arXiv subjects

Kentaro Nomura

Publications and source records attributed to Kentaro Nomura.

At least 91 records · Page 5Linked to original sources

arXiv2016

Chiral Magnetic Effect and Anomalous Hall Effect in Antiferromagnetic Insulators with Spin-Orbit Coupling

We search for dynamical magnetoelectric phenomena in three-dimensional correlated systems with spin-orbit coupling. We focus on the antiferromagnetic insulator phases where the dynamical axion field is realized by the fluctuation of the antiferromagnetic order parameter. It is shown that the dynamical chiral magnetic effect, an alternating current generation by magnetic fields, emerges due to the time dependences of the order parameter such as antiferromagnetic resonance. It is also shown that the anomalous Hall effect arises due to the spatial variations of the order parameter such as antiferromagnetic domain walls. Our study indicates that spin excitations in antiferromagnetic insulators with spin-orbit coupling can result in nontrivial charge responses. Moreover, observing the chiral magnetic effect and anomalous Hall effect in our system is equivalent to detecting the dynamical axion field in condensed matter.

cond-mat.str-el↗

arXiv2015

Spin textures and spin-wave excitations in doped Dirac-Weyl semimetals

We study correlations and magnetic textures of localized spins, doped in three-dimensional Dirac semimetals. An effective field theory for magnetic moments is constructed by integrating out the fermionic degrees of freedom. The spin correlation shows a strong anisotropy, originating from spin-momentum locking of Dirac electrons, in addition to the conventional Heisenberg-like ferromagnetic correlation. The anisotropic spin correlation allows topologically nontrivial magnetic excitation textures such as a transient hedgehog state, as well as the ferromagnetic ground state. The spin-wave dispersion in ferromagnetic Weyl semimetal also becomes anisotropic, being less dispersed perpendicular to the magnetization.

cond-mat.mes-hall↗

arXiv2015

Charge-Induced Spin Torque in Anomalous Hall Ferromagnets

We demonstrate that spin-orbit coupled electrons in a magnetically doped system exert a spin torque on the local magnetization, without a flowing current, when the chemical potential is modulated in a magnetic field. The spin torque is proportional to the anomalous Hall conductivity, and its effective field strength may overcome the Zeeman field. Using this effect, the direction of the local magnetization is switched by gate control in a thin film. This charge-induced spin torque is essentially an equilibrium effect, in contrast to the conventional current-induced spin-orbit torque, and, thus, devices using this operating principle possibly have higher efficiency than the conventional ones. In addition to a comprehensive phenomenological derivation, we present a physical understanding based on a model of a Dirac-Weyl semimetal, possibly realized in a magnetically doped topological insulator. The effect might be realized also in nanoscale transition materials, complex oxide ferromagnets, and dilute magnetic semiconductors.

cond-mat.mes-hall↗

arXiv2015

Disorder effect on magneto-transport on the surface of a topological insulator

We study the magneto-transport properties on the disordered surface of a topological insulator attached with a ferromagnet/ferromagnet junction. Since, in the surface Dirac Hamiltonian, out-of-plane magnetization induces a mass gap, while in-plane magnetization has a role of the effective vector potential, the mechanism of magneto-transport is different between these two cases. The former is similar to the conventional one in ferromagnetic metals, while the latter is due to the shift of Fermi circles in momentum space. Our numerical calculations show that the magnetoconductance in in-plane configuration is robust against disorder compared to that in out-of-plane configuration.

cond-mat.mes-hall↗

arXiv2015

Quantum thermal Hall effect of Majorana fermions on the surface of superconducting topoloigcal insulators

We study the quantum anomalous thermal Hall effect in a topological superconductor which possesses an integer bulk topological number, and supports Majorana excitations on the surface. To realize the quantum thermal Hall effect, a finite gap at the surface is induced by applying an external magnetic field or by the proximity effects with magnetic materials or $s$-wave superconductors with complex pair-potentials. Basing on the lattice model Hamiltonian for superconducting states in Cu-doped Bi$_2$Se$_3$, we compute the thermal Hall conductivity as a function of various parameters such as the chemical potential, the pair-potential, and the spin-orbit coupling induced band gap. It is argued that the bulk topological invariant corresponds to the quantization rule of the thermal Hall conductivity induced by complex $s$-wave pair-potentials.

cond-mat.mes-hall↗

arXiv2014

Axionic Antiferromagnetic Insulator Phase in a Correlated and Spin-Orbit Coupled System

We study theoretically a three-dimensional correlated and spin-orbit coupled system, the half-filled extended Fu-Kane-Mele-Hubbard model on a diamond lattice, focusing on the topological magnetoelectric response of the antiferromagnetic insulator phase. In the antiferromagnetic insulator phase, the Dirac-like low-energy effective Hamiltonian is obtained. Then the theta term, which results in the magnetoelectric response, is derived as a consequence of the chiral anomaly. The realization of the dynamical axion field in our model is discussed. The relation with a symmetry broken phase induced by interactions in lattice quantum chromodynamics is also discussed.

cond-mat.str-el↗

arXiv2014

Stability of Multinode Dirac Semimetals against Strong Long-Range Correlations

We study the stability of Dirac semimetals with $N$ nodes in three spatial dimensions against strong $1/r$ Coulomb interactions. We particularly study the cases of $N=4$ and $N=16$, where the $N=4$ Dirac semimetal is described by the staggered fermions and the $N=16$ Dirac semimetal is described by the doubled lattice fermions. We take into account the $1/r$ long-range Coulomb interactions between the bulk electrons. Based on the U(1) lattice gauge theory, we analyze the system from the strong coupling limit. It is shown that the Dirac semimetals survive in the strong coupling limit when the out-of-plane Fermi velocity anisotropy of the Dirac cones is weak, whereas they change to Mott insulators when the anisotropy is strong. A Possible global phase diagram of correlated multinode Dirac semimetals is presented. Implications of our result to the stability of Weyl semimetals and three-dimensional topological insulators are discussed.

cond-mat.str-el↗

arXiv2014

Magnetoelectric effect in topological insulator films beyond the linear response regime

We study the response of topological insulator films to strong magnetic and electric fields beyond the linear response theory. As a model, we use the three-dimensional lattice Wilson-Dirac Hamiltonian where we simultaneously introduce both magnetic field through the Peierls substitution and electric field as a potential energy depending on lattice coordinate. We compute the electron energy spectrum by numerically diagonalizing this Hamiltonian and obtain quantized magnetoelectric polarizability. In addition, we find that the magnetoelectric effect vanishes as the film width decreases due to the hybridization of surface wave functions. Furthermore, applying a gate voltage between the surfaces, we observe several quantized plateaus of $θ$ term, which are mainly determined by the Landau level structures on the top and bottom surfaces.

cond-mat.mes-hall↗

arXiv2014

Weyl Semimetal in the Strong Coulomb Interaction Limit

We study the effects of strong $1/r$ long-range Coulomb interactions in a Weyl semimetal. We consider a three-dimensional (3D) Dirac fermion system on a lattice with a time-reversal symmetry breaking term, and take into account $1/r$ long-range Coulomb interactions between the bulk electrons. This model is regarded as the case where magnetic impurities are doped into the bulk of a 3D topological insulator. With the use of the strong coupling expansion of the lattice gauge theory and the mean-field approximation, we analyze the system from the strong coupling limit. It is shown that parity symmetry of the system is spontaneously broken in the strong coupling limit, and a different type of the Weyl semimetal, in which time-reversal and parity symmetries are broken, appears in the strong coupling limit. A possible global phase diagram of a correlated Weyl semimetal is presented.

cond-mat.str-el↗

arXiv2014

Weyl semimetal phase in solid-solution narrow-gap semiconductors

We theoretically investigate ferromagnetic ordering in magnetically doped solid-solution narrow-gap semiconductors with the strong spin-orbit interaction such as Cr-doped Bi$_2$(Se$_x$Te$_{1-x}$)$_3$. We compute the spontaneous magnetization of impurities and itinerant electrons, and estimate the critical temperature as a function of the concentration of magnetic dopants and the strength of the spin-orbit interaction. It is found that the critical temperature is proportional to the concentration of dopants and enhanced with the strong spin-orbit interaction. It is also found that the ferromagnetic transition could make the system turn to the Weyl semimetal which possesses a pair of Weyl points separating in the momentum space.

cond-mat.mes-hall↗

arXiv2014

Disorder effects on thermal transport on the surface of topological superconductors by the self-consistent Born approximation

We have studied the longitudinal thermal conductivity of the surface of a three-dimensional time-reversal symmetric topological superconductor with random disorder. Majorana fermions on the surface of topological superconductors have a response to the gravitational field, which is realized as a thermal response to the temperature gradient inside of the material. Because of the presence of both time-reversal symmetry and particle-hole symmetry, disorder on the surface emerges in the Hamiltonian only as spatial deformations of the pair potential. In terms of the gravitational field, the disorder results in spatial fluctuations of the metric. We consider disorder effects on the thermal conductivity perturbatively within the self-consistent Born approximation. The density of states is calculated with the Green's function technique and the thermal conductivity of the surface modes is derived through the electronic conductivity using Wiedemann-Franz law for the Majorana fermions.

cond-mat.supr-con↗

arXiv2013

Phase structure of topological insulators by lattice strong-coupling expansion

The effect of the strong electron correlation on the topological phase structure of 2-dimensional (2D) and 3D topological insulators is investigated, in terms of lattice gauge theory. The effective model for noninteracting system is constructed similarly to the lattice fermions with the Wilson term, corresponding to the spin-orbit coupling. Introducing the electron-electron interaction as the coupling to the gauge field, we analyze the behavior of emergent orders by the strong coupling expansion methods. We show that there appears a new phase with the in-plane antiferromagnetic order in the 2D topological insulator, which is similar to the so-called "Aoki phase" in lattice QCD with Wilson fermions. In the 3D case, on the other hand, there does not appear such a new phase, and the electron correlation results in the shift of the phase boundary between the topological phase and the normal phase.

cond-mat.str-el↗

arXiv2013

Criticality of the metal-topological insulator transition driven by disorder

Employing scaling analysis of the localization length, we deduce the critical exponent of the metal-topological insulator (TI) transitions induced by disorder. The obtained exponent nu~2.7 shows no conspicuous deviation from the value established for metal-ordinary insulator transitions in systems of the symplectic class. We investigate the topological phase diagram upon carrier doping to reveal the nature of the so-called topological Anderson insulator (TAI) region. The critical exponent of the metal-TAI transition is also first estimated, shown to be undistinguishable from the above value within the numerical error. By symmetry considerations we determine the explicit form of Rashba spin-orbit coupling in systems of C4v point group symmetry.

cond-mat.mes-hall↗

arXiv2013

Strong Coupling Expansion in a Correlated Three-Dimensional Topological Insulator

Motivated by recent studies which show that topological phases may emerge in strongly correlated electron systems, we theoretically study the strong electron correlation effect in a three-dimensional topological insulator, which effective Hamiltonian can be described by the Wilson fermions. We adopt 1/r long-range Coulomb interaction as the interaction between the bulk electrons. Based on the U(1) lattice gauge theory, the strong coupling expansion is applied by assuming that the effective interaction is strong. It is shown that the effect of the Coulomb interaction is equivalent to the renormalization of the bare mass of the Wilson fermions, and that as a result, the topological insulator phase survives in the strong coupling limit.

cond-mat.str-el↗

arXiv2012

Electron Correlation Induced Spontaneous Symmetry Breaking and Weyl Semimetal Phase in a Strongly Spin-Orbit Coupled System

We study theoretically the electron correlation effect in a three-dimensional Dirac fermion system which describes a topologically nontrivial state. It is shown within the mean-field approximation that time-reversal and inversion symmetries of the system are spontaneously broken in the region where both spin-orbit coupling and electron correlation are strong. This phase is considered as an analog of that in the lattice quantum chromodynamics. It is also shown that in the presence of magnetic impurities, electron correlation enhances the appearance of the Weyl semimetal phase between the topological insulator phase and the normal insulator phase.

cond-mat.str-el↗

arXiv2012

Electromagnetic and thermal responses in topological matter: topological terms, quantum anomalies and D-branes

We discuss the thermal (or gravitational) responses in topological superconductors and in topological phases in general. Such thermal responses (as well as electromagnetic responses for conserved charge) provide a definition of topological insulators and superconductors beyond the single-particle picture. In two-dimensional topological phases, the Strěda formula for the electric Hall conductivity is generalized to the thermal Hall conductivity. Applying this formula to the Majorana surface states of three-dimensional topological superconductors predicts cross-correlated responses between the angular momentum and thermal polarization (entropy polarization). We also discuss a use of D-branes in string theory as a systematic tool to derive all such topological terms and topological responses. In particular, we relate the $\mathbb{Z}_2$ index of topological insulators introduced by Kane and Mele (and its generalization to other symmetry classes and to arbitrary dimensions) to the K-theory charge of non-BPS D-branes, and vice versa. We thus establish a link between the stability of non-BPS D-branes and the topological stability of topological insulators.

cond-mat.mes-hall↗

arXiv2012

Quasi-Particle Tunneling in Anti-Pfaffian Quantum Hall State

We study tunneling phenomena at the edge of the anti-Pfaffian quantum Hall state at the filling factor $ν=5/2$. The edge current in a single point-contact is considered. We focus on nonlinear behavior of two-terminal conductance with the increase in negative split-gate voltage. Expecting the appearance of the intermediate conductance plateau we calculate the value of its conductance by using the renormalization group (RG) analysis. Further, we show that non-perturbative quasi-particle tunneling is effectively described as perturbative electron tunneling by the instanton method. The two-terminals conductance is written as a function of the gate voltage. The obtained results enable us to distinguish the anti-Pfaffian state from the Pfaffian state experimentally.

cond-mat.mes-hall↗

arXiv2011

Cross-Correlated Responses of Topological Superconductors and Superfluids

We study nontrivial responses of topological superconductors and superfluids to the temperature gradient and rotation of the system. In two-dimensional gapped systems, the Strěda formula for the electric Hall conductivity is generalized to the thermal Hall conductivity. Applying this formula to the Majorana surface states of three-dimensional topological superconductors predicts cross-correlated responses between the orbital angular momentum and thermal polarization (entropy polarization). These results can be naturally related to the gravitoelectromagnetism description of three-dimensional topological superconductors and superfluids, analogous to the topological magnetoelectric effect in ${\mathbb{Z}}_2$ topological insulators.

cond-mat.supr-con↗