SearcharxivSearch

arXiv subjects

Masaru Onoda

Publications and source records attributed to Masaru Onoda.

At least 19 recordsLinked to original sources

Analytical solutions of topological surface states in a series of lattice models

We derive the analytical solutions of surface states in a series of lattice models for three-dimensional topological insulators and their nontopological counterparts based on an ansatz. A restriction on the spin-flip matrices in nearest-neighbor hopping characterizes the series. This restriction affords the ansatz and favors analytical solvability of surface-state eigenvectors. Despite the restriction, the series retains sufficient designability to describe various types of surface states. We also describe how it can serve as a tractable tool for elucidating unique phenomena on topological surfaces.

cond-mat.mes-hall

Photo-induced voltage in nano-porous gold thin film

We report an experimental study of generation of photo-induced voltage in nano-porous gold (NPG) thin film under the radiation of obliquely incident nanosecond laser light in visible regions. For s- polarized light, negative voltage is observed along the incident plane for positive incident angles, while for p- polarized light, positive voltage is observed for wavelength longer than 510 nm, while it turns to negative for shorter wavelengths. The transverse voltage for various polarized light is explained in terms of symmetry of configuration and that of microscopically random but macroscopically isotropic NPG.

cond-mat.mtrl-sci

Temperature dependent magnetotransport around $ν$= 1/2 in ZnO heterostructures

The sequence of prominent fractional quantum Hall states up to $ν$=5/11 around $ν$=1/2 in a high mobility two-dimensional electron system confined at oxide heterointerface (ZnO) is analyzed in terms of the composite fermion model. The temperature dependence of $\Rxx$ oscillations around $ν$=1/2 yields an estimation of the composite fermion effective mass, which increases linearly with the magnetic field. This mass is of similar value to an enhanced electron effective mass, which in itself arises from strong electron interaction. The energy gaps of fractional states and the temperature dependence of $\Rxx$ at $ν$=1/2 point to large residual interactions between composite fermions.

cond-mat.mes-hall

Photonic analog of graphene model and its extension -- Dirac cone, symmetry, and edge states --

This paper presents a theoretical analysis on bulk and edge states in honeycomb lattice photonic crystals with and without time-reversal and/or space-inversion symmetries. Multiple Dirac cones are found in the photonic band structure and the mass gaps are controllable via symmetry breaking. The zigzag and armchair edges of the photonic crystals can support novel edge states that reflect the symmetries of the photonic crystals. The dispersion relation and the field configuration of the edge states are analyzed in detail in comparison to electronic edge states. Leakage of the edge states to free space is inherent in photonic systems and is fully taken into account in the analysis. A topological relation between bulk and edge, which is analogous to that found in quantum Hall systems, is also verified.

cond-mat.mes-hall

Optical tornadoes in photonic crystals

Based on an optical analogy of spintronics, the generation of optical tornadoes is theoretically investigated in two-dimensional photonic crystals without space-inversion symmetry. We address its close relation to the Berry curvature in crystal momentum space, which represents the non-trivial geometric property of a Bloch state. It is shown that the Berry curvature is easily controlled by tuning two types of dielectric rods in a honeycomb photonic crystal. Then, Bloch states with large Berry curvatures appear as optical tornadoes in real space. The radiation force of such a configuration is analyzed, and its possible application is also discussed.

cond-mat.other

Tuning phase transition between quantum spin Hall and ordinary insulating phases

An effective theory is constructed for analyzing a generic phase transition between the quantum spin Hall and the insulator phases. Occurrence of degeneracies due to closing of the gap at the transition are carefully elucidated. For systems without inversion symmetry the gap-closing occurs at \pm k_0(\neq G/2) while for systems with inversion symmetry, the gap can close only at wave-numbers k=G/2, where G is a reciprocal lattice vector. In both cases, following a unitary transformation which mixes spins, the system is represented by two decoupled effective theories of massive two-component fermions having masses of opposite signs. Existence of gapless helical modes at a domain wall between the two phases directly follows from this formalism. This theory provides an elementary and comprehensive phenomenology of the quantum spin Hall system.

cond-mat.mes-hall

Left-handed Ferromagnet

The dynamics of the total magnetization in metallic ferromagnet is studied theoretically taking into account the relativistic spin-orbit interaction. It is found that its quantum dynamics is seriously influenced by the band crossings near the Fermi energy, and sometimes the direction of the precession can be reversed from what expected from the commutation relation $[S^{x},S^{y}]= i\hbar S^{z}$ ($h = 2π\hbar$: Planck constant), i.e., the left-handed ferromagnet can be realized.

cond-mat.mes-hall

Geometrical Aspects in Optical Wavepacket Dynamics

We construct a semiclassical theory for propagation of an optical wavepacket in non-conducting media with periodic structures of dielectric permittivity and magnetic permeability, i.e., non-conducting photonic crystals. We employ a quantum-mechanical formalism in order to clarify its link to those of electronic systems. It involves the geometrical phase, i.e., Berry phase, in a natural way, and describes an interplay between orbital motion and the internal rotation. Based on the above theory, we discuss the geometrical aspects of the optical Hall effect. We also consider a reduction of the theory to a system without periodic structure and apply it to the transverse shift at an interface reflection/refraction. For generic incident beams with elliptic polarizations, an identical result for the transverse shift of each reflected/transmitted beam is given by the following different approaches; (i) analytic evaluation of wavepacket dynamics, (ii) total angular momentum (TAM) conservation {\it for individual photons}, and (iii) numerical simulation of wavepacket dynamics. It is consistent with a result by classical electrodynamics. This means that the TAM conservation for individual photons is already taken into account in wave optics, i.e, classical electrodynamics. Finally, we show an application of our theory to a two-dimensional photonic crystal, and propose an optimal design for the enhancement of the optical Hall effect in photonic crystals.

physics.optics

Localization in a quantum spin Hall system

Localization problem of electronic states in a two-dimensional quantum spin Hall system (QSH - a symplectic model with a non-trivial topological structure) is studied by the transfer matrix method. The phase diagram in the plane of energy and disorder strength is exposed, and demonstrates "levitation" and "pair-annihilation" of the domains of extended states analogous to that of the integer quantum Hall system. The critical exponent $ν$ for the divergence of the localization length is estimated as $ν\cong 1.6$ which is distinct from both exponents pertaining to the conventional symplectic and the unitary quantum Hall systems. This strongly suggests a different universality class related to the non-trivial topology of the QSH system.

cond-mat.mes-hall

Disorder effect on the localization/delocalization in incommensurate potential

The interplay between incommensurate (IC) and random potentials is studied in a two-dimensional symplectic model with the focus on localization/delocalization problem. With the IC potential only, there appear wavefunctions localized along the IC wavevector while extended perpendicular to it. Once the disorder potential is introduced, these turn into two-dimensional anisotropic metallic states beyond the scale of the elastic mean free path, and eventually becomes localized in both directions at a critical strength of the disorder. Implications of these results to the experimental observation of the IC-induced localization is discussed.

cond-mat.mes-hall

Dynamics of localized spins coupled to the conduction electrons with charge/spin currents

The effects of the charge/spin currents of conduction electrons on the dynamics of the localized spins are studied in terms of the perturbation in the exchange coupling $J_{K}$ between them. The equations of motion for the localized spins are derived exactly up to $O(J_{K}^2)$, and the equations for the two-spin system is solved numerically. It is found that the dynamics depends sensitively upon the relative magnitude of the charge and spin currents, i.e., it shows steady state, periodic motion, and even chaotic behavior. Extension to the multi-spin system and its implications including possible ``spin current detector'' are also discussed.

cond-mat.other

Role of relaxation in spin Hall effect

The role of the relaxation due to the impurity scattering and/or the contacts to leads/electrodes are studied for the spin Hall effect (SHE). Relaxation is essential to attain the steady state and also to the spin accumulation, but has been considered to be harmful for the intrinsic SHE (ISHE). These issues are examined quantitatively on two types of 2D models, i.e., (a) Rashba model for $n$-type GaAs, and (b) Luttinger model for $p$-type GaAs. It is found that ISHE is robust against the realistic strength of the disorder producing the observable amount of the spin accumulation. Especially in model (b) the spin current and the accumulation are an order of magnitude larger than those in model (a). Experimental observations are discussed quantitatively from these results.

cond-mat.mes-hall

Spin current and accumulation generated by spin Hall insulator

Spin current and accumulation generated by the electric field in spin Hall insulator (SHI) are investigated theoretically in terms of the Keldysh formalism. In contrast to the quantum Hall system, there are no massless edge modes in general. The spin current is generated near the contacts to the electrodes by the hybridization between the metallic states and the conduction/valence bands of the SHI, but is truncated by the sink and source of the spin. However, one can produce the spin current flowing out to the conductors, which is attached to the SHI, and also the spin accumulation there due to the leakage charge current which breaks the time-reversal symmetry.

cond-mat.mes-hall

Hall Effect of Light

We derive the semiclassical equation of motion for the wave-packet of light taking into account the Berry curvature in the momentum space. This equation naturally describes the interplay between the orbital and spin angular momenta, i.e., the conservation of the total angular momentum of light. This leads to the shift of the wave-packet motion perpendicular to the gradient of the dielectric constant, i.e., the polarization-dependent Hall effect of light. An enhancement of this effect in the photonic crystal is also proposed.

cond-mat.other

Anomalous Hall Effect and Skyrmion Number in Real- and Momentum-space

We study the anomalous Hall effect (AHE) for the double exchange model with the exchange coupling $|J_H|$ being smaller than the bandwidth $|t|$ for the purpose of clarifying the following unresolved and confusing issues: (i) the effect of the underlying lattice structure, (ii) the relation between AHE and the skyrmion number, (iii) the duality between real and momentum spaces, and (iv) the role of the disorder scatterings; which is more essential, $σ_H$ (Hall conductivity) or $ρ_H$ (Hall resistivity)? Starting from a generic expression for $σ_H$, we resolve all these issues and classify the regimes in the parameter space of $J_H τ$ ($τ$: elastic-scattering time), and $λ_{s}$ (length scale of spin texture). There are two distinct mechanisms of AHE; one is characterized by the real-space skyrmion-number, and the other by momentum-space skyrmion-density at the Fermi level, which work in different regimes of the parameter space.

cond-mat.mes-hall

Quantized Anomalous Hall Effect in Two-Dimensional Ferromagnets - Quantum Hall Effect from Metal -

We study the effect of disorder on the anomalous Hall effect (AHE) in two-dimensional ferromagnets. The topological nature of AHE leads to the integer quantum Hall effect from a metal, i.e., the quantization of $σ_{xy}$ induced by the localization except for the few extended states carrying Chern number. Extensive numerical study on a model reveals that Pruisken's two-parameter scaling theory holds even when the system has no gap with the overlapping multibands and without the uniform magnetic field. Therefore the condition for the quantized AHE is given only by the Hall conductivity $σ_{xy}$ without the quantum correction, i.e., $|σ_{xy}| > e^2/(2h)$.

cond-mat.mes-hall

Particle-hole symmetry and transport properties of the flux state in underdoped cuprates

Transport properties are studied for the flux state with the gauge flux $ϕ$ per plaquett, which may model the underdoped cuprates, with the emphasis on the particle-hole and parity/chiral symmetries.This model is reduced to the Dirac fermions in (2+1)D with a mass gap introduced by the antiferromagnetic (AF) long range order and/or the stripe formation. Without the mass gap, the Hall constant $R_H$ and the thermopower $S$ obey two-parameter scaling laws, and show the strong temperature dependence due to the recovery of the particle-hole symmetry at high temperature. The $x$-dependences of $σ_{xx} (\propto \sqrt{x})$ and $σ_{xy}$ (independent of $x$) are in a sharp contradiction with the experiments. (Here $x$ is the hole concentration.) Therefore there is no signature of the particle-hole symmetry or the massless Dirac fermions in the underdoped cuprates even above the Neel temperature $T_N$. With the mass gap introduced by the AF order, there occurs the parity anomaly for each of the Dirac fermions. However the contributions from different valleys and spins cancel with each other to result in no spontaneous Hall effect even if the time-reversal symmetry is broken with $ϕ\ne π$. The effects of the stripes are also studied. The diagonal and vertical (horizontal) stripes have quite different influence on the transport properties. The suppression of $R_H$ occurs at low temperature only when (i) both the AF order and the vertical (horizontal) stripe coexist, and (ii) the average over the in-plane direction is taken. Discussions on the recent experiments are given from the viewpoint of these theoretical results.

cond-mat.supr-con

Topological Nature of Anomalous Hall Effect in Ferromagnet

The anomalous Hall effect in two-dimensional ferromagnets is discussed to be the physical realization of the parity anomaly in (2+1)D, and the band crossing points behave as the topological singularity in the Brillouin zone. This appears as the sharp peaks and the sign changes of the transverse conductance $σ_{xy}$ as a function of the Fermi energy and/or the magnetization. The relevance to the experiments including the three dimensional systems is also discussed.

cond-mat.str-el