RESISTIVE NOISE OF YBCO HIGH-T-C GRANULAR SUPERCONDUCTORS IN MAGNETIC-FIELD

Resistance and resistance-noise experimental results, obtained in the transition region of two YBCO bulk superconductors submitted to a DC m agnetic field are reported. The magnetic field was found to influence only the second stage of the transition, enlarging the transition-temp erature interval and determining a shift of the noise peak towards low er temperatures. The sample is modelled as a percolating network forme d by Josephson junctions and resistance noise is ascribed to the rando m switching of certain junctions (p-noise) [L.B. Kiss et al. Physica C 207(1993)318]. A junction may be enabled to switch either by thermal fluctuations or by random motion of intergranular vortices, The square of the temperature derivative of the resistance was used to control t he thermal-fluctuations effect in producing the noise, The magnetic fi eld is believed to influence the resistance and resistance noise via a parameter p (percentage of phase-locked junctions) and p-noise behavi or is considered an intrinsic property of the network.