arXiv subjects

Naoto Nagaosa

Publications and source records attributed to Naoto Nagaosa.

At least 289 records · Page 16Linked to original sources

arXiv2006

Dynamical Diffraction Theory for Wave Packet Propagation in Deformed Crystals

We develop a theory for the trajectory of an x ray in the presence of a crystal deformation. A set of equations of motion for an x-ray wave packet including the dynamical diffraction is derived, taking into account the Berry phase as a correction to geometrical optics. The trajectory of the wave packet has a shift of the center position due to a crystal deformation. Remarkably, in the vicinity of the Bragg condition, the shift is enhanced by a factor $ω/Δω$ ($ω$: frequency of an x ray, $Δω$: gap frequency induced by the Bragg reflection). Comparison with the conventional dynamical diffraction theory is also made.

cond-mat.mtrl-sci↗

arXiv2006

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↗

arXiv2006

Electron localization/delocalization in incommensurate helical magnets

The electronic states in incommensurate (IC) helical magnets are studied theoretically from the viewpoint of the localization/delocalization. It is found that in the multi-band system with relativistic spin-orbit interaction, the electronic wavefunctions show both the extended and localized natures along the helical axis depending on the orbital, helical wavenumber, and the direction of the plane on which spins rotate. Possible realization of this localization is discussed.

cond-mat.str-el↗

arXiv2006

Dynamical magneto-electric coupling in helical magnets

Collective mode dynamics of the helical magnets coupled to electric polarization via spin-orbit interaction is studied theoretically. The soft modes associated with the ferroelectricity are not the transverse optical phonons, as expected from the Lyddane-Sachs-Teller relation, but are the spin waves hybridized with the electric polarization. This leads to the Drude-like dielectric function $ε(ω)$ in the limit of zero magnetic anisotropy. There are two more low-lying modes; phason of the spiral and rotation of helical plane along the polarization axis. The roles of these soft modes in the neutron scattering and antiferromagnetic resonance are revealed, and a novel experiment to detect the dynamical magneto-electric coupling is proposed.

cond-mat.str-el↗

arXiv2006

Voltage dependence of Landau-Lifshitz-Gilbert damping of a spin in a current driven tunnel junction

We present a theory of Landau-Lifshitz-Gilbert damping $α$ for a localized spin ${\vec S}$ in the junction coupled to the conduction electrons in both leads under an applied volatege $V$. We find the voltage dependence of the damping term reflecting the energy dependence of the density of states. We find the effect is linear in the voltage and cotrolled by particle-hole asymmetry of the leads.

cond-mat.mes-hall↗

arXiv2005

The generalized equation of motion for the orbital dynamics in the presence of current

The orbital dynamics induced by the charge current is studied theoretically. The equation of motion for the isospin vector $T^A$ in the SU(N) case is derived in the presence of the current, and is applied to the cases of $e_g$ (N=2) and $t_{2g}$ (N=3) orbitals. In spite of the anisotropic transfer integrals between orbitals, the dynamics is found to be isotropic for $e_g$-orbitals similarly to the spin case, while it is anisotropic for $t_{2g}$-orbitals. The implication of this result to the current driven orbital domain wall motion is discussed.

cond-mat.str-el↗

arXiv2005

Optical Magnetoelectric Effect in Multiferroic Materials: Evidence for a Lorentz Force Acting on a Ray of Light

We theoretically propose that the optical analog of a Lorentz force acting on a ray of light is realized in multiferroic materials such as GaFeO$_3$ showing the magnetoelectric effect. The toroidal moment plays the role of a ``vector potential" while its rotation corresponds to a "magnetic field" for photons. Hence, the light is subject to the Lorentz force when propagating through the domain wall region of the ferromagnetic or ferroelectric order. A realistic estimate on the magnitude of this effect is given.

cond-mat.mes-hall↗

arXiv2005

Interfaces of correlated electron systems: Proposed mechanism for colossal electroresistance

Mott's metal-insulator transition at an interface due to band bending is studied by the density matrix renormalization group (DMRG). We show that the result can be recovered by a simple modification of the conventional Poisson's equation approach used in semi-conductor heterojunctions. A novel mechanism of colossal electroresistance is proposed, which incorporates the hysteretic behavior of the transition in higher dimensions.

cond-mat.str-el↗

arXiv2005

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↗

arXiv2005

Gigantic Enhancement of Magneto-Chiral Effect in Photonic Crystals

We theoretically propose a method to enhance dramatically a magneto-chiral(MC) effect by using the photonic crystals composed of a multiferroic material. The MC effect, the directional birefringence even for unpolarized light, is so small that it has been difficult to observe experimentally. Two kinds of periodic structures are investigated; (a) a multilayer and (b) a stripe composed of a magneto-chiral material and air. In both cases, the difference in reflectivity between different magnetization directions is enhanced by a factor of hundreds compared with a bulk material.

cond-mat.mes-hall↗

arXiv2005

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↗

arXiv2005

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↗

arXiv2005

Geometrical aspects of Gigantic Magneto-Electric effect and Quantum Pump

We study the Magneto-Electric (ME) effect from the viewpoint of the Berry phase connection and quantum adiabatic charge transport (QAPT). The linear response theory for the electronic polarization $\vec{P}_{\rm{el}}$ can be interpreted in terms of the flux or the fictitious magnetic field related to the Berry phase in the generalized space which is composed of a crystal momentum and some external parameters. An applied magnetic field modifies the spin configuration, which induces adiabatic deformation of the Bloch wavefunction and results in the electronic polarization given by the total flux penetrating the plaquette spanned by this external parameter and crystal mometum. This provides a new mechanism for the gigantic ME effect. For a cyclic change of the applied magnetic field, even a quantized charge transport is possible.

cond-mat.str-el↗

arXiv2004

Spin current and magneto-electric effect in non-collinear magnets

A new microscopic mechanism of the magneto-electric (ME) effect based on the spin supercurrent is theoretically presented for non-collinear magnets. The close analogy between the superconductors (charge current) and magnets (spin current) is drawn to derive the distribution of the spin supercurrent and the resultant electric polarization. Application to the spiral spin structure is discussed.

cond-mat.str-el↗

arXiv2004

Doping a Mott Insulator: Physics of High Temperature Superconductivity

This article reviews the effort to understand the physics of high temperature superconductors from the point of view of doping a Mott insulator. The basic electronic structure of the cuprates is reviewed, emphasizing the physics of strong correlation and establishing the model of a doped Mott insulator as a starting point. A variety of experiments are discussed, focusing on the region of the phase diagram close to the Mott insulator (the underdoped region) where the behavior is most anomalous. We introduce Anderson's idea of the resonating valence bond (RVB) and argue that it gives a qualitative account of the data. The importance of phase fluctuation is discussed, leading to a theory of the transition temperature which is driven by phase fluctuation and thermal excitation of quasiparticles. We then describe the numerical method of projected wavefunction which turns out to be a very useful technique to implement the strong correlation constraint, and leads to a number of predictions which are in agreement with experiments. The remainder of the paper deals with an analytic treatment of the t-J model, with the goal of putting the RVB idea on a more formal footing. The slave-boson is introduced to enforce the constraint of no double occupation. The implementation of the local constraint leads naturally to gauge theories. We give a rather thorough discussion of the role of gauge theory in describing the spin liquid phase of the undoped Mott insulator. We next describe the extension of the SU(2) formulation to nonzero doping. We show that inclusion of gauge fluctuation provides a reasonable description of the pseudogap phase.

cond-mat.str-el↗

arXiv2004

Topological nature of polarization and charge pumping in ferroelectrics

Electric polarization or transferred charge due to an adiabatic change of external parameters $\vec{Q}$ is expressed in terms of a vector field defined in the $\vec{Q}$ space. This vector field is characterized by strings, i.e., trajectories of band-crossing points. In particular, the transverse component is given by the Biot-Savart law in a nonlocal way. For a cyclic change of $\vec{Q}$ along a loop C, the linking number between this string and C represents the amount of the pumped charge, which is quantized to be an integer as discussed by Thouless.

cond-mat.mtrl-sci↗

arXiv2004

On the stability of U(1) spin liquids in two dimensions

We establish that spin liquids described in terms of gapless fermionic (Dirac) spinons and gapless U(1) gauge fluctuations can be stable in two dimensions, at least when the physical SU(2) spin symmetry is generalized to SU(N). Equivalently, we show that compact QED3 has a deconfined phase for a large number of fermion fields, in the sense that monopole fluctuations can be irrelevant at low energies. A precise characterization is provided by an emergent global topological U(1) symmetry corresponding to the conservation of gauge flux. Beginning with an SU(N) generalization of the S=1/2 square lattice Heisenberg antiferromagnet, we consider the pi-flux spin liquid and, via a systematic analysis of all operators, show that there are NO relevant perturbations (in the renormalization group sense) about the large-N spin liquid fixed point, which is thus a stable phase. We provide a further illustration of this conclusion with an approximate renormalization group calculation that treats the gapless fermions and the monopoles on an equal footing. This approach directly points out some of the flaws in the erroneous "screening" argument for the relevance of monopoles in compact QED3.

cond-mat.str-el↗

arXiv2004

Quantum Versus Jahn-Teller Orbital Physics in YVO$_3$ and LaVO$_3$

We argue that the large Jahn-Teller (JT) distortions in YVO$_3$ and LaVO$_3$ should suppress the quantum orbital fluctuation. The unusual magnetic properties can be well explained based on LDA+$U$ calculations using experimental structures, in terms of the JT orbital. The observed splitting of the spin-wave dispersions for YVO$_3$ in C-type antiferromagnetic state is attributed to the inequivalent VO$_2$ layers in the crystal structure, instead of the ``orbital Peierls state''. Alternative stacking of $ab$-plane exchange couplings produces the c-axis spin-wave splitting, thus the spin system is highly three dimensional rather than quasi-one-dimensional. Similar splitting is also predicted for LaVO$_3$, although it is weak.

cond-mat.str-el↗