Driven disordered polymorphic solids: phases and phase transitions, dynamical coexistence and peak effect anomalies

Abstract

We study a simple model for the depinning and driven steady-state phases of a solid tuned across a polymorphic phase transition between ground states of triangular and square symmetry. The competition between the underlying structural phase transition in the pure system and the effects of the underlying disorder, as modified by the drive, stabilizes a variety of unusual dynamical phases. These include pinned states which may have dominantly triangular or square correlations, a plastically flowing liquidlike phase, a moving phase with hexatic correlations, flowing triangular and square states and a dynamic coexistence regime characterized by the complex interconversion of locally square and triangular regions. We locate these phases in a dynamical phase diagram and study them by defining and measuring appropriate order parameters and their correlations. We demonstrate that the apparent power-law orientational correlations we obtain in our moving hexatic phase arise from circularly averaging an orientational correlation function which exhibits long-range order in the (longitudinal) drive direction and short-range order in the transverse direction. This calls previous simulation-based assignments of the driven hexatic glass into question. The intermediate coexistence regime exhibits several distinct properties, including substantial enhancement in the current noise, an unusual power-law spectrum of current fluctuations and striking metastability effects. We show that this noise arises from the fluctuations of the interface separating locally square and triangular ordered regions by demonstrating a correlation between enhanced velocity fluctuations and local coordinations intermediate between the square and triangular. We demonstrate the breakdown of effective "shaking temperature" treatments in the coexistence regime by showing that such shaking temperatures are nonmonotonic functions of the drive in this regime. Finally we discuss the relevance of these simulations to the anomalous behavior seen in the peak effect regime of vortex lines in the disordered mixed phase of type-II superconductors. We propose that this anomalous behavior is directly linked to the behavior exhibited in our simulations in the dynamical coexistence regime thus suggesting a possible solution to the problem of the origin of peak effect anomalies.