Relaxation-Time Ansatz for Quantum Transport Theory: Spin Effects

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Abstract
A relaxation-time ansatz which treats both orbital and spin relaxation in quantum electron transport theory is presented. For orbital relaxation we use an ansatz which conserves both charge and spin density, and which is a modification of the treatment in which the collision processes are assumed to relax the system to a state of instantaneous local thermal equilibrium. For spin relaxation the ansatz conserves only the charge density. The general method yields gauge-invariant charge-conserving results for the response of the system to an electromagnetic perturbation, and yields physically correct results for the spin resonance response of the electronic system. General results are derived for the linear-response part of the one-electron density matrix for the case of a space- and time-varying perturbation corresponding to a single Fourier component. The results are applied to the calculation of the electrical conductivity, the magnetic susceptibility, and the cross section for inelastic light scattering from semiconductor magnetoplasmas.