Spin-disorder-induced Raman scattering from phonons in europium chalcogenides. II. Theory

Abstract

A general expression for the light-scattering cross section in a magnetic semiconductor is developed in terms of a Raman tensor and a spin-correlation function. This expression is evaluated for the high-temperature, paramagnetic phase of the europium chalcogenides. In particular we show that the cross section can be approximately factored into a term which determines the frequency dependence and the selection rules and a term which determines the scattered spectrum. By use of experimental data for the linear susceptibility and a power expansion in the spin-orbit coupling constant of the $f$ holes the theoretical frequency dependence and the relative ratios of the various scattering components are shown to be in excellent agreement with experiment. In particular, the origin and the dominance of the antisymmetric component in the experimental spectra are explained. In order to calculate the Raman spectrum the phonon dispersion curves for all europium chalcogenides are determined using a shell model with macroscopic input data. The Raman spectrum is then obtained as a weighted one-phonon density of states where the major contribution arises from longitudinal phonons near the $L$ point. The calculated spectra are in good agreement with experiment.