FLUX PINNING AND DISSIPATION IN HIGH-TEMPERATURE SUPERCONDUCTORS

The effect of anisotropy on the field-induced broadening of resistive transitions in the highly-anisotropic high-temperature superconductors (HTS) is considered. For the applied field, H, parallel to the superconducting Cu-O layers the absence of a Lorentz-force, together with intrinsic pinning of the insulating region between layers, leads to an explanation other than flux motion. However, for H parallel to the c-axis, the lack of intrinsic pinning implies that the much greater broadening is due to thermally-activated flux motion. We show experimental evidence that the associated flux motion occurs as a result of a crossover from three dimensional (3D) vortex lines to 2D independent pancake-like vortices, residing in the Cu-O layers. This 3D to 2D crossover occurs when k(B)T exceeds the Josephson coupling energy. For YBa2Cu3O7, this dimensional crossover does not occur below H(c)2, presumably because the conducting Cu-O chains short circuit the Josephson interlayer coupling.