An Equilibrium Rotator Glass for Long-Ranged Repulsive Colloidal Rods

Glasses, amorphous solid phases nearly always out of equilibrium, remain poorly understood despite recent progress. Here we show by quantitative real-space experiments and computer simulations the existence of a new equilibrium solid phase which forms due to a subtle interplay between the rotational and translational degrees of freedom in a system of charged colloidal rods. In this rotational glass, the positional coordinates are glassy, while the rotations remain liquid-like. This phase can be reversibly switched into a crystalline solid through a first-order phase transition with minimal particle rearrangements by an external electric field. We speculate that this rotator glass forms due to the anisotropic particle interactions at higher volume fractions, destabilizing the crystal. Finding an equilibrium rotator glass will lead to new insights on how translations and rotations affect phase behavior, including glass formation and, additionally, allow new theoretical approaches to be used to study this amorphous solid.