Acoustic phonon instabilities and structural phase transitions

摘要: Structural phase transitions are considered in which the order parameter is a homogeneous deformation of crystal. The fluctuations at these acoustic modes, and it shown that an effective Hamiltonian may be constructed describing deformations their fluctuations. There three cases result, those there no with wavelengths less than crystal dimensions, modes have strongly temperature-dependent velocities for wave vectors on particular lines reciprocal space, temperature dependent within planes space. In many cases, expected to first because presence cubic invariants Hamiltonian. continuous, behavior by use renormalization-group theory classical Landau theory, possibility logarithmic corrections one instance. Unfortunately, we unaware any examples this case, but other results accord experimental results.