Magnetoresistance of a layered high-T_c superconductor below the Berezinskii-Kosterlitz-Thouless transition

The mechanism of the resistivity of a weakly coupled layered supecocoductor 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 Bi₂Sr₂CaCu₂O_x single crystals at temperatures 70-90 K in a magnetic field H< 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∝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 the 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.