SQUID picovoltometry of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ 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 ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ 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 ${10}^5$ vortices near the transition, combined with the qualitative failure of thermal-activation models to fit our data, support the existence of a phase transition.