Phase transitions and vortex structure evolution in two-level high-temperature granular superconductor YBa2Cu3O7 – δ under temperature and magnetic field

Abstract

Temperature dependences of the resistivity ρ(T) of samples of granular high-temperature superconductor YBa₂Cu₃O_{7 – δ} are measured at various transverse external magnetic fields at 0 < H_ext < 1900 Оe in the temperature range from the upper Josephson critical temperature of “weak bonds” T_c2J to temperatures slightly exceeding the superconducting transition temperature T_c. Based on the data obtained, the behavior of the field dependences of the critical temperatures of superconducting grains and “weak bonds,” and temperature and field dependences of the magnetic contribution to the resistivity \(\left[ {\Delta \rho \left( {T,H} \right) = \rho {{\left( T \right)}_{{H_{ext}} = const}} - \rho {{\left( T \right)}_{{H_{ext}} = 0}}} \right]\). It is shown that the behavior of the magnetic contribution to the resistivity Δρ along the line of the phase transition related to the onset of the magnetic field penetration in the form of Abrikosov vortices into the subsystem of superconducting grains T_c1g(H_ext) is anomalous. The concepts on the magnetic flux redistribution between both subsystems of two-level HTSC near in the vicinity of T_c1g: the Josephson vortex decreases, and the Abrikosov vortex density increases.