Low-temperature current-voltage characteristics of YBa₂Cu₃O_7-δ films in a magnetic field: direct evidence for a vortex-glass phase

Evidence for a vortex-glass phase is found from nonlinear current-voltage (I-V) characteristics measured on YBa2Cu3O7-δ thin films in high magnetic fields and at low temperatures. Well below the superconducting transition temperature the I-V curves are well described by E/J = ρ{variant}0 exp[-(J0/J)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 J are characteristic of a vortex glass and constitute the first direct evidence that the low-temperature state of a disordered superconductor is a vortex glass. The results are at variance with predictions from the theory of collective flux creep. Near the phase transition, both ρ{variant}0 and J0 exhibit critical behaviour. © 1993.