New excitations in systems undergoing displacive antiferrodistortive structural phase transitions?

Abstract

A two-dimensional model system exhibiting antiferrodistortive structural phase transitions is considered. It covers the displacive and order-disorder regime. Using a molecular-dynamics technique, various spectral densities were calculated and new excitation branches were found close to the transition temperature and close to the displacive limit. These new branches appear in addition to the expected phonon resonances. One branch is traced back to the cluster dynamics and the other one to a two-phonon process. At the appropriate wavevectors they collapse into a central peak.