In-plane and out-of-plane transport properties of Bi2Sr2CaCu2O8+x epitaxial films: Fluctuations and transition into a vortex solid state

The in-plane (rho(ab)) and out-of-plane (rho(c)) resistivities of Bi2Sr2CaCu2O8+x films have been simultaneously measured in magnetic fields parallel to the films c axis (B less than or equal to 1 T). The rho(ab)(T,B) and rho(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 rho(ab) and p(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.