THEORY OF LIGHT SCATTERING FROM ELECTRONIC EXCITATIONS IN SOLIDS

The effect of the periodic potential and the residual Coulomb interaction on the inelastic scattering of light from pure electronic excitations in solids has been investigated by using the diagrammatic perturbation theory. It is shown that for a degenerate electron gas single-particle excitations with no spin reversals are heavily screened and almost all the scattering is from the collective plasma mode for momentum transfers small compared to Fermi-Thomas momentum. However, there is no screening of single-particle spin-density fluctuations. This is an important intra-band process for III-V semiconductors with an appreciable spin-orbit splitting of the valence band. For high concentration of carriers, the nonparabolicity of the band structure also contributes to the intra-band single-particle scattering. For low concentrations or at high temperatures, when plasma mode is no longer a well-defined normal mode of the system, these two processes are not important. In this case there is no screening of the single-particle excitations from electrons in a spherical parabolic band. For inter-band excitations, the effect of Coulomb interaction is shown to be negligible in the long-wavelength limit. © 1969 Società Italiana di Fisica.