Electrical behavior of YBa2Cu3O7−x grain boundary junctions under low magnetic field

Abstract

The characterization of high temperature superconductor grain boundary junctions under a magnetic field in the mT range is reported. The devices were obtained by patterning narrow stripes in a YBa2Cu3O7−x (YBCO) film deposited on SrTiO3 and MgO symmetric bi-crystal substrates. To allow the investigation of devices having different current density, two grain boundary disorientations were considered: 24° and 45° for SrTiO3 and 24° for MgO. Current-voltage characteristics as a function of the applied field for several temperatures have been collected. The experimental data are discussed on the basis of the electrical parameters obtained for the different substrates. The periodic modulation of the Josephson critical current with the magnetic field indicates that the behavior of YBCO grain boundary junctions can be approximated by the standard overlap junctions model. A comparison between the experimental variation of the critical current with theoretical behavior allows for the determination of the current density distribution in the grain boundary as a function of the ratio of the largest junction dimension L and the Josephson penetration depth λJ. The nature of the barrier and the transport mechanism across the grain boundary is, therefore, investigated; good agreement between the experimental results and the expected behavior of a superconductor-insulator-superconductor structure where the barrier is intrinsically defective is observed.