Contribution of superconducting grains and grain boundaries to the magnetoresistance of ceramic YBa_2Cu_3O_(7 - δ) high-temperature superconductors in weak magnetic fields

摘要

The current-voltage characteristics of granular YBa_2Cu_3O_(6.95) high-temperature superconductor samples have been measured at a temperature of 77.3 K in external transverse magnetic fields H_(ext) with a strength of up to H_(ext) ≈ 500 Oe for low transport current densities (0.1 A/cm~2 ≤ j ≤ 0.6 A/cm~2). The current-voltage characteristics obtained have been used to construct dependences of the magnetoresistance ρ on the quantities j (ρ(j)_(Hext=const)) and H_(ext)(ρ(H_(ext))_(j = const)). has been revealed that the current and field dependences of the magnetoresistance exhibit anomalies at H_(ext) ≥ H_(c1g), where H_(c1g) is the lower critical field of superconducting grains. A comparative analysis of the dependences ρ(j)H_(ext = const) and ρ(H_(ext))_(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 H_(c1J) and with the breaking of a continuous chain of Josephson junctions in the magnetic field H_(c2J).