arXiv subjects

Masao Ogata

Publications and source records attributed to Masao Ogata.

At least 91 records · Page 5Linked to original sources

arXiv2015

Meissner Effect of Dirac Electrons in Superconducting State due to Inter-band Effect

Dirac electrons in solids show characteristic physical properties due to their linear dispersion relation and two-band nature. Although the transport phenomena of Dirac electrons in a normal state have intensively been studied, the transport phenomena in a superconducting state have not been fully understood. In particular, it is not clear whether Dirac electrons in a superconducting state show Meissner effect (ME), since a diamagnetic term of a current operator is absent as a result of the linear dispersion. We investigate the ME of three dimensional massive Dirac electrons in a superconducting state on the basis of Kubo formula, and clarify that Meissner kernel becomes finite by use of the inter-band contribution. This mechanism of the ME for Dirac electrons is completely different from that for the electrons in usual metals. Our result shows that the Meissner kernel remains finite even when the superconducting gap vanishes. This is an unavoidable problem in the Dirac electron system as reported in the previous works. Thus, we use a prescription in which we subtract the normal state contribution. In order to justify this prescription, we develop a specific model where the Meissner kernel is obtained by the prescription. We also derive the result for the electron gas by taking the non-relativistic limit of Dirac Hamiltonian, and clarify that the diamagnetic term of the Meissner kernel can be regarded as the inter-band contribution between electrons and positrons in terms of the Dirac model.

cond-mat.supr-con↗

arXiv2015

Distribution of Chern number by Landau level broadening in Hofstadter's butterfly

We discuss the relationship between the quantum Hall conductance and a fractal energy band structure, Hofstadter's butterfly, on a square lattice under a magnetic field. At first, we calculate the Hall conductance of Hofstadter's butterfly on the basis of the linear responce theory. By classifying the bands into some groups with a help of continued fraction expansion, we find that the conductance at the band gaps between the groups accord with the denominators of fractions obtained by aborting the expansion halfway. The broadening of Landau levels is given as an account of this correspondance.

cond-mat.str-el↗

arXiv2015

Longitudinal conductivity of massless fermions with tilted Dirac cone in magnetic field

We investigate a longitudinal conductivity of a two-dimensional relativistic electron gas with a tilted Dirac cone in magnetic field. It is demonstrated that the conductivity behaves differently in the directions parallel and perpendicular to the tilting of the cone. At high magnetic fields the conductivity at non-zero Landau levels in the direction perpendicular to the tilting modifies non-trivially, in contrast to the parallel case. At zero temperature the crossover of the conductivity at the Dirac point from high to low magnetic field is studied numerically. It is found that that the tilting produces anisotropy of the conductivity which changes with the magnetic field which is different from the anisotropy coming from the Fermi velocity. We also discuss the conductivity at finite temperatures and finite magnetic fields which can be directly compared with the experiments in $α$-(BEDT-TTF)$_2$I$_3$ organic conductor. We find that the tilting does not affect so much the magnetic-filed dependence of the conductivity except for the prefactor. We discuss the interpretation of recent experimental data and make some proposals to detect the effect of the tilting in future experiments.

cond-mat.mes-hall↗

arXiv2015

Negative Coulomb Drag in Coupled Quantum Wires

We present a theory of negative Coulomb drag in capacitively coupled quantum wires based on the commensurability of the electron density and the long-range nature of the Coulomb interaction in the Tomonaga-Luttinger liquid. The commensurability introduces a notion of doped particles and holes. We point out that the long-range interaction allows a particle-hole pairing over the wires and that the particle-hole pairing brings about the positive drag of holes, that is, the negative drag.

cond-mat.str-el↗

arXiv2015

Defect-Induced Kondo Effect in graphene: Role of Localized State of $π$ Electrons

We discuss a role of the localized $π$ orbital, which exists around the defect, on the defect-induced Kondo effect in graphene by a numerical renormalization group study. We find that the localized $π$ orbital assists this Kondo effect, and the Kondo temperature is sensitive to the broadening of the localized $π$ orbital. Secondly, we focus on the negative magnetoresistance of this Kondo effect. In the experimental result, it has been shown that the negative magnetoresistance is ten times larger than the usual Kondo effect. In order to clarify the mechanism of the "magnetic sensitive" Kondo effect, as a first step, we study an orbital magnetic field dependence of the localized $π$ orbital by a tight-binding model with a Peierls phase. We find that as the magnetic field increases, the spectral width of the localized $π$ orbital increases and the local DOS at the Fermi level decreases. Since the Kondo temperature is strongly dependent of the broadening of the localized $π$ orbital, it is expected that this Kondo effect is sensitive to the orbital magnetic field as observed in the experiment.

cond-mat.str-el↗

arXiv2014

Long-range Coulomb interaction effects on the surface Dirac electron system of a three-dimensional topological insulator

The surface state of a three-dimensional topological insulator forms a two-dimensional massless Dirac electron system. In Dirac electron systems, Coulomb interaction is not screened due to the small density of states at the Fermi energy and thus the long-range Coulomb interaction (LRCI) plays a crucial role. In this paper, we investigate the surface state with chemical potential $μ=0$ in the presence of the LRCI using the Wilsonian renormalization group. We first check the Fermi velocity enhancement in the surface Dirac system, which also occurs in a usual Dirac electron system. The most remarkable feature of the surface Dirac system is that the Dirac Hamiltonian contains not pseudo spin but real spin Pauli matrices. Because of this feature, we find the g-factor enhancement, which is a unique property of the surface Dirac system. We also investigate the explicit form of the spin susceptibility and find that the spin susceptibility is enhanced in the presence of the LRCI.

cond-mat.mes-hall↗

arXiv2014

Transport Properties and Diamagnetism of Dirac Electrons in Bismuth

Bismuth crystal is known for its remarkable properties resulting from particular electronic states, e. g., the Shubnikov-de Haas effect and the de Haas-van Alphen effect. Above all, the large diamagnetism of bismuth had been a long-standing puzzle soon after the establishment of quantum mechanics, which had been resolved eventually in 1970 based on the effective Hamiltonian derived by Wolff as due to the interband effects of a magnetic field in the presence of a large spin-orbit interaction. This Hamiltonian is essentially the same as the Dirac Hamiltonian, but with spatial anisotropy and an effective velocity much smaller than the light velocity. This paper reviews recent progress in the theoretical understanding of transport and optical properties, such as the weak-field Hall effect together with the spin Hall effect, and ac conductivity, of a system described by the Wolff Hamiltonian and its isotropic version with a special interest of exploring possible relationship with orbital magnetism. It is shown that there exist a fundamental relationship between spin Hall conductivity and orbital susceptibility in the insulating state on one hand, and the possibility of fully spin-polarized electric current in magneto-optics. Experimental tests of these interesting features have been proposed.

cond-mat.mes-hall↗

arXiv2014

Spin-Hall Effect and Diamagnetism of Anisotropic Dirac Electrons in Solids

Spin-Hall conductivity $σ_{{\rm s}xy}$ and orbital susceptibility $χ$ are investigated for the anisotropic Wolff Hamiltonian, which is an effective Hamiltonian common to Dirac electrons in solids. It is found that, both for $σ_{{\rm s}xy}$ and $χ$, the effect of anisotropy appears only in the prefactors, which is given as the Gaussian curvature of the energy dispersion, and their functional forms are equivalent to those of the isotropic Wolff Hamiltonian. As a result, it is revealed that the relationship between the spin Hall conductivity and the orbital susceptibility in the insulating state, $σ_{{\rm s}xy}=(3mc^2/\hbar e)χ$, which was firstly derived for the isotropic Wolff Hamiltonian, is also valid for the anisotropic Wolff Hamiltonian. Based on this theoretical finding, the magnitude of spin-Hall conductivity is estimated for bismuth and its alloys with antimony by that of orbital susceptibility, which has good correspondence between theory and experiments. The magnitude of spin-Hall conductivity turns out to be as large as $eσ_{{\rm s}xy} \sim 10^4 Ω^{-1}{\rm cm}^{-1}$, which is about 100 times larger than that of Pt.

cond-mat.mes-hall↗

arXiv2014

A Poor Man's Derivation of Quantum Compass-Heisenberg Interaction: Superexchange Interaction in J-J Coupling Scheme

The exchange interaction between 5d electrons in t$_{2g}$ orbitals is derived in the J-J coupling scheme which is the appropriate basis in the case of strong spin-orbit interaction. From simple calculations, it is found that ferromagnetic Ising interaction (quantum compass model) occurs due to a selection rule of hybridization between $Γ_7$ and $Γ_8$ orbitals, while antiferromagnetic Heisenberg interaction appears from hybridization between $Γ_7$ orbitals. It is also found that the ferromagnetic Ising interaction becomes small as the spin-orbit interaction increases. Thus, the sign of exchange interaction changes from ferromagnetic Ising to antiferromagnetic Heisenberg interaction as the spin-orbit interaction increases.

cond-mat.str-el↗

arXiv2013

Thermoelectric Transport Coefficients for Massless Dirac Electrons in Quantum Limit

We perform an analytic calculation of thermoelectric transport coefficients for massless Dirac electrons using the approach based on the Kubo--Středa formula and generalized Mott's relation. The main focus of the letter is made on the properties of the Nernst coefficient in the vicinity of the Dirac point in quantum limit. We calculate magnetic field and temperature dependencies of the Nernst coefficient and compare our results with recent experiments in $α$-(BEDT-TTF)$_{2}$I$_{3}$ organic conductor. We argue that the Zeeman splitting is important to understand the experimental data at high magnetic fields.

cond-mat.mes-hall↗

arXiv2013

Competition between spin fluctuations in Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ around $x=0.5$

We study the static susceptibilities for charge and spin sectors in paramagnetic states for Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ in $0.5\leq x \leq 2$ within random phase approximation on the basis of an effective Ru $t_{2g}$ orbital Hubbard model. We find that several modes of spin fluctuation around $\boldq=(0,0)$ and $\boldq\sim(0.797π,0)$ are strongly enhanced for the model of $x=0.5$. This enhancement arises from the increase of the corresponding susceptibilities for the $d_{xy}$ orbital due to the rotation-induced modifications of the electronic structure for this orbital (i.e., the flattening of the bandwidth and the increase of the density of state near the Fermi level). We also find that the ferromagnetic spin fluctuation becomes stronger for a special model than for the model of $x=0.5$, while the competition between the modes of spin fluctuation at $\boldq=(0,0)$ and around $\boldq\sim (π,0)$ is weaker for the special model; in this special model, the van Hove singularity (vHs) for the $d_{xy}$ orbital is located on the Fermi level. These results indicate that the location of the vHs for the $d_{xy}$ orbital, which is controlled by substitution of Ca for Sr, is a parameter to control this competition. We propose that the spin fluctuations for the $d_{xy}$ orbital around $\boldq=(0,0)$ and $\boldq\sim (π,0)$ play an important role in the electronic states around $x=0.5$ other than the criticality approaching the usual Mott transition where all electrons are localized.

cond-mat.str-el↗

arXiv2012

Unexpected weak spatial variation of local density of sates induced by individual Co impurity atoms in Na(Fe{1-x}Cox)As as revealed by scanning tunneling spectroscopy

We use spatially resolved scanning tunneling spectroscopy in Na(Fe{1-x}Cox)As to investigate the impurity effect induced by Co dopants. The Co impurities are successfully identified, and the spatial distributions of local density of state at different energies around these impurities are investigated. It is found that the spectrum shows negligible spatial variation at different positions near the Co impurity, although there is a continuum of the in-gap states which lifts the zero-bias conductance to a finite value. Our results put constraints on the S+- and S++ models and sharpen the debate on the role of scattering potentials induced by the Co dopants.

cond-mat.supr-con↗

arXiv2012

Theory of Defect-Induced Kondo Effect in Graphene: Effect of Zeeman Field

The effect of the Zeeman field on the defect-induced Kondo effect in graphene is investigated. The effective model of the Kondo effect is derived, and is analyzed on the basis of the numerical renormalization group method. It is found that, under the Zeeman field, at the border of the spin polarized state and the Kondo-Yosida singlet state, there is an unusual fixed point where the entropy of the defect is k_B ln2 and the expectation value of S^2_z is 1/8.

cond-mat.mes-hall↗

arXiv2012

Crossover between BCS Superconductor and Doped Mott Insulator of d-wave Pairing State in Two-Dimensional Hubbard Model

With high-Tc cuprates in mind, properties of correlated dx2-y2-wave superconducting (SC) and antiferromagnetic (AF) states are studied for the Hubbard (t-t'-U) model on square lattices, using a variational Monte Carlo method. We employ simple trial wave functions including only crucial parameters, such as a doublon-holon binding factor indispensable to describe correlated SC and normal states as doped Mott insulators. U/t, t'/t and δ(doping rate) dependence of relevant quantities are systematically calculated. As U/t increases, a sharp crossover of SC properties occurs at U_co/t \sim 10 from a conventional BCS type to a kinetic-energy-driven type for any t'/t. As δdecreases, U_co/t is smoothly connected to the Mott transition point at half filling. For U/t\lsim 5, steady superconductivity corresponding to the cuprates is not found, whereas the d-wave SC correlation function Pd^\infty rapidly increases for U/t\gsim 6 and becomes maximum at U=U_co. Comparing the δdependence of Pd^\infty with experimentally observed dome-shaped Tc and condensation energy, we find that the effective value of $U$ for the cuprates should be larger than the band width, for which the t-J model is valid. Analyzing the kinetic energy, we reveal that for U>U_co only doped holes (electrons) become charge carriers, which will make a small Fermi surface (hole pocket), but for U U_co is determined by two factors, i.e., the AF spin correlation, which creates singlet pairs (pseudogap), and the charge mobility dominated by Mott physics.

cond-mat.supr-con↗

arXiv2012

Origin of the Heavy Fermion Behavior in Ca_{2-x}Sr_{x}RuO_{4}: Roles of Coulomb Interaction and the Rotation of RuO_{6} octahedra

We study the electronic states for Ca_{2-x}Sr_{x}RuO_{4} in $0.5\leq x \leq 2$ within the Gutzwiller approximation (GA) on the basis of the three-orbital Hubbard model for the Ru t_{2g} orbitals. The main effects of the Ca substitution are taken account as the changes of the $dp$ hybridizations between the Ru 4d and O 2p orbitals. Using the numerical minimization of the energy obtained in the GA, we obtain the renormalization factor (RF) of the kinetic energy and total RF, which estimates the inverse of the mass enhancement, for three cases with the effective models of x=2 and 0.5 and a special model. We find that the inverse of the total RF becomes the largest for the case of x=0.5, and that the van Hove singularity, which is located on (below) the Fermi level for the special model (the effective model of x=0.5), plays a secondary role in enhancing the effective mass. Our calculation suggests that the heavy fermion behavior around x=0.5 comes from the cooperative effects between moderately strong Coulomb interaction compared to the total bandwidth and the modification of the electronic structures due to the rotation of RuO_{6} octahedra (i.e., the variation of the $dpπ$ hybridizations and the downward shift for the $d_{xy}$ orbital). We propose that moderately strong electron correlation and the orbital-dependent modifications of the electronic structures due to the lattice distortions play important roles in the electronic states for Ca_{2-x}Sr_{x}RuO_{4}.

cond-mat.str-el↗

arXiv2012

Spin-Hall Effect and Diamagnetism of Dirac Electrons

Spin-Hall conductivity (SHC) of fully relativistic (4x4 matrix) Dirac electrons is studied based on the Kubo formula aiming at possible application to bismuth and bismuth-antimony alloys. It is found that there are two distinct contributions to SHC, one only from the states near the Fermi energy and the other from all the occupied states. The latter remains even in the insulating state, i.e., when the chemical potential lies in the band-gap, and turns to have the same dependences on the chemical potential as the orbital susceptibility (diamagnetism), a surprising fact. These results are applied to bismuth-antimony alloys and the doping dependence of the SHC is proposed.

cond-mat.mes-hall↗

arXiv2012

Theory of Mechanism of $π$-d Interaction in Iron-Phthalocyanine

Transition metal-phtahalocyanine(Pc) compound, TPP[Fe(Pc)(CN)$_2$]$_2$, which is one of molecular conductors of charge transfer type with 3/4-filled conduction band consisting of LUMO of Pc molecules, shows characteristic features in transport and magnetic properties resulting from localized magnetic moments $S=1/2$ associated with Fe$^{+3}$ atoms. We construct an effective tight-binding model of this system and study the mechanism of exchange interaction, $J$, between $d$ and $π$ electrons based on both second order perturbation of transfer integrals between $d$ and $π$ orbitals and numerical diagonalization. It is found that there is no hybridization between $d$-orbitals and LUMO of $π$-orbitals and then super-exchange interaction in the Anderson model does not exist. Instead, processes associated with Hund's rule both on $d$ and $π$ orbitals, which may be called "the double Hund's exchange mechanism", turn out to play important roles and the sign of resultant $J$ can be either ferromagnetic or antiferromagnetic depending on model parameters because of competition among various processes. By taking account of magnetic anisotropy due to spin-orbit interactions and comparing with experimental results, it is indicated that $J$ is antiferromagnetic and of the order of 100K.

cond-mat.str-el↗

arXiv2012

Theory of Defect-Induced Kondo Effect in Graphene: Numerical Renormalization Group Study

An effective model that describes the Kondo effect due to a point defect in graphene is developed, taking account of the electronic state and the lattice structure of the defect. It is shown that this model can be transformed into a single-channel pseudogap Anderson model with a finite chemical potential. On the basis of the numerical renormalization group method, it is clarified that the experimentally observed gate-voltage dependence of the Kondo temperature is understood in this framework.

cond-mat.mtrl-sci↗