Evidence for vortex pinning induced by fluctuations in the transition temperature of (formula presented) superconductors

The field-dependent critical current density (formula presented) bulk sample has been obtained using magnetic measurements. The (formula presented) curves at different temperatures demonstrate a crossover from single vortex pinning to small-bundle vortex pinning, when the field is larger than the crossover field (formula presented) The temperature dependence of (formula presented) is in agreement with a model of randomly distributed weak pinning centers via the spatial fluctuations of the transition temperature (formula presented) pinning), while pinning due to the mean-free-path fluctuations (formula presented) pinning) is not observed. © 2002 The American Physical Society.