Modified flux flow in polycrystalline YBCO: temperature and current dependence

DC resistivity of a zero-field-cooled granular YBCO sample has been studied as a function of angle between the applied field and transport current, at various fields, currents and temperatures. In addition to the large angle-independent dissipation, a small angle-dependent part has been observed which has a sin2θ behaviour. This part varies with increasing fields according to a modified flux flow expression, in line with the reported microwave surface resistance data. Interestingly, the exponent, n, of the field dependence obtained from a modified flux flow expression gives values close to the one at low currents and low temperatures, whereas it reduces towards 0.5 for large values of currents and temperatures close to Tc. With increasing current and temperature, the behaviour of the flux flow in YBCO approaches that which has been reported for BSCCO. The behaviour is considered to be due to a decrease of the coherence of vortex segments at higher currents and temperatures. Furthermore, the deviation of the voltage from sin2θ at low angles has also been studied at different currents and temperatures, which further supports our previous conjecture of Lorentz-force-mediated flux redistribution between inter- and intragrain regions. (author)