Electrons and optic phonons in solids-the effects of longitudinal optical lattice vibrations on the electronic excitations of solids

The properties of charge carriers in solids can be modified greatly by their real and virtual interactions with optical phonons. These effects differ in two particular ways from the changes induced by interactions with acoustic lattice modes. Firstly, those optical modes which interact with carriers are closely monochromatic. Sharply defined structure can therefore appear in many experimental observations, including optical properties, photoconductivity, tunnelling, the magnetoresistance observed under quantum-limit conditions and a range of coupled mode phenomena embracing boht single carriers and plasma modes. Secondly, in materials of a polar nature the coupling of carriers to the optical modes is particularly strong and the resulting scattering probabilities and changes in effective mass can be large. Nevertheless, significant effects produced by the optical modes may be observed in homopolar semiconductors such as silicon and germanium. This article reviews the consequences of the coupling between the carriers and the optical phonons. The theory of such interactions is developed at length and then applied to a wide range of experimental observations in semiconducting and in insulating solids.