In-plane and out-of-plane transport properties ofBi2Sr2CaCu2O8+xepitaxial films: Fluctuations and transition into a vortex solid state

Abstract

The in-plane (${\mathrm{\ensuremath{\rho}}}_{\mathrm{ab}}$) and out-of-plane (${\mathrm{\ensuremath{\rho}}}_{\mathrm{c}}$) resistivities of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{x}}$ films have been simultaneously measured in magnetic fields parallel to the films c axis (B\ensuremath{\leqslant}1 T). The ${\mathrm{\ensuremath{\rho}}}_{\mathrm{ab}}$(T,B) and ${\mathrm{\ensuremath{\rho}}}_{\mathrm{c}}$(T,B) curves are well described both above and below the zero-field critical temperature by the fluctuation theory in the Hartree approximation, using a single set of microscopic parameters for both directions. This agreement extends down to temperatures at which the dissipation is usually attributed to vortex motion. In the low-temperature region, however, both ${\mathrm{\ensuremath{\rho}}}_{\mathrm{ab}}$ and ${\mathrm{\ensuremath{\rho}}}_{\mathrm{c}}$ exhibit a thermally activated behavior, and the I-V characteristics change from ohmic to nonohmic. We attribute this effect to the pinning-induced phase transition from a strongly fluctuating normal phase into a superconducting vortex solid phase.