ISOTOPE EFFECTS ON THE ELECTRONIC EXCITATIONS AND PHONONS IN SEMICONDUCTORS

Changes in the average isotopic mass, MBAR, produce a trivial effect in the phonon spectra of monoatomic solids, i.e., the variation of phonon frequencies such as M-1/2BAR. More sophisticated effects also appear. Effects on the frequencies of phonons and electronic excitations due to changes in the mean squared amplitude, <u2>, which also varies like M-1/2BAR, and effects of the mass fluctuations in isotopically mixed crystals are discussed. The latter can be represented by a complex self-energy whose real part corresponds to a frequency shift and its imaginary part to an increase in phonon linewidth.