Evidence for two vortex phase transitions in Tl2Ba2CaCu2O8 thin films.

Abstract

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 film of the high-temperature superconductor ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$. Based on electric field as a function of current density E(J) behavior, the temperature dependencies of the resistivity \ensuremath{\rho}(T) and dc magnetization M(T), as well as the frequency dependence of the ac susceptibility ${\mathrm{\ensuremath{\chi}}}_{\mathrm{ac}}$(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, \ensuremath{\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.