Vortex avalanches at one thousandth the superconducting transition temperature

We study the nonequilibrium dynamics of vortex motion at millikelvin temperatures in untwinned single crystals of YBa2Cu3O7-delta Above threshold fields of many tesla, flux jumps appear in the magnetic hysteresis B(H). These jumps correspond to a change of 750 vortices, on average, under our micrometer-sized Bi Hall probes. Not only are the threshold fields large, but they, and the characteristics of the flux jumps themselves, are essentially independent of magnet ramp rate or sample thickness, militating against a thermally triggered instability. Moreover, the threshold differs significantly on the ascending and descending branches of the hysteresis loop. We argue for a dynamical origin for these vortex avalanches and invoke a sandpile analogy to guide our detailed explorations of the H-T plane.