Optical and electrical enhancement of flux creep in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ epitaxial films

We report two novel flux-creep-related phenomena in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films at elevated currents in presence of magnetic fields: a sharp onset of nonequilibrium optical response and a current-dependent thermally activated electrical resistivity. In this regime the activation energy of the resistivity scales uniquely with current density as log(${\mathrm{J}}_0$/J) with ${\mathrm{J}}_0$≊3×${10}^6$ A/${\mathrm{Cm}}^2$. This nonlinear current dependence is significantly different from the predictions of the standard flux-creep model. At currents ≲7×${10}^3$ A/${\mathrm{cm}}^2$ the resistivity is current independent and the optical response is purely bolometric.