Superconducting resistive transit ion and vortex-pair excitation of bulk Bi2Sr2CaCu2O8 crystals

Abstract

The measurements of temperature dependence of the resistive transition in Bi2Sr2CaCu2O8 bulk single crystals in external magnetic field up to 7T were made. The starting compounds of Bi2O3, СаСО3, SrCO3, CuO taken in the ratio 2:1:2:2 in metal and 10–30 wt% NaCl–RCl were melted in а Аl2O3 crucible homogenized for 30 hours at 900 °C and then cooled at a rate of 0.5 °C/hour. The samples formed as a parallelepipeds typically with dimensions 5x5x12 mm consisted of thin single crystal plates with the с-axis normal to the plate and disoriented in about three degrees. Resistivity data were taken with a standard ac phase sensitive technique with 0.1–5 mA excitation current at 37 Hz. An external magnetic field was oriented both parallel and perpendicular to the c-axis. The dissipation observed at the superconducting transition is explained quantitatively in terms of the Kosterlitz-Thouless (RT) theory of vortex-antivortex pair excitations within the Cu–O planes. This conclusion is made from the observation of an exponential square-root singularity in the resistivity. The KT coupling parameter, RT phase transition temperature of 82.2 К and mean-field transition T = 84.7 К were obtained from the magnetoresistance data.