Collective Pinning and Vortex Dynamics in Type II Superconducting Thin Films with Varying Magnetic Field at <i>T</i> → 0

Abstract When a perpendicular magnetic field penetrates a thin slab of a type-II superconductor it produces vortices, with one vortex per flux quantum, h /2 e . The vortices interact repulsively and form an ordered array (Abrikosov lattice) in clean systems, while strong disorder changes the lattice into a vortex glass. The collective vortex dynamics is extremely vulnerable to external perturbations. Consequently, although of great importance, experimental observation is limited. Here we investigate type-II superconducting films (PdBi 2 and NbSe 2 ) 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 H c1 . We propose 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 ∼30 s relaxation after the jump. Our observation enriches the limited experimental evidence on the important topic of real-time vortex dynamics in superconductors.