Contribution of superconducting grains and grain boundaries to the magnetoresistance of ceramic YBa2Cu3O7 − δ high-temperature superconductors in weak magnetic fields

Abstract

The current-voltage characteristics of granular YBa2Cu3O6.95 high-temperature superconductor samples have been measured at a temperature of 77.3 K in external transverse magnetic fields Hext with a strength of up to Hext ≈ 500 Oe for low transport current densities (0.1 A/cm2 ≤ j ≤ 0.6 A/cm2). The current-voltage characteristics obtained have been used to construct dependences of the magnetoresistance ρ on the quantities j (ρ(j)Hext=const) and Hext(ρ(Hext)j = const). It has been revealed that the current and field dependences of the magnetoresistance exhibit anomalies at Hext ≥ Hc1g, where Hc1g is the lower critical field of superconducting grains. A comparative analysis of the dependences ρ(j)Hext = const and ρ(Hext)j = const has made it possible to develop concepts regarding the influence of the processes of redistribution of the magnetic field between grain boundaries and superconducting grains on the transport and galvanomagnetic properties of granular high-temperature superconductors. It has been established that the field dependences of the magnetoresistance exhibit specific features associated with the beginning of penetration of Josephson vortices into grain boundaries in the magnetic field Hc1J and with the breaking of a continuous chain of Josephson junctions in the magnetic field Hc2J.