Vortex configurations and dynamics in elliptical pinning sites for high matching fields

TL;DR

This study uses numerical simulations to analyze vortex configurations, dynamics, and melting in elliptical pinning sites at high matching fields, revealing complex behaviors and anisotropic depinning thresholds.

Contribution

It introduces a detailed simulation model of vortex behavior in elliptical pinning arrays, matching experimental observations and exploring high-field effects.

Findings

Strong pinning sites can trap multiple vortices.

Vortex saturation increases with applied field.

Interstitial vortices melt at lower temperatures than pinned vortices.

Abstract

Using numerical simulations we study the configurations, dynamics, and melting properties of vortex lattices interacting with elliptical pinning sites at integer matching fields with as many as 27 vortices per pin. Our pinning model is based on a recently produced experimental system [G. Karapetrov et al., Phys. Rev. Lett. 95, 167002 (2005)], and the vortex configurations we obtain match well with experimental vortex images from the same system. We find that the strong pinning sites capture more than one vortex each, and that the saturation number of vortices residing in a pin increases with applied field due to the pressure from the surrounding vortices. At high matching fields, the vortices in the intestitial regions form a disordered triangular lattice. We measure the depinning thresholds for both the x and y directions, and find distinctive dynamical responses along with highly anisotropic thresholds. For melting of the vortex configurations under zero applied current, we find multi-step melting transitions in which the interstitial vortices melt at a much lower temperature than the pinned vortices. We associate this with signatures in the specific heat.