Random bonds and random fields in two-dimensional orientational glasses

Abstract

Two-dimensional orientational glasses are studied using Monte Carlo simulation. The glasses are modelled using rigid rotors, with fixed positional distribution, chosen using either a random parking algorithm, or by randomly diluting a triangular lattice. The glass-like behaviour has its origin in the coupling, inherent in all anisotropic intermolecular pair potentials, between the rotational degrees of freedom and the quenched random positions. The authors show how this coupling can be broken down into random bond terms that lead to spin glass-like behaviour and random field terms that favour single-particle freezing. The authors study in detail the quadrupolar interaction and illustrate this behaviour by an ad hoc variation of the relative strength of the two types of term. For small random field a rich ground-state structure is observed, with the existence of many nonsymmetry related equivalent orientational states. At finite temperature they observe two temperature regimes for the spin glass order parameter. There is a field dependent regime giving some spin glass-like order out to high temperatures, while at low temperatures a cross-over to a field independent regime is observed, which the authors interpret as cooperative freezing to a low-temperature disordered phase. Comparison is made between their results and experiment, as well as with theoretical models for spin and orientational glasses. Finally they investigate the time dependence of the cooperative freezing, and find behaviour consistent with dynamical freezing, and loss of ergodicity on a finite observation time.