Collective vortex phases in periodic plus random pinning potential

TL;DR

This paper investigates how combined regular and random pinning potentials influence vortex lattice structures, revealing complex phase transitions and defect formations through analytical and molecular dynamics methods.

Contribution

It introduces a phase diagram for vortex lattice behavior under mixed pinning conditions and characterizes defect formation and disordering processes.

Findings

Disorder can occur in one or two steps depending on pinning strength.

Weak pinning leads to fractal-like vortex chain structures.

Total lattice depinning can happen gradually through intermediate states.

Abstract

We study theoretically the simultaneous effect of a regular and a random pinning potentials on the vortex lattice structure at filling factor of 1. This structure is determined by a competition between the square symmetry of regular pinning array, by the intervortex interaction favoring a triangular symmetry, and by the randomness trying to depin vortices from their regular positions. Both analytical and molecular-dynamics approaches are used. We construct a phase diagram of the system in the plane of regular and random pinning strengths and determine typical vortex lattice defects appearing in the system due to the disorder. We find that the total disordering of the vortex lattice can occur either in one step or in two steps. For instance, in the limit of weak pinning, a square lattice of pinned vortices is destroyed in two steps. First, elastic chains of depinned vortices appear in the film; but the vortex lattice as a whole remains still pinned by the underlying square array of regular pinning sites. These chains are composed into fractal-like structures. In a second step, domains of totally depinned vortices are generated and the vortex lattice depins from regular array.