Relaxation and Propagation of Nonequilibrium High-Frequency Acoustic Phonons in Thin Crystalline Plates

Abstract

There is an important problem in the low temperature physics of solid state connected with the relaxation of nonequilibrium phonons. In the most practical situations the phonon system is not in the thermal equilibrium (it’s usually so after interaction of the laser light with solid or after excitation by a current pulse). In this case space-frequency distribution function of the occupation numbers of phonons n(t,ω,r) may depends on r or even being spatially homogeneous, may deviate from the well-known Plankian function of ω. The way how the nonequilibrium phonons establish equilibrium temperature in the whole crystal space is one of the most interesting problem in phonon physics. Conditionally the study of this process may be reduced to the following two problems. The first is how does nonequilibrium frequency phonons distribution reach equilibrium one, this process is usually called relaxation. The second is that of propagation of the heat from excited region to cold parts of the crystal (heat pulses, heat conductivity).