Flux penetration in Bi(2212) single crystals at low magnetic fields

Abstract

We report high-resolution measurements of the magnetization for a series of Bi(2212) single crystals during continuous heating up in a low magnetic field after zero-field-cooling. We find a thermally activated behavior for the first flux penetration with a thermal activation energy decreasing logarithmically with increasing field below an internal field of about 150 Oe and being nearly field independent at higher fields. This behavior suggests that thermal activation of pancake vortices across Bean-Livingston barriers at the surface determines the first flux penetration at low fields, crossing over to a flux-flow process determining the flux penetration at higher fields. In the flux-flow regime we can resolve large and irregular jumps in the magnetization, indicative of a self-organized-critical (SOC) state.