LIGHT-SCATTERING FROM ELECTRON-DENSITY FL UCTUATIONS IN MANY-VALLEY SEMICONDUCTORS AND METALS

A review or basic theoretical concepts in light scattering from free c arriers in semiconductors and metals is presented. Various type-s of e lementary excitations are treated as sources of fluctuations which sca tter the light. They are connected with charge density fluctuations (b oth single particle and plasmon), with scalar and tensor fluctuations of the electron distribution function, with spin density fluctuations, energy density fluctuations, angular momentum density fluctuations. T hese elementary excitations are the same for different solids: metals, semimetals, semiconductors and superconductors. A macroscopic approac h is delivered as an adequate mathematical method which reflects the u niversal nature of Raman spectra of these elementary excitations. Two different mechanisms of relaxation are distinguished in such an approa ch. They are a diffusional mechanism in which a relaxation takes place through diffusion currents of different nature, and the Mandel'shtam- Leoniovich one in which a retarded adiabatic relaxation takes place. T hese mechanisms coexist in many-valley semiconductors giving additive contributions to the inverse lifetime of excitations while only one of them dominates in single valley semiconductors, metals and supercondu ctors. This one depends on the details of the electronic band structur e. There is a possibility to determine by a contactless manner differe nt band structure parameters and kinetic coefficients of electron gas.