Dynamics of a Nonlinear Diatomic Chain. II. Thermodynamic Properties

Abstract

We compute the free energy density of a nonlinear diatomic model for a solid which may undergo a displacive structural phase transition using (i) a two-component transfer integral operator equation and (ii) an ideal gas phenomenology incorporating the stable solutions of the coupled Euler-Lagrange equations as elementary excitations. The agreement between the two calculations formally establishes that the low temperature excitation spectrum is dominated by both the familiar linearized phonon solutions and by nonlinear domain wall or kink soliton solutions. The ideal gas phenomenology is then used to compute the kink density, order parameter correlation functions, and the kink contribution to the dynamical structure factor. The dynamical structure factor is found to exhibit a central peak.