Absorptions of Electromagnetic Waves in Electron-Phonon Systems

A study of the absorption of electromagnetic waves in an electron-phonon system is described, for waves whose frequencies are high compared to the collision frequency and whose wavelengths are long compared to the Bohr (Debye) radius. The treatment rests on the introduction of the temperature-dependent Green's function and Kubo's formula for the conductivity. An exact expression for the conductivity, where collective effects are properly taken into account, is obtained, assuming the number of electrons in a Bohr (Debye) sphere is large compared with one. The application of this theory to degenerate semiconductors is suggested.