MAGNETORESISTANCE OF A LAYERED HIGH-TC SUPERCONDUCTOR BELOW THE BEREZINSKII-KOSTERLITZ-THOULESS TRANSITION

The mechanism of the resistivity of a weakly coupled layered superconductor below the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature T(BKT) in a magnetic field perpendicular to the layers is studied experimentally and theoretically. The resistance of Bi2Sr2CaCu2Ox single crystals at temperatures 70-90 K in a magnetic field H < 2 kOe was measured. At T < T(BKT) in a field H larger than 10-30 Oe the resistance is characterized by the thermoactivated behaviour down to some characteristic temperature T*(H), below which power-law I-V curves V is proportional-to I(aH(T) + 1) were observed. We account for these results by the creation of 2D-vortices due to the breaking of Abrikosov vortex lines by thermal fluctuations and transport current. Calculated dependences of the power index a(H) on temperature and magnetic field agree with the experimental ones. The observed T*(H) dependence is described by the condition that the mean thermal fluctuations of the positions of the 2D-vortices forming flux lines are of the order of a mean distance between flux lines.