MEASUREMENT OF THE EXPONENT-MU IN THE LOW-TEMPERATURE PHASE OF YBA2CU3O7-DELTA FILMS IN A MAGNETIC-FIELD - DIRECT EVIDENCE FOR A VORTEX-GLASS PHASE

Current-voltage (I-V) characteristics were measured for YBa2Cu3O7-delta thin films in high magnetic fields and at temperatures well below the superconducting transition temperature. The functional form of the I-V curves is described by E/J is-proportional-to exp[- (J0/J)mu], with E the electric field and J the current density. The exponent-mu is 0.19 +/- 0.05 for low values of current density and mu = 0.94 +/- 0.1 at high values. The results at low current density are characteristic of a vortex glass and constitute the first direct evidence that the low-temperature phase of a disordered superconductor is a vortex glass.