Observing the spin-Coulomb drag in spin-valve devices

The Coulomb interaction between electrons of opposite spin orientations in a metal or in a doped semiconductor results in a negative off-diagonal component of the electrical resistivity matrix -- the so-called "spin-drag resistivity". It is generally quite difficult to separate the spin-drag contribution from more conventional mechanisms of resistivity. In this paper I discuss two methods to accomplish this separation in a spin-valve device.Comment: 11 pages, 5 figure

Equilibrium Current and Orbital Magnetization in a Quantum Hall Fluid

We present a general theory for the equilibrium current distribution in an interacting two-dimensional electron gas subjected to a perpendicular magnetic field, and confined by a potential that varies slowly on the scale of the magnetic length. The distribution is found to consist of strips or channels of current, which alternate in direction, and which have universal integrated strength.Comment: 13 pages, Revtex, to appear in the proceedings of the "Workshop on Novel Physics in Low-Dimensional Electron Systems" held in Madra

Spin Coulomb drag beyond the random phase approximation

We study the spin Coulomb drag in a quasi-two-dimensional electron gas beyond the random phase approximation (RPA). We find that the finite transverse width of the electron gas causes a significant reduction of the spin Coulomb drag. This reduction, however, is largely compensated by the enhancement coming from the inclusion of many-body local field effects beyond the RPA, thereby restoring good agreement with the experimental observations by C. P. Weber \textit{et al.}, Nature, \textbf{437}, 1330 (2005).Comment: 3 figures, accepted for publication in Phys. Rev. Let

Bosonization of the two-dimensional electron gas in the lowest Landau level

We develop a bosonization scheme for the collective dynamics of a spinless two-dimensional electron gas (2DEG) in the lowest Landau level. The system is treated as a continuous elastic medium, and quantum commutation relations are imposed between orthogonal components of the elastic displacement field. This theory provides a unified description of bulk and edge excitations of compressible and incompressible phases, and explains the results of recent tunneling experiments at the edge of the 2DEG.Comment: 4 pages, includes 1 figur

Violation of the Wiedemann-Franz law in clean graphene layers

The Wiedemann-Franz law, connecting the electronic thermal conductivity to the electrical conductivity of a disordered metal, is generally found to be well satisfied even when electron-electron (e-e) interactions are strong. In ultra-clean conductors, however, large deviations from the standard form of the law are expected, due to the fact that e-e interactions affect the two conductivities in radically different ways. Thus, the standard Wiedemann-Franz ratio between the thermal and the electric conductivity is reduced by a factor $1+\tau/\tau_{\rm th}^{\rm ee}$, where $1/\tau$ is the momentum relaxation rate, and $1/\tau_{\rm th}^{\rm ee}$ is the relaxation time of the thermal current due to e-e collisions. Here we study the density and temperature dependence of $1/\tau_{\rm th}^{\rm ee}$ in the important case of doped, clean single layers of graphene, which exhibit record-high thermal conductivities. We show that at low temperature $1/\tau_{\rm th}^{\rm ee}$ is $8/5$ of the quasiparticle decay rate. We also show that the many-body renormalization of the thermal Drude weight coincides with that of the Fermi velocity.Comment: 6 pages, 5 appendices (13 pages

Electronic viscosity in a quantum well: A test for the local density approximation

In the local density approximation (LDA) for electronic time-dependent current-density functional theory (TDCDFT) many-body effects are described in terms of the visco-elastic constants of the homogeneous three-dimensional electron gas. In this paper we critically examine the applicability of the three-dimensional LDA to the calculation of the viscous damping of 1-dimensional collective oscillations of angular frequency $\omega$ in a quasi 2-dimensional quantum well. We calculate the effective viscosity $\zeta(\omega)$ from perturbation theory in the screened Coulomb interaction and compare it with the commonly used three-dimensional LDA viscosity $Y(\omega)$. Significant differences are found. At low frequency $Y(\omega)$ is dominated by a shear term, which is absent in $\zeta(\omega)$. At high frequency $\zeta(\omega)$ and $Y(\omega)$ exhibit different power law behaviors ($\omega^{-3}$ and $\omega^{-5/2}$ respectively), reflecting different spectral densities of electron-hole excitations in two and three dimensions. These findings demonstrate the need for better approximations for the exchange-correlation stress tensor in specific systems where the use of the three-dimensional functionals may lead to unphysical results.Comment: 10 pages, 7 figures, RevTex

On the "Causality Paradox" of Time-Dependent Density Functional Theory

I show that the so-called causality paradox of time-dependent density functional theory arises from an incorrect formulation of the variational principle for the time evolution of the density. The correct formulation not only resolves the paradox in real time, but also leads to a new expression for the causal exchange-correlation kernel in terms of Berry curvature. Furthermore, I show that all the results that were previously derived from symmetries of the action functional remain valid in the present formulation. Finally, I develop a model functional theory which explicitly demonstrates the workings of the new formulation.Comment: 21 page

Time-dependent current density functional theory for the linear response of weakly disordered systems

This paper develops a quantitatively accurate first-principles description for the frequency and the linewidth of collective electronic excitations in inhomogeneous weakly disordered systems. A finite linewidth in general has intrinsic and extrinsic sources. At low temperatures and outside the region where electron-phonon interaction occurs, the only intrinsic damping mechanism is provided by electron-electron interaction. This kind of intrinsic damping can be described within time-dependent density-functional theory (TDFT), but one needs to go beyond the adiabatic approximation and include retardation effects. It was shown previously that a density-functional response theory that is local in space but nonlocal in time has to be constructed in terms of the currents, rather than the density. This theory will be reviewed in the first part of this paper. For quantitatively accurate linewidths, extrinsic dissipation mechanisms, such as impurities or disorder, have to be included. In the second part of this paper, we discuss how extrinsic dissipation can be described within the memory function formalism. We first review this formalism for homogeneous systems, and then present a synthesis of TDFT with the memory function formalism for inhomogeneous systems, to account simultaneously for intrinsic and extrinsic damping of collective excitations. As example, we calculate frequencies and linewidths of intersubband plasmons in a 40 nm wide GaAs/AlGaAs quantum well.Comment: 20 pages, 3 figure