arXiv subjects

Naoto Nagaosa

Publications and source records attributed to Naoto Nagaosa.

At least 307 records · Page 17Linked to original sources

arXiv2004

Spin Hall Insulator

Recent theories predict dissipationless spin current induced by an electric field in doped semiconductors. Nevertheless, the charge current is still dissipative in these systems. In this work, we theoretically predict the dissipationless spin Hall effect, without any accompanying charge current, in some classes of band insulators, including zero-gap semiconductors such as HgTe and narrow-gap semiconductors such as PbTe. This effect is similar to the quantum Hall effect in that all the states below the gap contribute and there occurs no dissipation. However the spin Hall conductance is not quantized even in two dimensions. This is the first example of a nontrivial topological structure in a band insulator without any magnetic field.

cond-mat.mes-hall↗

arXiv2004

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↗

arXiv2004

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↗

arXiv2004

Entropy-Driven Reentrant Behavior in CMR Manganites

We discuss the origin of the reentrant behaviors of insulating states above the Curie temperature of the colossal magnetoresistance manganites. We consider a system where charge ordering and ferromagnetism compete with each other. In the presence of randomness which pins charge order fluctuations, entropy-driven reentrant behaviors will appear, which explains the typical temperature dependence of the resistivity for CMR manganites.

cond-mat.str-el↗

arXiv2004

An Origin of CMR: Competing Phases and Disorder-Induced Insulator-to-Metal Transition in Manganites

We theoretically explore the mechanism of the colossal magnetoresistance in manganese oxides by explicitly taking into account the phase competition between the double-exchange ferromagnetism and the charge-ordered insulator. We find that quenched disorder causes a drastic change of the multicritical phase diagram by destroying the charge-ordered state selectively. As a result, there appears a nontrivial phenomenon of the disorder-induced insulator-to-metal transition in the multicritical regime. On the contrary, the disorder induces a highly-insulating state above the transition temperature where charge-ordering fluctuations are much enhanced. The contrasting effects provide an understanding of the mechanism of the colossal magnetoresistance. The obtained scenario is discussed in comparison with other theoretical proposals such as the polaron theory, the Anderson localization, the multicritical-fluctuation scenario, and the percolation scenario.

cond-mat.str-el↗

arXiv2004

Re-entrant Behavior and Gigantic Response in Disordered Spin-Peierls System

Effects of disorder and external field on the competing spin-Peierls and antiferromagnetic states are studied theoretically in terms of the numerical transfer matrix method applied to a quasi one-dimensional spin 1/2 Heisenberg model coupled to the lattice degree of freedom. We show that, at temperatures above the impurity-induced antiferromagnetic phase, inhomogeneous spin-Peierls lattice distortions remain to exist showing a re-entrant behavior. This feature can be drastically altered by very weak perturbations, e.g., the staggered magnetic field or the change in interchain exchange coupling $J_\perp$, leading to a huge response, which is analogous to the colossal magnetoresistance phenomenon in perovskite manganese oxides.

cond-mat.str-el↗

arXiv2004

SU(2) Non-Abelian Holonomy and Dissipationless Spin Current in Semiconductors

Following our previous work [S. Murakami, N. Nagaosa, S. C. Zhang, Science 301, 1348 (2003)] on the dissipationless quantum spin current, we present an exact quantum mechanical calculation of this novel effect based on the linear response theory and the Kubo formula. We show that it is possibxle to define an exactly conserved spin current, even in the presence of the spin-orbit coupling in the Luttinger Hamiltonian of p-type semiconductors. The light- and the heavy-hole bands form two Kramers doublets, and an SU(2) non-abelian gauge field acts naturally on each of the doublets. This quantum holonomy gives rise to a monopole structure in momentum space, whose curvature tensor directly leads to the novel dissipationless spin Hall effect, i.e., a transverse spin current is generated by an electric field. The result obtained in the current work gives a quantum correction to the spin current obtained in the previous semiclassical approximation.

cond-mat.mes-hall↗

arXiv2004

Orbital Dependent Phase Control in Ca2-xSrxRuO4

We present first-principles studies on the orbital states of the layered perovskites Ca$_{2-x}$Sr$_x$RuO$_4$. The crossover from antiferromagnetic (AF) Mott insulator for $x < 0.2$ to nearly ferromagnetic (FM) metal at $x=0.5$ is characterized by the systematic change of the $xy$ orbital occupation. For the AF side ($x < 0.2$), we present firm evidence for the $xy$ ferro-orbital ordering. It is found that the degeneracy of $t_{2g}$ (or $e_g$) states is lifted robustly due to the two-dimensional (2D) crystal-structure, even without the Jahn-Teller distortion of RuO$_6$. This effect dominates, and the cooperative occupation of $xy$ orbital is concluded. In contrast to recent proposals, the resulting electronic structure explains well both the observed X-ray absorption spectra and the double peak structure of optical conductivity. For the FM side ($x=0.5$), however, the $xy$ orbital with half filling opens a pseudo-gap in the FM state and contributes to the spin $S$=1/2 moment (rather than $S$=1 for $x$=0.0 case) dominantly, while $yz,zx$ states are itinerant with very small spin polarization, explaining the recent neutron data consistently.

cond-mat.str-el↗

arXiv2004

Two-dimensional charge order in layered 2-1-4 perovskite oxides

Monte Carlo simulations are performed on the three-dimensional (3D) Ising model with the 2-1-4 layered perovskite structure as a minimal model for checkerboard charge ordering phenomena in layered perovskite oxides. Due to the interlayer frustration, only 2D long-range order emerges with a finite correlation length along the c axis. Critical exponents of the transition change continuously as a function of the interlayer coupling constant. The interlayer long-range Coulomb interaction decays exponentially and is negligible even between the second-neighbor layers. Instead, monoclinic distortion of a tetragonal unit cell lifts the macroscopic degeneracy to induce a 3D charge ordering. The dimensionality of the charge order in La$_{0.5}$Sr$_{1.5}$MnO$_4$ is discussed from this viewpoint.

cond-mat.stat-mech↗

arXiv2003

Interplay of Electron-Phonon Interaction and Electron Correlation in High Temperature Superconductivity

We study the electron-phonon interaction in the strongly correlated superconducting cuprates. Two types of the electron-phonon interactions are introduced in the $t-J$ model; the diagonal and off-diagonal interactions which modify the formation energy of the Zhang-Rice singlet and its transfer integral, respectively. The characteristic phonon-momentum $(\vec q)$ and electron-momentum $(\vec k)$ dependence resulted from the off-diagonal coupling can explain a variety of experiments. The vertex correction for the electron-phonon interaction is formulated in the SU(2) slave-boson theory by taking into account the collective modes in the superconducting ground states. It is shown that the vertex correction enhances the attractive potential for the d-wave paring mediated by phonon with $\vec q=(π(1-δ), 0)$ around $δ\cong 0.3$ which corresponds to the half-breathing mode of the oxygen motion.

cond-mat.str-el↗

arXiv2003

Dissipationless Quantum Spin Current at Room Temperature

While microscopic laws of physics are invariant under the reversal of the arrow of time, the transport of energy and information in most devices is an irreversible process. It is this irreversibility that leads to intrinsic dissipations in electronic devices and limits the possibility of quantum computation. We theoreticallypredict that the electric field can induce a substantial amount of dissipationless quantum spin current at room temperature, in hole doped semiconductors such as Si, Ge and GaAs. Based on a generalization of the quantum Hall effect, the predicted effect leads to efficient spin injection without the need for metallic ferromagnets. Principles found in this work could enable quantum spintronic devices with integrated information processing and storage units, operating with low power consumption and performing reversible quantum computation.

cond-mat.mes-hall↗

arXiv2003

Mott Transition vs Multicritical Phenomenon of Superconductivity and Antiferromagnetism -- Application to $κ$-(BEDT-TTF)$_2$X --

Interplay between the Mott transition and the multicritical phenomenon of d-wave superconductivity (SC) and antiferromagnetism (AF) is studied theoretically. We describe the Mott transition, which is analogous to a liquid-gas phase transition, in terms of an Ising-type order parameter $η$. We reveal possible mean-field phase diagrams produced by this interplay. Renormalization group analysis up to one-loop order gives flows of coupling constants, which in most cases lead to fluctuation-induced first-order phase transitions even when the SO(5) symmetry exists betwen the SC and AF. Behaviors of various physical quantities around the Mott critical point are predicted. Experiments in $κ$-(BEDT-TTF)$_2$X are discussed from this viewpoint.

cond-mat.str-el↗

arXiv2003

Competing Orders and Disorder-induced Insulator to Metal Transition in Manganites

Effects of disorder on the two competing phases, i.e., the ferromagnetic metal and the commensurate charge/lattice ordered insulator, are studied by Monte Carlo simulation. The disorder suppresses the charge/lattice ordering more strongly than the ferromagnetic order, driving the commensurate insulator to the ferromagnetic metal near the phase boundary in the pure case. Above the ferromagnetic transition temperature, on the contrary, the disorder makes the system more insulating, which might cause an enhanced colossal magnetoresistance as observed in the half-doped or Cr-substituted manganites. No indication of the percolation or the cluster formation is found, and there remain the charge/lattice fluctuations instead which are enhanced toward the transition temperature.

cond-mat.str-el↗

arXiv2003

New Collective Modes in the Superconducting Ground States in the Gauge Theory Description of the Cuprates

In the slave boson mean field treatment of the t-J model, the ground state for small doping is a d-wave superconductor. A conventional superconductor has collective modes associated with the amplitude and phase of the pairing order parameter. Here, the hopping matrix element $χ_{ij}$ is also a mean field order parameter. We therefore expect that new collective modes will be introduced with these additional complex degrees of freedom. We compute the new collective modes and their spectral functions by numerically diagonalizing the matrix which describes the fluctuations about the mean field solution. We also show that the SU(2) gauge theory formulation allows us to predict and classify these collective modes. Indeed, the SU(2) formulation is essential in order to avoid spurious collective modes as the doping $x$ goes to zero. The most important new collective modes are the $θ$ mode and the longitudinal and transverse $ϕ$ gauge modes. The $θ$ mode corresponds to fluctuations of the staggered flux phase and creates orbital current fluctuations. The $ϕ$ gauge modes correspond to out-of-phase fluctuations of the {\em amplitudes} of the pairing and the hopping matrix elements. We compute the neutron scattering cross-section which couples to the $θ$ mode and inelastic X-ray scattering cross-section which couples to the fluctuation in the real part of $χ_{ij}$. In addition, we show that the latter fluctuation at $(π,π)$ may be coupled to the buckling phonon mode in the LTO phase of LSCO and may be detected optically. Experimental searches of these collective modes will serve as important tests of this line of attack on the high T$_c$ problem.

cond-mat.str-el↗

arXiv2003

Randomness Effect on Multicritical Phenomena in Double-Exchange Systems

Double-exchange model interacting with adiabatic phonons is studied in the presence of randomness by using the Monte-Carlo method and the systematic size-scaling analysis. A bicritical behavior is found between the ferromagnetic metal and the charge-ordered insulator. We find the distinct response to the randomness between these two states, which agrees well with the experimental results in the colossal magnetoresistance manganites.

cond-mat.str-el↗

arXiv2003

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↗

arXiv2002

Berry phase in Magnetic Superconductors

In magnetic systems, electronic bands often acquire nontrivial topological structure characterized by gauge flux distribution in momentum (k)-space. It sometimes follows that the phase of the wavefunctions cannot be defined uniquely over the whole Brillouin zone. In this Letter we develop a theory of superconductivity in the presence of this gauge flux both in two- and three-dimensional systems. It is found that the superconducting gap has "nodes" as a function of k where the Fermi surface is penetrated by a gauge string.

cond-mat.supr-con↗

arXiv2002

One-Dimensional Confinement and Enhanced Jahn-Teller Instability in LaVO$_3$

Ordering and quantum fluctuations of orbital degrees of freedom are studied theoretically for LaVO$_3$ in spin-C-type antiferromagnetic state. The effective Hamiltonian for the orbital pseudospin shows strong one-dimensional anisotropy due to the negative interference among various exchange processes. This significantly enhances the instability toward lattice distortions for the realistic estimate of the Jahn-Teller coupling by first-principle LDA+$U$ calculations, instead of favoring the orbital singlet formation. This explains well the experimental results on the anisotropic optical spectra as well as the proximity of the two transition temperatures for spin and orbital orderings.

cond-mat.str-el↗