Evidence for a Kosterlitz-Thouless transition in a simulation ofCD4adsorbed on MgO

A realistic model of a two-dimensional molecular crystal of methane adsorbed on MgO(100) has been studied by molecular dynamics. The low-temperature phase has quasi-long-range order in the molecular orientations, in which there proliferates a dilute gas of vortex-antivortex pairs. Finite-size scaling calculations indicate that an unbinding transition takes place at ${\mathit{T}}_{\mathit{c}}$≊27 K. In addition, the specific heat exhibits a marked anomaly at T≊32 K, in good agreement with the Kosterlitz-Thouless theory.