Thermodynamic approach to quasielastic light scattering from crystals

Abstract

Abstract Nonequilibrium thermodynamics (NETD) is utilized in theanalysis of the spectrum of light scattered quasielastically by crystals. This thermodynamic approach involves: (i) determination fo the equations satisfied by the crystal's thermodynamic variables, taking irreversible processes into account; (ii) solution of such equations for the chosen scattering vector and the relevant crystalline symmetry, and (iii) determination of the spectrum by means of the Onsager hypothesis relating the soluitons obtained and the correlation functions of the same variables. Several applications of the methodoloy are discussed. The ferroelectric KH2PO4 is considered both as a typical crystal exhibiting the standard spectrum consisting of a Rayleigh line and two Brillouin lines and also as a piezoelectric crystal, in which case an additional line a polarization relaxation line–is present in the spectrum. Another relaxation process is also considered: that associated with hopping diffusion in the superionic conductor RbAg4I5. The critical region is also discussed, in the context of the Ornstein-Zernike approximation. The applications illustrate the power and basic simplicity of the NETD approach: in many situations of interest, the parameters that come into play possess transplrent physical meaning and the spectra can be completely determined by trivial algebraic manipulations. This is to be contrasted with the standard phenomenological coupled-mode approch to quasielastic light scattering, invovling more complex manipulations and parameters whose physical meaning is often unclear and which are usually only very indirectly related to quantities measured using other experimental techniques.