Collective Pinning and Vortex Dynamics in type 2 superconducting thin films with Varying Magnetic Field

TL;DR

This study investigates vortex dynamics in thin type-II superconducting films using surface acoustic waves, revealing vortex entry and exit instabilities and relaxation phenomena at very low temperatures and slow magnetic field sweeps.

Contribution

It provides experimental evidence of vortex entry/exit instabilities and relaxation processes in superconducting films under varying magnetic fields using SAWs.

Findings

Observation of spikes in SAW attenuation and velocity at specific magnetic fields.

Identification of vortex entry and exit instabilities linked to edge barriers.

Relaxation of vortex configurations over approximately 30 seconds after jumps.

Abstract

A perpendicular magnetic field penetrating a thin type-II superconductor slab produces vortices, with one vortex per flux quantum, h/2e. The vortices interact repulsively and form an ordered array (Abrikosov lattice) in clean systems, while strong disorder changes the lattice into a vortex glass. Here we investigate type-II superconducting films (PdBi2 and NbSe2) with surface acoustic waves (SAWs) at mK temperature. When sweeping the magnetic field at an extremely slow rate, we observe a series of spikes in the attenuation and velocity of the SAW, on average separated in field by approximately Hc1. We suspect the following scenario: The vortex-free region at the edges of the film produces an edge barrier across which the vortices can enter or leave. When the applied field changes, the induced supercurrents flowing along this edge region lowers this barrier until there is an instability. At that point, vortices avalanche into (or out of) the bulk and change the vortex crystal, suggested by the sharp jump in each such spike. The vortices then gradually relax to a new stable pinned configuration, leading to a ~30s relaxation after the jump. Our observation enriches the limited experimental evidence on the important topic of real-time vortex dynamics in superconductors.