Evidence for vortex pinning induced by fluctuations in the transition temperature of MgB2 superconductors -: art. no. 132508

The field-dependent critical current density j(c)(B) of a MgB2 bulk sample has been obtained using magnetic measurements. The j(c)(B) 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 B-sb. The temperature dependence of B-sb(T) is in agreement with a model of randomly distributed weak pinning centers via the spatial fluctuations of the transition temperature (deltaT(c) pinning), while pinning due to the mean-free-path fluctuations (deltal pinning) is not observed.