Transition from activated to diffusive behavior in the vortex-liquid state in YBa2Cu3O7

In the vortex-liquid state in YBa2Cu3O7, there exist two distinct dissipative states separated by a field H(k). Below H(k), the resistivity is strongly field activated, with a prefactor that is anomalously enhanced over the normal-state resistivity rho(N) by about 12 orders of magnitude. At H(k), the prefactor collapses to a temperature-independent value equal to rho(N), and the vortex motion becomes diffusive. The existence of the H(k) boundary is also reflected in the anomalous behavior of the Hall resistivity. (Author)