Vibrational spectra and thermal properties of a two-dimensional triangular lattice, where first-neighbor atoms interact with a Lennard-Jones potential, are calculated using both classical molecular dynamics (MD) and leading-order anharmonic perturbation theory (PT). The phonon quasiparticle spectra (QPS), obtained nonperturbatively through MD, depend linearly on $T$ at low temperatures and are in good agreement with the QPS calculated by PT. However, noticeable deviations from the linear $T$ dependence are observed at high $T$, which are attributed to higher order anharmonic effects. We find that when the QPS obtained by leading-order PT are used in the quasiparticle entropy formula, the first-order anharmonic corrections to the thermal properties are correctly generated. Higher order anharmonic corrections are also described, if one uses the nonperturbative QPS obtained from MD. The success of quasiparticle theory is somewhat surprising in light of strong deviations of the thermal conductivity from Peierls-Boltzmann theory, as found in the companion paper.

• Received 5 August 2010

DOI:https://doi.org/10.1103/PhysRevB.82.224304