Two-Stage Rotational Disordering of a Molecular Crystal Surface: $C_{60}$

We propose a two-stage mechanism for the rotational surface disordering phase transition of a molecular crystal, as realized in ${\mathrm{C}}_{60}$ fullerite. Our study, based on Monte Carlo simulations, uncovers the existence of a new intermediate regime, between a low-temperature ordered $(2\times 2)$ state, and a high-temperature $(1\times 1)$ disordered phase. In the intermediate regime there is partial disorder, strongest for a subset of particularly frustrated surface molecules. These concepts and calculations provide a coherent understanding of experimental observations, with possible extension to other molecular crystal surfaces.