SURFACE PHONONS IN LAYERED CRYSTALS - THEORETICAL ASPECTS

Layered compounds are characterized by special phenomena such as phase changes, critical phenomena and charge-density waves, in which the phonons play the very important role of fingerprints. Moreover, the surface lattice dynamics of layered compounds provides a deep insight into the structural anisotropy of these technologically important materials. Therefore, it is important to support the large amount of experimental data with a careful theoretical analysis. Due to the presence of large structures such as the ones associated to the charge-density waves, the layered compounds are systems which are still unaccessible to the powerful ab initio methods and are investigated by means of phenomenological models. The theoretical analysis of the surface-phonon modes in several layered crystals is reviewed in this article. The calculations are performed on the basis of both force-constant and bond-charge models for graphite, the multipolar model for TaS2, and with the dispersive linear-chain (DLC) method for GaSe and TaSe2. For all the systems the agreement between experimental and theoretical results is very good.