Orientational glass transition in a rotator model.

Abstract

Orientational dynamics in a simple model of infinitely thin hard needles, initially proposed by Rubinstein and Obukhov [1], are investigated by molecular dynamics simulations. The center-of-mass coordinates of the needles are fixed on a regular fcc lattice. The statics of the system is analytically known, yielding a global stability of the disordered phase. The dynamical properties, however, are nontrivial. As the ratio l of needle length to lattice constant is increased, the time scale for the decay of orientational correlations increases rapidly. A ``computer glass transition'' is observed in the vicinity of l\ensuremath{\approxeq}2.7, with the orientational correlations being frozen-in on the time scale of our simulation. The scenario for this glass transition is very similar to that observed in conventional structural or orientational glasses. The glass transition in the rotator model, however, is necessarily a purely dynamical feature, since the static properties of the system are known to be independent of l. The same glass transition is also studied in a system confined between two parallel plates, for various boundary conditions at the plates. The transition is found to take place for the same value of l as in the bulk, provided the number of possible collision partners near the boundary is the same as in the bulk.