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

Consistent with various models of quasi-two-dimensional behavior in st rongly anisotropic superconductors, evidence for two separate vortex p hase transitions is observed for an epitaxial thin sim of the high-tem perature superconductor Tl2Ba2CaCu2O8. Based on electric field as a fu nction of current density E(J) behavior, the temperature dependencies of the resistivity rho(T) and de magnetization M(T), as well. as the f requency dependence of the ac susceptibility chi(ac)(f), there is stro ng evidence for a lower-temperature vortex-solid (glass) phase transit ion to a true superconducting state that is characterized by a loss of linear resistivity. In addition, rho(T) and M(T) results provide evid ence for a higher-temperature transition, or crossover, at which the v ortex dissipation processes undergo distinctive changes. This transiti on or crossover may be associated with the coupling of unbound two-dim ensional vortices (pancake-vortex liquid) into a liquid of vortex line s as temperature decreases.