from a simple harmonic potential for the non-interacting case to a double-well potential for the strong electron-phonon coupling. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.098

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We investigate the two-orbital periodic Anderson model with f(2)-configuration by using the dynamical mean-field theory in which the effective impurity Anderson model is solved using the exact diagonalization method. The renormalization factor Z and the local moment < S-2 > are obtained as functions of the Coulomb interaction U, the exchange (Hund's rule) coupling J and the crystal-field splitting J. When the ionic ground state is triplet for Delta <Delta(c) = 3J, the heavy fermion state with the mass enhancement m*/m = Z(-1) >= 100 together with (S-2)similar to 2 is realized in the strong correlation regime for U >= W, where W is the conduction bandwidth. When the ionic ground state is singlet for Delta > Delta(c), m*/m similar to 1 and < S-2 >similar to 0 for any value of U. When the singlet ground state and the triplet excited state form a quasi-quartet for J the system shows a moderate mass enhancement m*/m similar to 10-100. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.224

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Recently, Matsukawa et al. discovered a new superconductor, Pr2Ba4Cu7O15-delta, in which metallic CuO double chains are responsible for the superconductivity. To investigate its superconductivity, we employ the d-p double chain model where the tight-binding parameters are determined so as to fit a band structure in the local density approximation (LDA). On the basis of the Tomonaga-Luttinger liquid theory, we obtain a phase diagram that includes the superconducting phase in the weak coupling limit. We also calculate the Luttinger liquid parameter K-p as a function of the electron density n using the Hartree-Fock approximation. With increasing n from quarter filling, K-p increases and then exceeds 1/2 when the superconducting correlation becomes most dominant. K-p has a maximum at an optimal density between quarter- and half-filling. These results are consistent with the experimental observation.

DOI： 10.1143/JPSJ.74.2885

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We investigate the periodic Anderson-Holstein model by using the dynamical mean-field theory combined with the exact diagonalization method. For the strong electron-phonon coupling g greater than or similar to g(c), the system shows an anomalous heavy-fermion behaviour which is accompanied by a large lattice fluctuation and an extreme phonon softening. We also calculate an effective potential for the ions and find that a simple harmonic potential for g g, changes into a double-well potential for g greater than or similar to g(c). The effective pairing interaction between the conduction electrons shows a maximum at g approximate to g(c) where the superconducting transition temperature is expected to be maximum. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2004.12.021

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We present the phase diagram and dynamical correlation functions for the Holstein-Hubbard model at half-filling and at zero temperature. The calculations are based on the dynamical mean field theory (DMFT). The effective impurity model is solved using exact diagonalization (ED) and the numerical renormalization group (NRG). Excluding long-range order, we find three different paramagnetic phases, metallic, bipolaronic and Mott insulating, depending on the Hubbard interaction U and the electron-phonon coupling g. We present the behaviour of the one-electron spectral functions and phonon spectra close to the metal-insulator transitions. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2005.01.230

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We examine the critical behavior near the metal-insulator transition (MIT) in the one-dimensional extended Hubbard model with the on-site and the nearest-neighbor interactions U and V at quarter filling using a combined method of the numerical diagonalization and the renormalization group (RG). The Luttinger-liquid parameter K-rho is calculated with the exact diagonalization for finite size systems and is substituted into the RG equation as an initial condition to obtain K-rho in the infinite size system. This approach also yields the charge gap Delta in the insulating state near the MIT. The results agree very well with the available exact results for U=infinity even in the critical regime of the MIT where the characteristic energy becomes exponentially small and the usual finite size scaling is not applicable. When the system approaches the MIT critical point V-->V-c for a fixed U, K-rho and Delta behave as \lnDelta\(-2)=c(Delta)(V/V-c-1) and (K-rho-1/4)(2)=c(K)(1-V/V-c), where the critical value V-c and the coefficients c(Delta) and c(K) are functions of U. These critical properties, which are known to be exact for U=infinity, are observed also for finite U case. We also observe the same critical behavior in the limit of the MIT critical point U-->U-c when U is varied for a fixed V.

DOI： 10.1103/PhysRevB.70.155102

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：E D P SCIENCES  

We investigate metal-insulator transitions in the half-filled Holstein-Hubbard model as a function of the on-site electron-electron interaction U and the electron-phonon coupling g. We use several different numerical methods to calculate the phase diagram, the results of which are in excellent agreement. When the electron-electron interaction U is dominant, the transition is to a Mott insulator; when the electron-phonon interaction dominates, the transition is to a localized bipolaronic state. In the former case, the transition is always found to be second order. This is in contrast to the transition to the bipolaronic state, which is clearly first order for larger values of U. We also present results for the quasiparticle weight and the double occupancy as functions of U and g.

DOI： 10.1209/epl/i2003-10228-6

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A novel nonlinear sigma model method is proposed for the two-dimensional J(1)-J(2) model, which is extended to include plaquette-type distortion. The nonlinear sigma model is properly derived without spoiling the original spin degrees of freedom. The method shows that a single disordered phase continuously extends from a frustrated uniform regime to an unfrustrated distorted regime. By the continuity and Oshikawa's commensurability condition, the disordered ground states for the uniform J(1)-J(2) model are plaquette states with fourfold degeneracy.

DOI： 10.1103/PhysRevLett.91.197202

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The multi-orbital Hubbard model in one dimension is studied using the numerical diagonalization method. Due to them effect of crystal-field splitting Delta, the fully polarized ferromagnetism, which is observed in the strong coupling regime, becomes unstable against the partially polarized ferromagnetism when Hund's rule coupling J is smaller than a certain critical value of order of Delta. In the vicinity of the partially polarized ferromagnetism, the orbital fluctuation develops due to the competition between Hund's rule coupling and crystal-field splitting. The superconducting phase with the Luttinger liquid parameter K-rho > 1 is observed for the singlet ground state in this region.

DOI： 10.1143/JPSJ.72.1847

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The Mott metal-insulator transition in the multi-band Hubbard model in infinite dimensions is studied by using the linearized dynamical mean-field theory. The critical interaction U-c is obtained analytically. For the symmetric case and for orbital degeneracy M we find U-c = (4M + 2)rootL(2) where L-2 is the second moment of the non-interacting density of states. We also derive an analytical expression for the discontinuity of the chemical potential mu(n) at the filling n = M for U > U-c. The findings are in good agreement with numerical results obtained from the exact diagonalization method. (C) 2003 Published by Elsevier Science B.V.

DOI： 10.1016/S0921-4526(02)02613-3

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We examine the one-dimensional (ID) d-p model in comparison with typical 1D models such as the 1D Hubbard model and the 1D t-J model using the numerical diagonalization method combined with the Luttinger liquid theory. We calculate the spin velocity nu(sigma), the charge velocity nu(rho) and the Luttinger liquid parameter K-rho for each model. Using these parameters, a relationship between the models is obtained unambiguously. We find that the d-p model can be described by the Hubbard model in the wide parameter region, while it can be described by the t-J model only in the strong coupling limit. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(02)02662-X

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

Mott metal-insulator transitions in an M-fold orbitally degenerate Hubbard model are studied by means of a generalization of the linearized dynamical mean-field theory. The method allows for an efficient and reliable determination of the critical interaction U-c for any integer filling n and different M at zero temperature. For half filling a linear dependence of U-c on M is found. Inclusion of the (full) Hund's rule exchange J results in a strong reduction of U-c. The transition turns out to change qualitatively from continuous for J=0 to discontinuous for any finite J. This result is confirmed using the exact-diagonalization method for M=2.

DOI： 10.1103/PhysRevB.67.035119

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Language：English   Publishing type：Research paper (other academic)   Publisher：The Physical Society of Japan  

We examine the 1D <italic>d</italic>–<italic>p</italic>model in comparison with a typical one-dimensional (1D) model such as the Hubbard model or the <italic>t</italic>–<italic>J</italic>model by the numerical diagonalization method combined with the Luttinger liquid theory. We calculate the Luttinger liquid parameter <italic>K</italic>_ρand the ratio of the spin velocity and the charge velocity <italic>v</italic>_s/<italic>v</italic>_cfor the three models. Analysis of these values gives the relationship between those models without ambiguity. We find that the <italic>d</italic>–<italic>p</italic>model is similar to not the <italic>t</italic>–<italic>J</italic>model but the Hubbard model for wide range of the parameters. Though the repulsion of the <italic>d</italic>–<italic>p</italic>model at Cu-sites is very strong, the Hubbard model corresponding to the <italic>d</italic>–<italic>p</italic>model has relatively weak repulsion for the small charge-transfer energy.

DOI： 10.1143/JPSJS.71S.353

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Other Link： http://hdl.handle.net/10191/26841

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

The metal-insulator (M-I) transition of the one-dimensional extended Hubbard model with both on-site U and nearest neighbor V interactions is studied by a combination of the numerical diagonalization and the renormalization group (RG) method. We diagonalize finite size systems numerically and calculate the Luttinger-liquid parameter K-p. Substituting the numerical result to the RG equations as the initial condition, we explicitly calculate the renormalized K-p and the renormalized umklapp scattering parameter G. This approach gives the critical properties of the K-p and the charge gap near the M-I transition beyond the usual finite-size scaling for the numerical diagonalization result. We also obtain a contour map of the charge gap by the numerical diagonalization method on the U-V plain. (C) 2002 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(02)00104-X

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We study a two-band Hubbard model using the dynamical mean-field theory combined with the exact diagonalization method. At the electron density n=2, a transition from a band insulator to a correlated semimetal occurs when the on-site Coulomb interaction U is varied for a fixed value of the charge-transfer energy A. At, low temperature, the correlated semimetal shows ferromagnetism or superconductivity. With increasing doping In 2', the ferromagnetic transition temperature rapidly decreases and finally bet owes zero at a critical value of n. The second-order phase transition occUTS at high temperature, while a phase separation of ferromagnetic and paramagnetic states takes place at low temperature. The superconducting transition temperature gradually decreases and finally becomes zero near a =3) where the system is Mott insulator which shows antiferromagnetism at low temperature.

DOI： 10.1143/JPSJS.71S.356

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The Mott metal-insulator transition in the two-band ifubbard model in infinite dimensions is studied by using the linearized dynamical mean-field theory. The discontinuity in the chemical potential for the change from hole to electron doping is calculated analytically as a function of the on-site Coulomb interaction U, and the charge-transfer energy Delta between the d- and p-orbitals, transfer integrals t(pd), t(pp), t(dd) between p-d, p-p and d-d sites respectively. The critical behaviour of the quasiparticle weight is also obtained analytically.

DOI： 10.1143/JPSJS.71S.217

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The Mott metal insulator transition in the two-band Hubbard model in infinite dimensions is studied by using the linearized dynamical mean-field theory. The discontinuity in the chemical potential for the change from hole to electron doping is calculated analytically as a function of the on-site Coulomb interaction U at the d-orbital and the charge-transfer energy Delta between the d-and p-orbitals. Critical behaviour of the quasiparticle weight is also obtained analytically as a function of U and Delta. The analytic results are in good agreement with the numerical results of the exact diagonalization method.

DOI： 10.1143/JPSJ.70.2989

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER-VERLAG  

We have studied the critical behaviour of a doped Mott insulator near the metal-insulator transition for the infinite-dimensional Hubbard model using a linearized form of dynamical mean-field theory. The discontinuity in the chemical potential in the change from hole to electron doping. for U larger than a critical value U-c, has been calculated analytically and is found to be in good agreement with the results of numerical methods. We have also derived analytic expressions for the compressibility, the quasiparticle weight. the double occupancy and the local spin susceptibility near half-filling as functions of the on-site Coulomb interaction and the doping.

DOI： 10.1007/PL00011147

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WORLD SCIENTIFIC PUBL CO PTE LTD  

In recent years the numerical renormalization group (NRG) method has been extended to the calculation of dynamic response functions and transport properties of magnetic impurity models. The approach can now be applied more widely to lattice models of strongly correlated electron systems by the use of dynamical mean field theory (DMFT), in which the lattice problem is transformed into one for an effective impurity with an additional self-consistency constraint. We review these developments and assess the potential for further applications of this approach. We also discuss an alternative approach to renormalization, renormalized perturbation theory, in which the leading asymptotically exact results for the low temperature regime for a number of magnetic impurity models can be obtained within finite order perturbation theory.

DOI： 10.1142/S0217979201006367

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The Mott metal-insulator transition in the two-band Hubbard model in infinite dimensions is studied by using the linearized dynamical mean-field theory recently developed by Bulla and Potthoff. The phase boundary of the metal-insulator transition is obtained analytically as a function of the on-site Coulomb interaction at the d-orbital, the charge-transfer energy between the d- and p-orbitals and the hopping integrals between p-d, d-d and p-p orbitals. The result is in good agreement with the numerical results obtained from the exact diagonalization method.

DOI： 10.1007/s100510170313

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Metal-insulator transition of the one-dimensional Hubbard model with both on-site U and nearest neighbor V interactions is studied by a combination of the numerical diagonalization method and the renormalization group (RG) method. Substituting the numerical result to the RG equation as the initial condition, we calculate the Luttinger-liquid parameter K-p at quarter tilling. Comparing the renormalized K-p with the known exact result, we find it to be reliable in the strong coupling regime. This approach gives an evident critical point of the metal-insulator transition beyond the usual finite-size scaling for the numerical diagonalization result. The charge gap is also estimated by the numerical diagonalization method in the insulator region. (C) 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(00)00146-3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Electronic states in the two-band Hubbard model with infinite dimensions are investigated by using the exact diagonalization method. In the half-filled case, the boundary separating the metallic and the insulating regimes is calculated as the function of the on-site repulsive interaction U and the charge-transfer energy Delta. The metal-insulator transition of Mott-Hubbard type is obtained for U &lt; &lt;Delta&gt; while that of charge-transfer type is obtained for U &gt; Delta. In the doped case, on-site paring susceptibility is calculated at finite temperature. The critical temperature of the superconductivity is found in a charge-transfer type but not in a Mott-Hubbard type. (C) 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(00)00147-5

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Electronic states in the single- and multi-channel two-band Hubbard models in infinite spatial dimensions are investigated by using the self-consistent 1/N-expansion method which is a conserving approximation based on the perturbation expansion from the large limit of the spin-orbital degeneracy N. It is shown that a metallic state in the single-channel model behaves as a Fermi liquid. where imaginary part of self-energy of quasi-particles is proportional to omega(2) + (pi T)(2). while a metallic stare in the multi-channel model behaves as a non-Fermi liquid, where the imaginary part of the self-energy is finite even at omega = T = 0. In each model with a half-filled band the Mott metal-insulator transition (MIT) is observed. when the charge transfer energy Delta is varied. When the system approaches the MIT point. effective mass of quasi-particles diverges in the single-channel model, while the total number of quasi-particles becomes zero in the multi-channel model. (C) 2000 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(99)00924-2

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We investigate electronic states in the M-channel N fold degenerate Anderson lattice with the help of 1/N-expansion. In the single-channel case, we showed that the imaginary part of the self-energy of the conduction electrons up to O(N-2) is exactly given by a Form of the Fermi-liquid-type. In the multichannel case, we find a T-linear term of O(N-1) in the imaginary part of the self-energy of the conduction electrons, in contrast to a constant term found in the infinite dimensions. Then we critically examine the results obtained in the infinite dimensions. (C) 2000 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(99)00998-9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOCIETY JAPAN  

We investigate electronic states in the multichannel degenerate Anderson lattice with the aid of the 1/N-expansion procedure. Inclusion of the self-energy in O(N-0) leads to heavy electrons with degeneracy of channel and spin-orbit. In O(N-1); we find a T-linear term in the imaginary part of the self-energy of the conduction electrons at the Fermi level, in contrast to a T-2-term as in the Fermi liquid or a constant term as in the non-Fermi liquid found in infinite dimensions. The disappearance of the imaginary Dart at T = O is due to the intersite correlation effects that reflect the translational invariance. The Luttinger sum rule is proved to hold.

DOI： 10.1143/JPSJ.68.2491

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We investigate the Fermi liquid in the infinite-U degenerate Anderson lattice using expansion from the limit of large spin-orbital degeneracy N. By collecting all diagrams up to O(N-2), we show that the imaginary part of the self-energy of the conduction electrons is given by a form proportional to (omega - mu)(2) + pi(2)T(2). We also show that the Luttlinger sum rule holds up to O(N-1). This implies that the literal 1/N-expansion procedure correctly describes the Fermi liquid in the U-infinite Anderson lattice.

DOI： 10.1143/JPSJ.68.1067

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We investigate the one-dimensional d-p model with the transfer energy t(pp) between the nearest neighbor p sites by the numerical diagonalization method. By calculating the critical exponent K-rho based on the Luttinger liquid relations and the paring correlation functions, we analyze the superconducting state induced by the effect of the large repulsion U-d at the Cu site in cooperation with the transfer energy t(pp). We obtain a phase diagram of the superconducting region on the filling n vs. U-d parameter plane. It shows that the superconducting region appears when U-d is larger than a certain critical value which decreases with increasing n. The results of paring correlation functions suggest that a singlet paring between the nearest neighbor p sites is most relevant to the superconductivity. We also discuss a relationship between the d-p model and the t-J model through an analysis of the dependence of K-rho on the parameters.

DOI： 10.1143/JPSJ.67.4151

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Metamagnetism of heavy fermions is investigated using the Anderson lattice model in which the anisotropy of the hybridization matrix element, together with electron-lattice coupling, is taken into account. Within the leading-order approximation in the 1/N-expansion, it is found that the incoherent part of the f electron becomes relevant above a critical field H-M, while it is irrelevant below H-M. Consequently, both magnetization and magnetostriction show abrupt increases with increasing H at H-M. The results are in good agreement with the experimental data of CeRu2Si2.

DOI： 10.1143/JPSJ.67.2197

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

In the previous studies, we examined roles of infrared divergences in pseudo-particle spectra in the impurity Anderson models with use of the expansion from the limit of the large spin-orbital degeneracy (1/N-expansion). Here, such studies are extended to obtain renormalized forms of frequency-/temperature-dependence of physical quantities such as scattering rate of conduction electrons and spin-, pseudo-spin- and charge-susceptibility? where the pseudo-spin represents the channel's degree of freedom, hy collecting the most infrared-divergent terms in the 1/N-expansion. The results reconfirm that in single channel models the infrared divergent terms in the pseudo-particle spectra are completely cancelled hy the vertex corrections at each order of the 1/N-expansion, while in multichannel models the cancelation becomes incomplete and the terms remain in the physical quantities.

DOI： 10.1143/JPSJ.67.2346

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Dynamical spin susceptibility chi(s)(q, omega) of the d-p model in the over doping region is investigated by using the auxiliary boson technique. It includes higher order terms of the 1/N-expansion within the random phase approximation (RPA) of the local vertex, where frequency dependence of the quasi-particle interaction is taken into account. The incommensurate spin fluctuation is obtained due to the nesting effect in the low energy region (omega less than or similar to omega*), whereas the commensurate one in the high energy region (omega greater than or similar to omega*), the characteristic energy omega* is estimated to be about 30 meV. Both of the spin-lattice relaxation rate 1/T-l and the spin-spin relaxation rate 1/T-g monotonically increase as T decreases, while the spin Knight shift K is almost independent of T. (C) 1998 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(98)00131-2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Using the numerical diagonalization method, we investigate the one-dimensional d-p model, simulating a Cu-O linear chain with strong Coulomb repulsions. Considering the effect of the transfer energy t(pp) between the nearest neighbor oxygen-sites, we calculate the critical exponent of the correlation functions K-p based on the Luttinger liquid relations and the ground state energy E-0(phi) as a function of an external flux phi. We find that the transfer t(pp) increases the charge susceptibility and the exponent K-rho in cooperation with the repulsion U-d at the Cu site. We also show that anomalous flux quantization occurs for K-rho &gt; 1. The superconducting region is presented on a phase diagram of U-d vs. t(pp) plane.

DOI： 10.1143/JPSJ.67.389

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Effects of d-site substitution in the d-p model simulating a CuO2-plane in high-T-c cuprates are investigated using the slave boson technique and the 1/N-expansion method. It is found that nonmagnetic impurities, e.g., Zn, substituted for Cu, strongly scatter the p-holes and drastically decrease hole densities at the neighboring O-sites. In the low doping region; the decrease reduces the Kondo temperature T-K at the neighboring Cu-sites. The repulsion U-pd between holes on the nearest neighbor Cu-and O-sites; which is known to enhance the charge susceptibility in the pure systems without impurities, turns out to promote the reduction of Tic due to the nonmagnetic impurity. Then, Cu d-spins in the vicinity of the impurity behave as local moments at temperatures higher than the reduced T-K.

DOI： 10.1143/JPSJ.67.29

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

As a straightforward extention of the study on infrared divergences in pseudo-particle spectra in the single-channel degenerate impurity Anderson model, those of the multichannel model are examined with use of the expansion from the large limit of the spin-orbital degeneracy N. Analysis with the most divergent terms in the expansion shows that exponents of spectra of the. slave boson and the pseudo-fermion in the M-channel model with 1 less than or equal to M much less than N are given respectively by 1 - n(f)(2)M/N and (2n(f)-n(f)(2))M/N, where n(f) is the average number of localized electrons at the impurity site. When M = 1, the results recover those obtained earlier in the single channel model. In the limits, n(f) --&gt; 1 and M-2 much less than 1, these results tend to those obtained by the non-crossing approximation (NCA) study. It also shows that the scattering rate of the conduction electrons in the M-channel model includes infrared divergent terms proportional to (1 - n(f))(1 - M-2) in contrast to the absence of infrared divergent terms in the single-channel model.

DOI： 10.1143/JPSJ.66.3528

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We investigate magnetization as a function of external magnetic field H in the U-infinite Anderson lattice model within the leading-order approximation in the 1/N-expansion. At T = 0, at H = H-M where the Zeeman energy is equal to a certain characteristic energy of the system, the magnetization curve has a kink and the differential susceptibility dM/dH shows a jump. At finite temperature, dM/dH shows a peak around H-M. Its maximum value increases with decreasing T and saturates to a finite value at T --&gt; 0. When H &lt; H-M, the f and conduction electrons form renormalized bands with a large Fermi surface determined by the Luttinger sum On the other hand, when H &gt; H-M, the bands reform such that they are free from the Luttinger sum rule, eventually leading to a small Fermi surface at H &gt;&gt; H-M. These results are consistent with the metamagnetic properties observed in the heavy fermion compound CeRu2Si2.

DOI： 10.1143/JPSJ.65.19

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Effects of a single impurity in the degenerate lattice Anderson model describing heavy elecron systems are investigated rigorously within the framework of the axiliary boson method and the perturbation expansion from the large limit of the spin-orbital degeneracy N(1/N-expansion). It is found that a nan-magnetic impurity leads to the unitarity limit value of the scattering rate of the conduction electrons at low temperature while it has no appreciable effect on the thermodynamic properties. These results are consistent with the experimental observations on the heavy electron systems such as Ce1-xLaxCu6.

DOI： 10.1143/JPSJ.64.3826

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The properties of the charge fluctuation are investigated in the d-p model with the repulsion U-pd between holes on the nearest-neighbor Cu and O sites and the infinite on-site repulsion U-d at the Cu site. We calculate the charge susceptibility chi(c)(q, i omega(n)) and the charge correlation function S-c(q)=T Sigma(omega n)chi(c)(q, i omega(n)). It is found that S-c(q) has a peak at the Gamma point and a maximum in a ring around the Gamma point. The former is due to T chi(c)(q, 0). Its intensity is proportional to temperature T and strongly enhanced by U-pd. The latter is due to T Sigma(omega n not equal 0)chi(c)(q, i omega(n)) and shows a weak T and U-pd dependence. The intensity of the diffuse X-ray scattering on taking the charge fluctuation into account is also calculated. The result is consistent with the experiments in La2-deltaSrdeltaCuO4.

DOI： 10.1016/0921-4534(95)00360-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We investigate finite temperature properties of the U-d-infinite d-p model in the auxiliary boson representation within the leading order approximation in the 1/N-expansion. We obtain a unified description of the system valid over the whole temperature range divided by the two characteristic energies, the binding energy of the resonance state of the auxiliary boson, omega(0), and the coherence temperature, T-0, of O (omega(0)/10). At T much less than T-0, a new coherent band (in-gap band) appears inside the charge-transfer gap upon carrier doping and behaves as Fermi liquid with large Fermi surface determined by the Luttinger sum rule. The band width is an increasing function of carrier doping rate and of O (omega(0)). At T &gt; omega(0), the chemical potential is below the in-gap band and the doped holes are mostly on the O-2p band forming small Fermi surface, while the Cu-3d holes behave like spin-1/2 localized moments. At T-0 less than or equal to T less than or equal to omega(0), crossover phenomena between the two limiting cases are observed. The in-gap band becomes quasi-coherent and the inverse life-time due to the incoherent scattering increases linearly with T. The observed crossover phenomena show significant contrast to the distinct phase transition obtained in the mean field theory.

DOI： 10.1143/JPSJ.64.1595

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We develop an approximation procedure to calculate the optical conductivity sigma(omega) and the electronic Raman scattering intensity in the d-p model by including terms of the leading order in the expansion from the large limit of the spin-orbital degeneracy N at T less than or similar to T-0, where the well-defined quasi-particle states called in-gap states emerge inside the charge-transfer gap upon hole doping to the half-filled case. Here, T-0 is the coherence temperature of 0.2 omega(0), where omega(0) is a half of the band width of the in-gap states at T = 0 and is proportional to the doped hole concentration delta at delta less than or similar to 0.1. The in-gap states are shown to contribute to the Drude components in sigma(omega) with an intensity also proportional to delta at delta less than or similar to 0.1, as has been observed in experiments. Both the omega and the T dependence in sigma(omega) are found to be scaled well by omega(0). The generalized Drude analysis (GDA) of sigma(omega) shows that both the optical mass m(opt)(omega) and the optical scattering rate tau(opt)(*)(omega(-1) are only weakly omega dependent in contradiction to the experimental observations, which seems to suggest that the in-gap states at T less than or similar to T-0 do not account for the anomalous optical properties of high-T-c superconductors. We show that the omega dependence of the quasi-particle scattering rate plays a crucial role for determining the omega dependence of the optical mass m(opt)(omega) and comment on the confusion regarding GDA seen among physicists.

DOI： 10.1016/0921-4534(95)00097-6

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Using the numerical diagonalization method and Luttinger liquid theory, we investigate the one-dimensional d-p model, simulating a Cu-O linear chain with strong Coulomb repulsion. In our previous work [Physica C 205 (1993) 170], we have found that the superconducting correlation is dominant in the existence both of a large on-site repulsion at the Cu-site, U-d, and an inter-site repulsion, U-pd, between the nearest-neighbor Cu- and O-sites. In this paper, to clarify the superconducting phase in more detail, we calculate various types of pairing correlation functions. The results indicate that the pairing on O-sites dominates over the pairing on Cu-sites and on the nearest-neighbor Cu- and O-sites. We also investigate the effect of the on-site repulsion at the O-site, U-p, on the pairing correlation and a critical exponent of correlation functions. These results suggest that the superconducting phase promoted by the repulsion U-pd remains against U-p.

DOI： 10.1016/0921-4534(94)02314-X

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

DOI： 10.1103/PhysRevB.51.1175

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We investigate transport phenomena in the normal state of the simplest d-p model using the auxiliary boson technique together with the 1/N expansion method. Analysis of the optical conductivity with the generalized Drude formula shows that omega-dependence of the optical lifetime tau(opt)(omega) is like non-Fermi liquid and that the optical mass m(opt)*(omega) is of order of the band mass m(b) and almost independent of omega and the doped hole concentration delta.

DOI： 10.1016/0921-4534(94)92368-X

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We investigated the one-dimensional extended Hubbard model with the on-site interaction U and the nearest-neighbor interaction V by using the numerical diagonalization method. Assuming the Luttinger liquid relations, we obtained the phase diagram of the ground state on the U-V plane. In the weak coupling region, the phase diagram agrees with the result of geology. On the other hand, in the strong coupling region for V much greater than \t\, a novel superconducting phase appears both for positive and negative U. We also investigated the charge gap and the spin gap. The results suggest that the superconducting phase extends over the Tomonaga-Luttinger region (without spin gap) and the Luther-Emery region (with spin gap).

DOI： 10.1143/JPSJ.63.1250

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Properties of the heavy-fermion valence-fluctuating compounds leading to insulating ground states (called Kondo insulators) are investigated theoretically by using the U-infinite degenerate lattice Anderson model in the auxiliary boson representation. The model is treated within the approximation including terms of the leading order in the expansion w.r.t. inverse of the spin-orbit degeneracy (1/N-expansion) under the strict local constraints guaranteeing equivalence of the bosonic version to the original U-infinite model. The results are compared with the experimental observations on the possible candidates for Kondo insulators. Differences between Kondo insulators and charge transfer type insulators now paid much attention as the mother systems for the high-T(c) superconductors are briefly discussed on a unified basis within the model described above.

DOI： 10.1143/JPSJ.63.1406

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Organic conductors, DCNQI-Cu salts, with a mixed valence Cu+4/3 on the average has been studied theoretically by applying the slave-boson method to the strongly correlated systems which consist of pi-electrons of DCNQI molecule and d-electrons of Cu atoms. The ground state is calculated by use of the 1/N(sigma) expansion method which treats the local constraint strictly. It is shown that the electron mass around the Fermi surface is enhanced in the presence of the charge density with the three-fold periodicity.

DOI： 10.1143/JPSJ.62.3244

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The one-dimensional two-band Hubbard model, simulating a Cu-O linear chain with infinite intra-site Coulomb repulsion among holes at the Cu-sites, is solved exactly by using the numerical diagonalization method. It is confirmed that inter-site Coulomb repulsion between holes at the nearest-neighbor Cu- and 0-sites enhances the charge fluctuation as well as the superconducting fluctuations, and when it exceeds a certain critical value the system becomes unstable towards the phase separation. By assuming the Luttinger liquid relations to hold in the present system, the critical exponents in the superconducting, CDW and SDW correlation functions are also calculated. The results suggest that in the proximity of the phase boundary towards the phase separation, the superconducting correlation is dominant compared with the CDW and SDW correlations.

DOI： 10.1016/0921-4534(93)90184-R

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We investigate effects of the repulsive interaction U(pd) between holes on the nearest-neighbor Cu- and O-sites in the d-p model with infinite repulsion on the Cu site using the auxiliary boson technique and the 1/N expansion method. It is found that the condition for a metal-insulator (M-1) transition to occur at the half-filled case is modified. We then investigate the charge susceptibility; it is found that the thermodynamic charge susceptibility is enhanced by U(pd) except for the region around the critical point of the M-1 transition. It is found to diverge at a certain value of U(pd). which implies an instability toward a phase separation. On the basis of these calculations, we obtain phase diagrams, which consist of the metallic, unstable (phase-separating) and insulating phases. Furthermore, we calculate the quasiparticle interaction and find that the charge fluctuation induces attractive interaction comparable in magnitude to the repulsive interaction originating from the on-site repulsion U(d) in both of the s- and d-wave channel; in the immediate vicinity of the instability, the attraction can overcome the repulsion.

DOI： 10.1143/JPSJ.61.649

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Elsevier  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Springer, Berlin, Heidelberg  

DOI： 10.1007/978-3-642-77154-5_23

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE  

The most important results obtained through a series of the present authors' studies on the bosonic versions of the infinite- U impurity- and lattice-Anderson models with use of the rigorous expansion with respect to inverse power of the spin-orbit degeneracy N (1/N-expansion) are reviewed and their physical implications are discussed in comparison with the results obtained in the other 1/N-expansion studies as well as their relating experimental observations.

DOI： 10.1143/PTPS.108.173

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The infinite-U Anderson lattice model is studied by using the 1/N-expansion method with taking account of the local electronic correlation rigorously. In order to understand T-dependence of properties of the system properly, another characteristic energy originated from the quasi-particle band structure is found to be introduced in addition to the coherence temperature and the Kondo temperature.

DOI： 10.1016/0921-4534(91)90961-W

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Electronic state of the d-p hybridization model with infinite repulsion on the Cu site is investigated with the help of the auxilliary boson technique and the 1/N expansion method. In particular, effects of the repulsion U(pd) between the holes on the nearest-neighbor Cu and O sites are clarified; it is shown that the charge fluctuation is enhanced by U(pd). Possible relevance to superconductivity is discussed.

DOI： 10.1016/0921-4534(91)90883-Z

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOCIETY JAPAN KIKAI-SHINKO BUILDING  

Electronic states in the d-p model, particularly those around the half-filled case, are investigated with the help of the auxiliary boson method and the 1/N-expansion. The critical value of the charge transfer energy for a metal-insulator transition to occur at the half-filled case is evaluated. The analytic expression for the charge susceptibility is given. Then characteristics of the charge fluctuation around the transition point are clarified; it is found that the charge susceptibility vanishes on the critical point. Furthermore, dependence of the charge susceptibility on the hole number and on the charge transfer energy is studied. Relevance to high temperature superconducting materials is then discussed.

DOI： 10.1143/JPSJ.60.2269

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOCIETY JAPAN  

We investigate effects of the repulsive interaction U(pd) between holes on the nearest-neighbor Cu- and O-sites in the d-p model, a model suitable to describe the CuO2 plane in high-temperature superconductors. It is found that the condition for a metal-insulator (M-I) transition to occur at the half-filled case is modified. Furthermore, the charge susceptibility is found to be enhanced by U(pd), except for the region around the critical point of the M-I transition; it indeed diverges at a certain value of U(pd). Possible relevance of the enhanced charge susceptibility to the mechanism of the high T(c) superconductivity is discussed.

DOI： 10.1143/JPSJ.60.1864

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Language：English   Publishing type：Doctoral thesis  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOCIETY JAPAN KIKAI-SHINKO BUILDING  

DOI： 10.1143/JPSJ.60.1142

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE  

The d-p model with the local constraint has been studied. With help of the slave boson and 1/N-expansion the electronic states are investigated. At T &lt; T0 (coherence temperature) a new band (mid-gap state) appears in the CT gap which behaves as the Fermi liquid with the large Fermi surface. Luttinger sum rule is proved. At T0 less-than-or-similar-to T less-than-or-similar-to omega(0) (resonance temperature) the mid-gap state behaves as the marginal Fermi liquid. At T greater-than-or-similar-to omega(0) such renormalized band collapses. These occur as the crossover phenomena in contrast to the phase transition in the mean field approximation. The anomalous physical properties are observed in the temperature region of the marginal Fermi liquid. The system is metallic or insulating at non-doping limit when DELTABAR = DELTA/2t(pd) &lt; 2.37 or DELTABAR &gt; 2.37. For DELTABAR = 2.37, the charge susceptibility becomes zero, which is a characteristic feature of the strongly correlated system. U(pd) changes the critical value of DELTABAR, enhances the charge fluctuation and causes the phase separation. The interactions mediated by the enhanced charge fluctuation may derive the superconductivity of the cuprate compounds.

DOI： 10.1143/PTP.106.51

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Physical Society of Japan  

Earlier we proposed a modified version of the Anderson lattice model where the in-tersite coherence effects can be properly included even in the large limit of the spin-orbital degeneracy N and investigated the low temperature Fermi liquid properties rigorously in the limit N→ ∞. In the present study, using the same model as above, we investigate properties over the whole temperature range through explicit numerical calculations for 7V=6. We find that a coherence temperature T0 of order of one tenth of the Kondo temperature TK exists
at lower temperature T≲T0 the system is in the Fermi liquid state, while at higher temperature T≫ T0 the electrons are scattered incoherently by the Kondo impurities. At intermediate temperature, with increasing T, the hybridization gap appeared in the Fermi liquid state decreases rapidly at T≳ T0, then smoothly turns into the pseudo gap at T⋍ TK, and finally the dense Kondo regime is realized at T. TK. © 1991, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.60.3475

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

DOI： 10.1016/0921-4534(89)90162-7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

DOI： 10.1016/0921-4534(89)90163-9

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPRINGER-VERLAG BERLIN  

DOI： 10.1007/978-3-642-83836-1_17

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