Measurement of the Exponent $\mu$ in the Low-Temperature Phase of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ Films in a Magnetic Field: Direct Evidence for a Vortex-Glass Phase

Current-voltage ($I-V$) characteristics were measured for $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ 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 $\frac{E}{J\propto \exp }[-{\left(\frac{J_0}{J}\right)}^{\mu }]$, 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 $\mu =0.94\mathrm{}\pm 0.1$ 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.