EFFECT OF ISOTOPIC DISORDER AND MASS ON THE ELECTRONIC AND VIBRONIC PROPERTIES OF 3-DIMENSIONAL, 2-DIMENSIONAL AND ONE-DIMENSIONAL SOLIDS

Most natural elements contain several isotopes. Single crystals grown from them are disordered systems in which the translational invariance is violated by the isotopic mass disorder. This disorder yields perha ps the simplest, and easiest to deal with, partial violation of the tr anslational invariance. Its effects on the vibrational properties of s olids of various dimensions are discussed and illustrated with example s involving germanium and diamond. Such effects are often the stronger the lower the dimensionality. In recent years highly perfect large si ngle crystals of strongly isotopically enriched elements have become a vailable as part of the 'peace dividend'. So far these crystals (mostl y germanium and diamond) have been grown in three-dimensional form. Th eir vibrational frequencies vary rather accurately as the 'average' is otopic mass M. Hence, the average zero-point (i.e. low-temperature) vi brational amplitude varies as M-1/2. This leads to changes in many phy sical properties with average isotopic mass, many of which have been c alculated and measured. Among them we discuss: the lattice constant, t he thermal conductivity, the phonon lifetimes and the electronic gap e nergies and Lorentzian widths. The possibility of doping superlattices and MQW with layers made of different isotopes is also discussed. Exp eriments involving isotopically enriched surfaces and thin films are s uggested.