Microscopic Linear Response Theory Research Articles

Anomalous Spin Transport Properties of Gapped Dirac Electrons with Tilting

The anomalous spin transport coefficients of gapped Dirac electrons are studied with application to a quasi-two-dimensional organic conductor $\alpha$-(BETS)$_2$I$_3$ in mind. In the presence of a gap induced by spin-orbit interaction, we show that the effective Hamiltonian is similar to the model considered by Kane and Mele with additional tilting. With this effective Hamiltonian, conductivity tensors up to the linear order of the applied magnetic field are obtained analytically using the microscopic linear response theory or Kubo formula. It is shown that spin Hall conductivity and anomalous diagonal spin conductivity proportional to the magnetic field become nonzero in this system, which are written in terms of the Berry curvature and orbital magnetic moment.The estimated values of spin conductivities using typical parameters turn out to be comparable to the spin Hall conductivity in Pt.

Formation of superheavy nuclei in S36+U238 and Ni64+U238 reactions

We describe the capture, fusion, fission, and evaporation residue formation cross sections of superheavy nuclei within the proposed earlier two-stage dynamical model. The approaching of the projectile nucleus to the target nucleus is described in the first stage of the model. In the second stage, the evolution of the system formed after the touching of the projectile and target nuclei is considered. The evolution of the system on both stages is described by the Langevin equations. The transport coefficients of these equations for the shape degrees of freedom are calculated within the microscopic linear-response theory. The mutual orientation of the colliding ions, the tunneling through the Coulomb barrier in the entrance channel and the shell effects in the potential energy on both stages of the calculations are taking into account. The obtained results are compared with the available experimental data and other theoretical predictions.

Microscopic Linear Response Theory of Spin Relaxation and Relativistic Transport Phenomena in Graphene

We present a unified theoretical framework for the study of spin dynamics and relativistic transport phenomena in disordered two-dimensional Dirac systems with pseudospin-spin coupling. The formalism is applied to the paradigmatic case of graphene with uniform Bychkov-Rashba interaction and shown to capture spin relaxation processes and associated charge-to-spin interconversion phenomena in response to generic external perturbations, including spin density fluctuations and electric fields. A controlled diagrammatic evaluation of the generalized spin susceptibility in the diffusive regime of weak spin-orbit interaction allows us to show that the spin and momentum lifetimes satisfy the standard Dyakonov-Perel relation for both weak (Gaussian) and resonant (unitary) nonmagnetic disorder. Finally, we demonstrate that the spin relaxation rate can be derived in the zero-frequency limit by exploiting the SU(2) covariant conservation laws for the spin observables. Our results set the stage for a fully quantum-mechanical description of spin relaxation in both pristine graphene samples with weak spin-orbit fields and in graphene heterostructures with enhanced spin-orbital effects currently attracting much attention.

Pumping of magnons in a Dzyaloshinskii-Moriya ferromagnet

We formulate a microscopic linear response theory of nonequilibrium magnonic torques and magnon pumping applicable to multiple-magnonic-band uniform ferromagnets with Dzyaloshinskii-Moriya interactions. From the linear response theory, we identify the extrinsic and intrinsic contributions where the latter is expressed via the Berry curvature of magnonic bands. We observe that in the presence of a time-dependent magnetization Dzyaloshinskii-Moriya interactions can act as fictitious electric fields acting on magnons. We study various current responses to this fictitious field and analyze the role of Berry curvature. After identifying the magnon-mediated contribution to the equilibrium Dzyaloshinskii-Moriya interaction, we also establish the Onsager reciprocity between the magnon-mediated torques and heat pumping. We apply our theory to the magnonic heat pumping and torque responses in honeycomb and kagome lattice ferromagnets.

Spin torque and Nernst effects in Dzyaloshinskii-Moriya ferromagnets

We predict that a temperature gradient can induce a magnon-mediated intrinsic torque and a transverse spin current in systems with non-trivial magnon Berry curvature. With the help of a microscopic linear response theory of nonequilibrium magnon-mediated torques and spin currents we identify the interband and intraband components that manifest in ferromagnets with Dzyaloshinskii-Moriya interactions and magnetic textures. To illustrate and assess the importance of such effects, we apply our theory to the magnon-mediated spin Nernst and torque responses in a kagome lattice ferromagnet.

Superfluid spin transport through easy-plane ferromagnetic insulators.

Superfluid spin transport-dissipationless transport of spin-is theoretically studied in a ferromagnetic insulator with easy-plane anisotropy. We consider an open geometry where the spin current is injected into the ferromagnet from one side by a metallic reservoir with a nonequilibrium spin accumulation and ejected into another metallic reservoir located downstream. Spin transport is studied using a combination of magnetoelectric circuit theory, Landau-Lifshitz-Gilbert phenomenology, and microscopic linear-response theory. We discuss how spin superfluidity can be probed in a magnetically mediated negative electron-drag experiment.

Strong‐Coupling Theory for a Polarizable Planar Colloid

AbstractWe propose a strong‐coupling analysis of a polarizable planar interface, in the spirit of a recently introduced Wigner‐Crystal formulation. The system is made up of two moieties: a semi‐infinite medium (z < 0) with permittivity ε ′ while the other half space in z > 0 is occupied by a solution with permittivity ε, and mobile counter‐ions (no added electrolyte). The interface at z = 0 bears a uniform surface charge. The counter‐ion density profile is worked out explicitly for both repulsive and attractive dielectric image cases (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Long‐Range Correlations of the Surface Charge Density Between Electrical Media with Flat and Spherical Interfaces

AbstractWe study the asymptotic long‐range behavior of the time‐dependent correlation function of the surface charge density induced on the interface between two media of distinct dielectric functions which are in thermal equilibrium with one another as well as with the radiated electromagnetic field. We start with a short review which summarizes the results obtained by using quantum and classical descriptions of media, in both non‐retarded and retarded regimes of particle interactions. The classical static result for the flat interface is rederived by using a combination of the microscopic linear response theory and the macroscopic method of electrostatic image charges. The method is then applied to the case of a spherically shaped interface between media (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin dynamics in the strong spin-orbit coupling regime

We study the spin dynamics in a high-mobility two-dimensional electron gas with generic spin-orbit interactions (SOI's). We derive a set of spin-dynamics equations that capture purely exponentially the damped oscillatory spin evolution modes observed in different regimes of SOI strength. Hence we provide a full treatment of the D'yakonov-Perel mechanism by using the microscopic linear-response theory from the weak to the strong SOI limit. We show that the damped oscillatory modes appear when the electron scattering time is larger than half of the spin precession time due to the SOI, in agreement with recent observations. We propose a way to measure the scattering time and the relative strength of Rashba and linear Dresselhaus SOI's based on these modes and optical grating experiments. We discuss the physical interpretation of each of these modes in the context of Rabi oscillation.

Theory of spin-charge-coupled transport in a two-dimensional electron gas with Rashba spin-orbit interactions

We use microscopic linear response theory to derive a set of equations that provide a complete description of coupled spin and charge diffusive transport in a two-dimensional electron gas (2DEG) with the Rashba spin-orbit (SO) interaction. These equations capture a number of interrelated effects including spin accumulation and diffusion, Dyakonov-Perel spin relaxation, magnetoelectric, and spin-galvanic effects. They can be used under very general circumstances to model transport experiments in 2DEG systems that involve either electrical or optical spin injection. We comment on the relationship between these equations and the exact spin and charge density operator equations of motion. As an example of the application of our equations, we consider a simple electrical spin injection experiment and show that a voltage will develop between two ferromagnetic contacts if a spin-polarized current is injected into a 2DEG, that depends on the relative magnetization orientation of the contacts. This voltage is present even when the separation between the contacts is larger than the spin diffusion length.

Polarizability and absorption of small conducting particles in a time-varying electromagnetic field

We study small conducting particles and thin films in an oscillating longitudinal electric field. We find the charge, current, and field distribution in the particle, the polarizability and the electric dipole absorption. We account for Thomas–Fermi screening by adding a Fick's diffusion term to Ohm's law. Alternatively, we describe a particle as a dielectric body with a non-local dielectric constant which is derived in a microscopic linear-response theory. We show that both approaches are equivalent.

Concerning the microscopic linear response theory

We present a procedure for dealing with the fluctuation–dissipation theorem (Kubo) without using Liouville–Neumann phase-space evolution. Thus, our procedure clarifies some objections posed against microscopic linear-response theory. As an application of our method, we revisit the calculation of the electric conductivity, in particular emphasis is made to establish the validity of the Greenwood–Peierls DC-conductivity when compared with the fundamental Kubo's formula.

Schottky barrier tuning at Al/GaAs(100) junctions

Using an ab initio pseudopotential approach, we have investigated the electronic structure of ideal Al/GaAs(100) and Al/Ga1−xAlxAs(100) junctions, and the change of the corresponding Schottky barrier height versus the alloy composition x and in the presence of ultrathin group-IV atom interlayers. We find large changes in the Schottky barrier height which agree well with the experimental data. In order to interpret the observed trends we have analyzed the charge density response to chemical substitutions near the junction. This allowed us to extend to metal/semiconductor interfaces a microscopic linear-response theory approach previously employed to interpret band-offset trends at semiconductor heterojunctions.

On the validity of a hydrodynamic description of laser-driven fusion

The validity of hydrodynamic approximations for non-equilibrium plasmas is examined, with emphasis on applications to laser-driven fusion pellets. Typical density–temperature trajectories of such pellets, as predicted by hydrodynamic calculations in the published literature, are shown to lie sometimes outside the region of validity of hydrodynamic theory, which is an unsatisfactory situation. In view of this, we discuss certain criteria which can be easily used to test the self- consistent validity of any hydrodynamic result. Finally, it is noted that in the low-density, low-temperature plasma region, hydrodynamics initially breaks down because of a breakdown in conventional microscopic linear response theory. A modified microscopic theory is proposed which continues to be valid in this region. This may correspondingly extend the validity of hydrodynamics to plasmas of somewhat lower temperatures and densities.

Nonlinear response to external magnetic fields

The purpose of this paper is to study conditions under which a system of itinerate spin- 1 2 fermions might exhibit a macroscopic linear response to external magnetic fields after long times. Exact expressions are obtained for the nonlinear response of the magnetization and the total energy. We find that for a constant field there is no response (our model contains no mechanism for the relaxation of spins). For an oscillatory field there is a response in which secular terms (in the time) appear which are associated both with nonlinear terms in the external field and with contributions from the background medium. The secular terms involving the magnetic field would not be seen if one used the usual approximations of microscopic linear response theory. They give rise to new conditions which must be satisfied if the system is to exhibit a macroscopic linear response in the long-time limit.