Internal avalanches in a pile of superconducting vortices

Using an array of miniature Hall probes, we monitored the spatiotemporal variation of the internal magnetic induction in a superconducting niobium sample during a slow sweep of external magnetic field. We found that a sizable fraction of the increase in the local vortex population occurs in abrupt jumps of many scales which propagate over macroscopic distances in the whole sample. The size distribution of these avalanches deviates significantly from a power-law behavior at scales much smaller compared to the system size. At low temperatures and low fields, huge avalanches with a typical size occur and the system does not display a well-defined macroscopic critical current.