Resistivity of high-temperature superconductors: Is the vortex state a liquid?

We discuss thermally activated resistivity of strongly layered high-temperature superconductors such as ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ in the limit in which the dominant pinning mechanism is the intrinsic pinning due to the layered structure. From a comparison of our theory with the experimental data we conclude that the vortex state of such superconductors must be disordered over a substantial range of parameters—a melted liquid. The magnetic field, the temperature, and the current dependence of the calculated activation energy are consistent with experiments.