Electric field dependence of the screening currents in the mixed state of high temperature superconductors investigated by an ac technique

The influence of the electric field on the shielding currents of melt textured YBa2Cu3O7−δ superconductors was investigated by low frequency ac magnetization measurements. The electric field was varied by changing the amplitude as well as the frequency of the applied ac field. These two dependencies were found to be consistent with each other and allowed to develop a method for the derivation of the E(J) characteristic of the superconductor from these measurements. E(J) was found to follow approximately a power law with an exponent, which decreases monotonically with increasing magnetic dc field. The results are compared with dc magnetization measurements, where the corresponding electric field is lower by up to four orders of magnitude. We find convincing agreement between the dc shielding currents and their relaxation and the current–voltage characteristic derived from the ac technique.