Observation of free flux flow at high dissipation levels in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ epitaxial films

The total resistivity (ρ=E/J not dE/dJ) was measured in epitaxial ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films up to high dissipation levels using a pulsed current source. In the reversible region, ρ(J) has an ssS shape; it is Ohmic at low J (thermally activated free-flux motion), goes through a nonlinear transition region (depinning), and becomes Ohmic again at the highest J (free flux flow, i.e., purely viscous motion). The free-flux-flow resistivity ${\mathrm{\rho }}_{\mathrm{fff}}$ obeys ${\mathrm{\rho }}_{\mathrm{fff}}$/${\mathrm{\rho }}_{\mathit{n}}$≃H/${\mathit{H}}_{\mathit{c}2}$(T), with ${\mathit{dH}}_{\mathit{c}2}$/dT≊2 I/K.