arXiv subjects

Naoto Nagaosa

Publications and source records attributed to Naoto Nagaosa.

At least 271 records · Page 15Linked to original sources

arXiv2008

Current-induced dynamics of spiral magnet

We study the dynamics of the spiral magnet under the charge current by solving the Landau-Lifshitz-Gilbert equation numerically. In the steady state, the current ${\vec j}$ induces (i) the parallel shift of the spiral pattern with velocity $v=(β/α)j$ ($α$, $β$: the Gilbert damping coefficients), (ii) the uniform magnetization $M$ parallel or anti-parallel to the current depending on the chirality of the spiral and the ratio $β/ α$, and (iii) the change in the wavenumber $k$ of the spiral. These are analyzed by the continuum effective theory using the scaling argument, and the various nonequilibrium phenomena such as the chaotic behavior and current-induced annealing are also discussed.

cond-mat.str-el↗

arXiv2008

Nematic and chiral orders for planar spins on triangular lattice

We propose a variant of the antiferromagnetic XY model on the triangular lattice to study the interplay between the chiral and nematic orders in addition to the magnetic order. The model has a significant bi-quadratic interaction of the planar spins. When the bi-quadratic exchange energy dominates, a large temperature window is shown to exist over which the nematic and the chiral orders co-exist without the magnetic order, thus defining a chiral-nematic state. The phase diagram of the model and some of its critical properties are derived by means of the Monte Carlo simulation.

cond-mat.str-el↗

arXiv2008

Field-induced metal-insulator transition and switching phenomenon in correlated insulators

We study the nonequilibrium switching phenomenon associated with the metal-insulator transition under electric field E in correlated insulator by a gauge-covariant Keldysh formalism. Due to the feedback effect of the resistive current I, this occurs as a first-order transition with a hysteresis of I-V characteristics having a lower threshold electric field (\sim 10^4 Vcm^{-1}) much weaker than that for the Zener breakdown. It is also found that the localized mid-gap states introduced by impurities and defects act as hot spots across which the resonant tunneling occurs selectively, which leads to the conductive filamentary paths and reduces the energy cost of the switching function.

cond-mat.other↗

arXiv2008

Quantum Dynamics of Multiferroic Helimagnets: a Schwinger-Boson Approach

We study the quantum dynamics/fluctuation of the cycloidal helical magnet in terms of the Schwinger boson approach. In sharp contrast to the classical fluctuation, the quantum fluctuation is collinear in nature which gives rise to the collinear spin density wave state slightly above the helical cycloidal state as the temperature is lowered. Physical properties such as the reduced elliptic ratio of the spiral, the neutron scattering and infrared absorption spectra are discussed from this viewpoint with the possible relevance to the quasi-one dimensional LiCu$_2$O$_2$ and LiCuVO$_4$.

cond-mat.str-el↗

arXiv2008

Quantum transport theory of anomalous electric, thermoelectric, and thermal Hall effects in ferromagnets

The mechanism of the anomalous Hall transport phenomena, if it is of the intrinsic or extrinsic origin, has been controversial. We present a unified theory of them for ferromagnetic metals with dilute impurities at the zero temperature, in terms of a quantum transport theory with the self-consistent T-matrix approximation. With the Fermi energy E_F and the spin-orbit interaction energy E_{SO} being fixed (E_F > E_{SO}), three regimes are found as a function of the scattering rate \hbar/τ. (i) In the superclean case \hbar/τ< u_{imp} E_{SO}D, the skew scattering from the vertex correction dominates the anomalous Hall conductivity σ_{xy}, where u_{imp} is the impurity potential strength and D is the density of states. With increasing \hbar/τ, this extrinsic skew-scattering contribution rapidly decays. (ii) In the moderately dirty regime u_{imp}E_{SO}D < \hbar/τ< E_{SO}, σ_{xy} is dominated by the intrinsic dissipationless Berry-phase contribution, which is resonantly enhanced to the order of e^2/\hbar when an accidental degeneracy of band dispersions around the Fermi level is lifted by the spin-orbit interaction. (iii) Further increasing \hbar/τ, a σ_{xy}\proptoσ_{xx}^{1.6} scaling appears, which has been verified by recent experiments. The themal and thermoelectric Hall conductivities are also discussed.

cond-mat.mes-hall↗

arXiv2007

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↗

arXiv2007

Chiral spin pairing in helical magnets

The Ginzburg-Landau Hamiltonian for incommensurate frustrated classical spin systems is analyzed. The coupling to phonons through the Dzyaloshinskii-Moriya interaction and/or the four-spin exchange interaction of the Coulomb origin under the egde-sharing network of magnetic and ligand ions drive chiral spin piarings, introducing two successive second-order phase transitions upon cooling. First, a vector spin-chiral order appears with an either parity, leaving an O(2) chiral spin liquid. Then, the O(2) symmetry is broken by the spin ordering into a helical magnetic state. Possible candidate materials are also discussed.

cond-mat.str-el↗

arXiv2007

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↗

arXiv2007

Quantum charge pumping and electric polarization in Anderson insulators

We investigate adiabatic charge pumping in disordered system in one dimension with open and closed boundary conditions. In contrast to the Thouless charge pumping, the system has no gap even though all the states are localized, i.e., strong localization. Charge pumping can be achieved by making a loop adiabatically in the two-dimensional parameter space of the Hamiltonian. It is because there are many $δ$-function-like fluxes distributed over the parameter space with random strength, in sharp contrast to the single $δ$-function in the pure case. This provides a new and more efficient way of charge pumping and polarization.

cond-mat.dis-nn↗

arXiv2007

Spin-polarization coupling in multiferroic transition-metal oxides

A systematic microscopic theory of magnetically induced ferroelectricity and lattice modulation is presented for all electron configurations of Mott-insulating transition-metal oxides. Various mechanisms of polarization are identified in terms of a strong-coupling perturbation theory. Especially, the spin-orbit interaction acting on the ligand p orbitals is shown to give the ferroelectric polarization of the spin-current form, which plays a crucial role particularly in eg systems. Semiquantitative agreements with the multiferroic TbMnO3 are obtained. Predictions for X-ray and neutron scattering experiments are proposed to clarify the microscopic mechanism of the spin-polarization coupling in different materials.

cond-mat.str-el↗

arXiv2006

Bond electronic polarization induced by spin

We study theoretically the electric polarization induced by noncollinear spin configurations in the limit of strong Hund coupling. We employ a model of two magnetic ions sandwitching an oxygen ion. It is shown that there appears a longitudinal polarization P_x along the bond, which is roughly proportional to (m_r^x)^2-(m_l^x)^2 where m_{r(l)}^x is the x-component of the spin orientation vector at right (left) magnetic ion. A numerical study of the model Hamiltonian yields both longitudinal and transverse electric dipole moments. The transverse polarization is shown to have a non-uniform as well as a uniform component, with the latter being consistent with the previous theory. The longitudinal polarization is non-uniform and oscillating with the period half that of the spin order, but the local magnitude is typically much larger than the uniform transverse polarization and may be detected by X-ray/neutron scattering experiments.

cond-mat.str-el↗

arXiv2006

Enhanced dielectric response by disordered nanoscale/mesoscopic insulators

Enhancement of the dielectric response of insulators by disorder is theoretically proposed, where the quantum interference of electronic waves through the nanoscale/mesoscopic system and its change due to external perturbations control the polarization. In the disordered case with all the states being localized, the resonant tunneling, which is topologically protected, plays a crucial role, and enhances the dielectric response by a factor 30~40 compared with the pure case. Realization of this idea with accessible materials/structures is also discussed.

cond-mat.mes-hall↗

arXiv2006

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↗

arXiv2006

Intrinsic vs. extrinsic anomalous Hall effect in ferromagnets

A unified theory of the anomalous Hall effect (AHE) is presented for multi-band ferromagnetic metallic systems with dilute impurities. In the clean limit, the AHE is mostly due to the extrinsic skew-scattering. When the Fermi level is located around anti-crossing of band dispersions split by spin-orbit interaction, the intrinsic AHE to be calculated ab initio is resonantly enhanced by its non-perturbative nature, revealing the extrinsic-to-intrinsic crossover which occurs when the relaxation rate is comparable to the spin-orbit interaction energy.

cond-mat.mes-hall↗

arXiv2006

Antiferromagnetism and singlet formation in underdoped high-Tc cuprates: Implications for superconducting pairing

The extended $t-J$ model is theoretically studied, in the context of hole underdoped cuprates. Based on results obtained by recent numerical studies, we identify the mean field state having both the antiferromagnetic and staggered flux resonating valence bond orders. The random-phase approximation is employed to analyze all the possible collective modes in this mean field state. In the static (Bardeen Cooper Schrieffer) limit justified in the weak coupling regime, we obtain the effective superconducting interaction between the doped holes at the small pockets located around $\bm{k}= (\pm π/2, \pm π/2)$. In contrast to the spin-bag theory, which takes into acccount only the antiferromagnetic order, this effective force is pair breaking for the pairing without the nodes in each of the small hole pocket, and is canceled out to be very small for the $d_{x^2-y^2}$ pairing with nodes which is realized in the real cuprates. Therefore we conclude that no superconducting instability can occur when only the magnetic mechanism is considered. The relations of our work with other approaches are also discussed.

cond-mat.supr-con↗

arXiv2006

Theory of Non-Equilibirum States Driven by Constant Electromagnetic Fields: Non-Commutative Quantum Mechanics in the Keldysh Formalism

We develop a general theory of non-equilibrium states based on the Keldysh formalism, in particular, for charged-particle systems under static uniform electromagnetic fields. The Dyson equation for the uniform stationary state is rewritten in a compact gauge-invariant form by using the Moyal product in the phase space of energy-momentum variables, which originally do not commute in the case of the conventional operator algebra. Expanding the Dyson equation in electromagnetic fields, a systematic method for the order-by-order calculation of linear and non-linear responses from the zeroth-order Green's functions is obtained. In particular, we find that for impurity problems, up to linear order in the electric field, the present approach provides a diagrammatic method for the Stureda formula. This approach also generalizes the semi-classical Boltzmann transport theory to fully quantum-mechanical and/or multi-component systems. In multi-component systems and/or for Hall transport phenomena, however, this quantum Boltzmann transport theory, constructed from the anti-symmetric combination of two different representations for the Dyson equation, does not uniquely specify the non-equilibrium state, but the symmetric combination is required. We demonstate the formalism to calculate longitudinal and Hall electric conductivities in an isotropic single-band electron system in the clean limit. It is found that the results are fully consistent with those obtained by Mott and Ziman in terms of the semi-classical Boltzmann transport theory.

cond-mat.stat-mech↗

arXiv2006

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↗

arXiv2006

Spin Hall effect of conserved current: Conditions for a nonzero spin Hall current

We study the spin Hall effect taking into account the impurity scattering effect as general as possible with the focus on the definition of the spin current. The conserved bulk spin current (Shi et al. [Phys. Rev. Lett. 96, 076604 (2006)]) satisfying the continuity equation of spin is considered in addition to the conventional one defined by the symmetric product of the spin and velocity operators. Conditions for non-zero spin Hall current are clarified. In particular, it is found that (i) the spin Hall current is non-zero in the Rashba model with a finite-range impurity potential, and (ii) the spin Hall current vanishes in the cubic Rashba model with a $δ$-function impurity potential.

cond-mat.mes-hall↗