Vortex avalanches in superconductors visualized by Magneto-Optical Imaging

The vortex diffusion in Type II superconductors is the dominant dissipative mechanism both in transport and in magnetization properties. The strong nonlinear relation between the current density and the electric field, along with nonlocal interactions, determines the complex behavior of the vortex matter, which also includes "catastrophic" phenomena such as vortex avalanches. Here, I present a brief review of the experimental studies devoted to these unstable vortex patterns, starting from the historical findings up to recent works, and focusing on the analysis of the vortex avalanches by means of Magneto-Optical Imaging. This technique enables the observation of the magnetic field distribution in real time, on the whole surface of the superconductor, with microscopic resolution. Hence, both dynamics and morphology of the vortex avalanches are reported in detail, along with theoretical efforts for understanding and for modeling this complex phenomenon. It turns out that vortex avalanches are ubiquitously occurring in superconductors, if certain conditions are satisfied: in particular, temperature, applied magnetic field, and applied field rate are of paramount importance for observing/avoiding these phenomena. It is worthy to note that, together with the interest in fundamental behavior of the vortex matter, this study is instrumental for assessing the limits of usage of superconductors in power applications