Measurement of the exponent μ in the low-temperature phase of YBa2Cu3O7-δ films in a magnetic field: Direct evidence for a vortex-glass phase

Current-voltage (I-V) characteristics were measured for YBa2Cu3O7-δ 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 EJexp[-(J0J)μ], with E the electric field and J the current density. The exponent μ is 0.19 ± 0.05 for low values of current density and μ=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. © 1992 The American Physical Society.