Anharmonic effects and the lattice dynamics of insulators

Abstract

Anharmonic effects manifest themselves in everyday properties of solids. Two of the more obvious examples are thermal expansion and heat conductivity. More sophisticated examples anharmonicity are provided by the direct measurement of the lifetimes and of the pressure and temperature dependence of modes of vibration solids by infrared, Raman, Brillouin, or neutron spectroscopy. The present article reviews current status of the theory of anharmonic effects in insulators. We will draw our examples almost exclusively from the simplest insulators, namely the fcc rare gas solids (RGS) and alkali-halides (AH) mostly of the rocksalt structure. Much what we have to say is applicable to other kinds solids also, but we shall not dwell on this aspect. We shall arbitrarily exclude perhaps the two most interesting classes of anharmonic solids, that ferroelectrics and quantum solids. These subjects are sufficiently developed as to warrant reviews their own and it would be impossible to do them justice in the space avdable here. No mention will be made of the vast subject of impurity modes which especially in the case of alkali-halides has developed into an autonomous branch of solidstate physics. For similar reasons we have chosen to exclude thermal conductivity attenuation sound and second sound. We are then left with what we feel to be a more manageable task reviewing the basic anharmonic properties of the simplest classes of perfect insulators. Our selection of material reflects our own biased and peculiar interests. Examples wdl be illustrative rather than exhaustive. Previous reviews and basic articles dealing with anharmonic effects are listed separately in the references at the end of this article.