Observation of a truly superconducting phase in the c-axis conduction of Bi2Sr2CaCu2O8+x single crystal in zero magnetic field

Abstract

We report results on isothermal current-voltage (IV) measurements of a Bi2Sr2CaCu2O8+x single crystal taken both in the ab-plane and along the c-axis in zero magnetic field. Decreasing the temperature, a bulk zero-resistance state develops in sequence characterized by three temperatures, T-c(c), T-g, and T-c(ab). In the temperature range of T > T-c(ab), the in-plane IV curves are all ohmic. The out-of-plane IV curves show very diverse behaviors; ohmic in the high-temperature region above T-c(c), highly nonlinear with an ohmic tail consistent with the effect of thermal fluctuations on Josephson coupling between Cu-O layers for T-g < T < T-c(c), and a down-turn behavior in logarithmic scale showing a true superconducting state with zero linear resistance for T < T-g. This down-turn of the c-axis IV curves is reminiscent of the vortex-glass behavior seen in the in-plane resistivities in an external magnetic field and cannot be explained by the conventional interlayer Josephson coupling model with thermal fluctuation effects.