arXiv subjects

Hidetoshi Fukuyama

Publications and source records attributed to Hidetoshi Fukuyama.

At least 55 records · Page 3Linked to original sources

arXiv1999

Interplay between Structure and Electronic Properties in Organic Conductors

It is known that the ground states of organic conductors have a diversity reflecting the spatial arrangement of the constituent molecules within the unit cell. A systematic theoretical search for the unifying view behind such possible ground states has been made based on the Hartree-Fock mean field approximation not only to the on-site but also to intersite Coulomb interactions with special emphasis on the families of typical organic conductors (TMTCF)$_2X$ and (ET)$_2X$.

cond-mat.str-el↗

arXiv1999

Low-Temperature Dephasing in Disordered Conductors: the Effect of ``1/f'' Fluctuations

Electronic quantum effects in disordered conductors are controlled by the dephasing rate of conduction electrons. This rate is expected to vanish with the temperature. We consider the very intriguing recently reported apparent saturation of this dephasing rate in several systems at very low temperatures. We show that the ``standard model'' of a conductor with static defects can {\em not} have such an effect. However, allowing some dynamics of the defects may produce it.

cond-mat.mes-hall↗

arXiv1999

Dephasing Effects by Ferromagnetic Boundary on Resistivity in Disordered Metallic Layer

The resistivity of disordered metallic layer sandwiched by two ferromagnetic layers at low-temperature is investigated theoretically. It is shown that the magnetic field acting at the interface does not affect the classical Boltzmann resistivity but causes a dephasing among electrons in the presence of the spin-orbit interaction, suppressing the anti-localization due to the spin-orbit interaction. The dephasing turns out to be stronger in the case where the magnetization of the two layers is parallel, contributing to a positive magnetoresistance close to a switching field at low temperature.

cond-mat.mes-hall↗

arXiv1999

Effects of Disorder on the Competition between Antiferromagnetism and Superconductivity

Motivated by the observation of unusual magnetism in Ce_xCu_2Si_2 ($x\sim 1$), we study the effect of disorder, such as Ce vacancy, on the competition between superconductivity (SC) and antiferromagnetism (AF) on the basis of the phenomenological Ginzburg-Landau theory. Assuming that the AF-SC transition is of first order in clean system, we show that a single impurity in the SC state can induce staggered magnetization by suppressing the SC around it. For finite concentration of impurities, the first-order AF-SC boundary in the clean case is replaced by a finite region where the SC and the induced AF moments coexist microscopically with spatially varying order parameters. We argue that spin excitation spectrum in the coexistent state has a dual structure of SC gapped mode and the low-energy spin-wave mode. In accordance with the emergence of AF out of SC ground state, the spectral weight will be transferred from the former mode to the latter, keeping the structure of both modes basically unchanged.

cond-mat.str-el↗

arXiv1998

Strongly Correlated Electrons and Neutron Scattering

Various aspects of close and interesting relationship between antiferromagnetism and singlet ground states are introduced for which neutron scattering have been playing vital roles. The special emphasis is on the disorder-induced antiferromagnetism in spin-Peierls systems, which can be viewed as a nucleation process of classical magnetic order in the background of singlet state whose origin is purely quantum-mechanical. It is then pointed out that similar features will be found in Ce_xCu_2Si_2 and high T_c cuprates. Finally the possible charge ordering process in NaV_2O_5 is discussed which leads to the quenching of localized spins.

cond-mat.str-el↗

arXiv1998

Effects of Charge Density Modulation on Incommensurate Antiferromagnetism: Ginzburg-Landau Study

In the vicinity of hole density, 1/8, La-based high-$T_{c}$ superconductors exhibit long-range order of incommensurate antiferromagnetism (IC-AF). Motivated by this observation, we explore the possible stabilization of IC-AF ordering due to the existence of static charge density modulation (CDM). We use Ginzburg-Landau free energy based on the mean field theory of the t-J model. It is shown numerically that three kinds of CDM including a stripe pattern can stabilize static IC-AF ordering. We also argue that such CDMs can cause the saturation of the degree of incommensurability ($η$) as a function of hole density for IC-AF fluctuation. In our framework a principal effect of low-temperature tetragonal (LTT) structure is just to increase $η$ of IC-AF slightly larger only for the stripe pattern.

cond-mat.supr-con↗

arXiv1998

Charge Ordering and Spin gap in NaV_2O_5

A possible ground state of NaV_2O_5 is proposed based on the Hartree approximation for both on-site and intersite Coulomb interactions. The results indicate that the intersite Coulomb interaction induces a zigzag type of charge disproportionation (i.e. charge ordering) along the ladders of V-ions resulting in the localized spins between neighboring ladders to form a spin gap. This new state, which is different from the spin-Peierls state so far believed, seems to be consistent with the existing experimental results.

cond-mat.str-el↗

arXiv1998

On the Spin Gap Phase in lambda-(BETS)_2GaX_zY_{4-z}

The nature of the insulating ground state in quasi-two-dimensional organic conductor lambda-(BETS)_2GaX_zY_{4-z}, where the existence of a spin gap is suggested by susceptibility measurement, has been studied theoretically. Hartree-Fock calculations at absolute zero temperature show that if the on-site Coulomb interaction exceeds some critical value, then antiferromagnetic spin ordering emerges and eventually leads to an insulating state which can be considered as a two-dimensional localized spin system. Based on the quantum Monte Carlo simulations of Katoh and Imada, we will show that this system locates near the boundary between the antiferromagnetic phase and the spin gap phase, so that experimental facts can be explained.

cond-mat.str-el↗

arXiv1998

Frustrated Spin System in theta-(BEDT-TTF)_2RbZn(SCN)_4

The origin of the spin gap behavior in the low-temperature dimerized phase of theta-(BEDT-TTF)_2RbZn(SCN)_4 has been theoretically studied based on the Hartree-Fock approximation for the on-site Coulomb interaction at absolute zero. Calculations show that, in the parameter region considered to be relevant to this compound, antiferromagnetic ordering is stabilized between dimers consisting of pairs of molecules coupled with the largest transfer integral. Based on this result an effective localized spin 1/2 model is constructed which indicates the existence of the frustration among spins. This frustration may result in the formation of spin gap.

cond-mat.str-el↗

arXiv1998

Backward Scattering and Coexistent State in Two-Dimensional Electron System

The results of the mean field studies on the effects of the backward scattering with large momentum transfer in a two-dimensional electron system are extended to the case with various types of the Fermi surface and coupling constants. It is found that the coexistent state of d-wave superconductivity, antiferromagnetism and $π$-triplet pair can be stabilized quite generally near half filling.

cond-mat.str-el↗

arXiv1998

Weak Field Magnetoresistance in Quasi-One-Dimensional Systems

Theoretical studies are presented on weak localization effects and magnetoresistance in quasi-one-dimensional systems with open Fermi surfaces. Based on the Wigner representation, the magnetoresistance in the region of weak field has been studied for five possible configurations of current and field with respect to the one-dimensional axis. It has been indicated that the anisotropy and its temperature dependences of the magnetoresistance will give information on the degree of one-dimensionality and the phase relaxation time.

cond-mat.dis-nn↗

arXiv1998

Effects of Umklapp Scattering on Electronic States in One Dimension

The effects of Umklapp scattering on electronic states are studied in one spatial dimension at absolute zero. The model is basically the Hubbard model, where parameters characterizing the normal ($U$) and Umklapp ($V$) scattering are treated independently. The density of states is calculated in the t-matrix approximation by taking only the forward and Umklapp scattering into account. It is found that the Umklapp scattering causes the global splitting of the density of states. In the presence of sufficiently strong Umklapp scattering, a pole in the t-matrix appears in the upper half plane, signalling an instability towards the '$G/2-$pairing' ordered state ($G$ is the reciprocal lattice vector), whose consequences are studied in the mean field approximation. It turns out that this ordered state coexists with spin-density-wave state and also brings about Cooper-pairs. A phase diagram is determined in the plane of $V$ and electron filling $n$.

cond-mat.str-el↗

arXiv1998

Effect of Local Inhomogeneity on Nucleation; Case of Charge Density Wave Depinning

The spatial inhomogeneities are expected to affect nucleation process in an essential way. These effects are studied theoretically by considering the case of the depinning of the charge density wave as a typical example. The threshold field of the depinning of the one-dimensional commensurate charge density wave with one impurity has been examined classically based on the phase Hamiltonian at absolute zero. It is found that the threshold field is lowered by a finite amount compared to that in the absence of an impurity.

cond-mat.dis-nn↗

arXiv1998

Effects of the Backward Scattering in Two-Dimensional Electron System

Effects of the backward scattering with large momentum transfer are examined in two-dimensional electron system with a special emphasis on electrons around ($π$,0), (0,$π$). The phase diagram is shown in the plane of temperature $T$ and hole doping $δ$ in the mean field approximation and it is found that d-wave superconductivity, antiferromagnetism and $π$-triplet pair can coexist near half filling.

cond-mat.str-el↗

arXiv1997

Quantized Conductance of One-Dimensional Doped Mott Insulator

The possible modification of quantized conductance of one-dimensional doped Mott insulator, where the Umklapp scattering plays an important role, is studied based on the method by Maslov-Stone and Ponomarenko. At T=0 and away from half-filling, the conductance is quantized as $g=2e^2/h$ and there is no renormalization by Umklapp scattering process. At finite temperatures, however, the quantization is affected depending on the gate voltage and temperature.

cond-mat↗

arXiv1997

Effects of Low Energy Excitations in SNS Junction on the Dynamics of Josephson Phase

The effect of low energy excitation on the dynamics of Josephson phase in SNS junction is investigated. From the microscopic Hamiltonian, the effective action for the phase variable is derived. The retardation effects due to low energy excitation in normal(N) region are seen to play important roles in the dynamics of the phase. By the self consistent harmonic approximation, renormalization of mass and dissipation constant are calculated, revealing the enhancement of the former and the suppression of the latter in general. Various situation appears depending on these renormalized values.

cond-mat.supr-con↗

arXiv1997

Antiferromagnetic Phases of One-Dimensional Quarter-Filled Organic Conductors

The magnetic structure of antiferromagnetically ordered phases of quasi-one-dimensional organic conductors is studied theoretically at absolute zero based on the mean field approximation to the quarter-filled band with on-site and nearest-neighbor Coulomb interaction. The differences in magnetic properties between the antiferromagnetic phase of (TMTTF)$_2$X and the spin density wave phase in (TMTSF)$_2$X are seen to be due to a varying degrees of roles played by the on-site Coulomb interaction. The nearest-neighbor Coulomb interaction introduces charge disproportionation, which has the same spatial periodicity as the Wigner crystal, accompanied by a modified antiferromagnetic phase. This is in accordance with the results of experiments on (TMTTF)$_2$Br and (TMTTF)$_2$SCN. Moreover, the antiferromagnetic phase of (DI-DCNQI)$_2$Ag is predicted to have a similar antiferromagnetic spin structure.

cond-mat.str-el↗

arXiv1997

One-Particle Excitation of the Two-Dimensional Hubbard Model

The real part of the self-energy of interacting two-dimensional electrons has been calculated in the t-matrix approximation. It is shown that the forward scattering results in an anomalous term leading to the vanishing renormalization factor of the one-particle Green function, which is a non-perturbative effect of the interaction U. The present result is a microscopic demonstration of the claim by Anderson based on the conventional many-body theory. The effect of the damping of the interacting electrons, which has been ignored in reaching above conclusion, has been briefly discussed.

cond-mat.str-el↗