OBSERVATION OF THE VORTEX-GLASS TRANSITION IN YBA2CU3O7 BY PHASE-RESOLVED CURRENT-VOLTAGE MEASUREMENTS

We report on investigations of the current-dependent ac impedance Z at 17 Hz of a YBa2Cu3O7 thin-film microbridge in magnetic fields from 1 to 8 T. A phase-sensitive lock-in technique was applied and supplement ed by a pulsed method for high current densities. Above the vortex-gla ss transition temperature T-g a constant Z(j) is observed with low cur rent densities j, and a power-law behavior at high currents. Below T-g , the curvature of the Re Z vs. j curve is negative in a log-log plot. The Re Z(j) isotherms can be collapsed onto two universal scaling fun ctions according to the predictions of the vortex-glass model. Close t o and below T-g, a phase lag between current and voltage appears, whic h saturates to finite values at low j and to zero at high j. The phase lag within the constant-Z(j) region remains below the critical value, predicted theoretically by Dorsey, if T > T-g and approaches 90 degre es at T < T-g indicating the freezing of the vortices into the dissipa tion-free vortex-glass state.