The Depinning Line in Bi-Based High- T c Superconductors

Abstract
We have measured the depinning line of vortices in polycrystalline samples of Bi2Sr2CaCu2O8 by d.c.- and a.c.-susceptibility and vibrating reed methods in a wide temperature, magnetic field and frequency range. The main observed features are consistent with the diffusion character of the flux motion and can be understood with the recently updated theory for thermally assisted flux flow. We provide clear evidence that the depinning observed in the vibrating reed has the same origin as in susceptibility measurements.