MAGNETORESISTIVITY MEASUREMENTS IN HTC SUPERCONDUCTORS - LAYERED BEHAVIOR AND VORTEX-GLASS-TO-LIQUID TRANSITION

We present a set of measurements of the resistivity in Bi2Sr2CaCu2Oy as a function of the temperature T at fixed magnetic field H, rho(H)(7), and as a function of H at fixed T, rho(T)(H), at different angles theta between H and the (a,b) plane. In the rho(T)(H) experiments an onset field for the resistivity H* is detectable, and it has the same angular behavior as the upper critical field H(c2) in traditional layered superconductors: H* presents a round or cusp-like maximum when the magnetic field is directed close to the (a,b) plane, depending on the temperature. H* in perpendicular orientation is found to lie on the irreversibility line. Assuming that the irreversibility line is a critical field line for a glass-to-liquid transition, we show that a phenomenological model where the leading dissipative term is the phase slippage of the flux lines over a disordered network of inhomogeneities of the order parameter, with an activation process given by a Vogel-Fulcher equation, can fit all the data from low temperatures up to T(c).