Brillouin scattering study of surface effect on the ferroelastic phase transition of KD₃(SeO₃)₂

Abstract

A soft acoustic mode in a ferroelastic KD₃(SeO₃)₂ crystal have been observed as a function of depth from the surface by the use of a microprobe Brillouin scattering system. Just above the phase transition temperature, the Brillouin frequency shift v, which is related to the hyper sound velocity, becomes hard as the observation area closes to the surface. It is concluded that the softening is restricted near the surface. The temperature dependence of v is also investigated at several depths from the surface. The critical exponents of the elastic susceptibility are given as classical values γ = γ′ = 1.0 at the surface in both para-and ferroelastic phases. The transition temperature at the surface, determined as the minimum position of v, shifts to lower side by about 0.1 K.