SQUID picovoltometry of YBa2Cu3O7 single crystals: Evidence for a finite-temperature phase transition in the high-field vortex state

Using a SQUID picovoltmeter, we have measured current-voltage curves of YBa2Cu3O7 microtwinned crystals as a function of temperature in fields from 1 to 6 T. The data constitute evidence for a finite-temperature phase transition in the vortex state. Exponents derived within the framework of the vortex-glass model are found to be similar to thin-film values. Strongly temperature- dependent correlations involving up to 105 vortices near the transition, combined with the qualitative failure of thermal-activation models to fit our data, support the existence of a phase transition.