Voltage asymmetry in the current-voltage characteristics of granular YBa2Cu3O7-delta at very low magnetic fields - art. no. 224511

We study the current-voltage characteristics (CVC's) in granular YBa2Cu3O7- delta under applied magnetic fields with an amplitude and a spatial distrib ution similar to the self-fields generated by the transport currents. By th is way, a proper compensating procedure between both fields has allowed us to obtain CVC's at a very low constant resultant field. To that end, hollow cylinder-shaped samples were axially threaded by an external wire in a cou nter-current setup. The measurement procedure lay in applying a variable ma gnetic field by the external wire compensating the increasing self-field. C VC's in the range 2-20 Oe, even below the sample self-field, were thus obta ined. with voltage spanning three orders of magnitude. A relevant result fr om these measurements is the different voltage values observed for resultan t field distributions of the same root-mean-square volume average but oppos ite azimuthal polarity. This asymmetry may find an explanation in terms of a concentration of the current in between the grains giving an effective ma gnetic field different from the macroscopic held. An expression for the eff ective magnetic field that accounts quantitatively for the data is put forw ard.