Dynamics of a Nonlinear Diatomic Chain. III. A Molecular Dynamics Study

Abstract

We investigate the dynamics and the statistical mechanics of a nonlinear diatomic model for a solid which may undergo a displacive structural phase transition (DSPT), using the molecular dynamics (MD) technique. Snapshots of the lattice displacement pattern reveal the presence of kinks at low temperatures. MD collision experiments show that the kinks exhibit soliton-like behaviour. Phonon wave packets are observed to pass through kinks and kinks and anti-kinks interact with one another with little distortion. The MD data are used to calculate the dynamical structure factor for the displacement fields in the diatomic chain. The dynamical structure factor is found to exhibit a central peak when the lattice displacement pattern is dominated by kinks. At small but finite wave vectors the central peak splits producing a new excitation branch in addition to the usual soft-mode phonon side-band. The central peak does not arise from self-consistent phonons but is dependent on the presence of kinks. The height of the peak increases and the width of the peak decreases as the temperature goes to zero.