Transition from activated to diffusive behavior in the vortex-liquid state in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

In the vortex-liquid state in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ there exist two distinct dissipative states separated by a field ${\mathit{H}}_{\mathit{k}}$. Below ${\mathit{H}}_{\mathit{k}}$, the resistivity is strongly field activated, with a prefactor that is anomalously enhanced over the normal-state resistivity ${\mathrm{\rho }}_{\mathit{N}}$ by ∼12 orders of magnitude. At ${\mathit{H}}_{\mathit{k}}$, the prefactor collapses to a temperature-independent value equal to ${\mathrm{\rho }}_{\mathit{N}}$, and the vortex motion becomes diffusive. The existence of the ${\mathit{H}}_{\mathit{k}}$ boundary is also reflected in the anomalous behavior of the Hall resistivity.