Using a molecular-dynamics technique simulating a canonical ensemble with nearly conserved energy, we studied the occurrence and some properties of second sound in terms of the calculated spectral densities. We considered two models: model I exhibiting a ferrodistortive phase, where displacement and energy fluctuations are coupled and third-order anharmonicity is dominant at low temperatures, and model II, where there is no ordered phase, and quartic anharmonicity is present only. Both models exhibit an optical-phonon branch only. Our molecular-dynamics technique makes it possible to study second sound and its damping in terms of a resonance in the appropriate spectral densities. The results confirm the existence of a temperature window, where well-defined second sound occurs. They also suggest that second sound might be a rather usual low-temperature phenomenon, provided the crystal is sufficiently clean.

• Received 10 April 1978

DOI:https://doi.org/10.1103/PhysRevB.18.6468