Phonons in isotopically modified semiconductors and high Tc superconductors

Abstract

Following the fall of the Iron Curtain there is an increasing availability of stable isotopes at affordable prices, a fact which has encouraged the production of single crystals with variable isotopic compositions. These samples have been copiously used to gain information on phonons. Here we briefly review the information obtained. It involves: (1) Dependence of the lattice parameters on isotopic masses. (2) Dependence of anharmonic self-energies on isotopic masses. (3) Thermal conductivity. (4) Dependence of frequencies and widths (self energies) on isotopic disorder. (5) Mass-disorder-induced first order Raman spectra. (6) Renormalization of electronic states through electron–phonon interaction. (7) Determination of phonon eigenvectors from the dependence of frequencies on isotopic mass. Not only elemental but also compound crystals have been studied; in the latter the mass dependence of a given physical property is different for the various constituent elements. Isotope substitution has also been used when one of the constituent atoms is a strong neutron absorber (e.g. 113Cd), to determine phonon dispersion relations by inelastic neutron scattering on crystals in which the absorbing isotope is absent. This work has been recently performed for CdS and CdSe.