EVIDENCE FOR 2 VORTEX PHASE-TRANSITIONS IN TL2BA2CACU2O8 THIN-FILMS

Consistent with various models of quasi-two-dimensional behavior in strongly anisotropic superconductors, evidence for two separate vortex phase transitions is observed for an epitaxial thin sim of the high-temperature superconductor Tl2Ba2CaCu2O8. Based on electric field as a function of current density E(J) behavior, the temperature dependencies of the resistivity rho(T) and de magnetization M(T), as well. as the frequency dependence of the ac susceptibility chi(ac)(f), there is strong evidence for a lower-temperature vortex-solid (glass) phase transition to a true superconducting state that is characterized by a loss of linear resistivity. In addition, rho(T) and M(T) results provide evidence for a higher-temperature transition, or crossover, at which the vortex dissipation processes undergo distinctive changes. This transition or crossover may be associated with the coupling of unbound two-dimensional vortices (pancake-vortex liquid) into a liquid of vortex lines as temperature decreases.