Peculiarities in galvanomagnetic properties of granular YBa2Cu3O7−δ high-temperature superconductors in very weak magnetic fields

Abstract

The character of the evolution of a system of weak links in granular high-temperature superconductors under the action of an external magnetic field Hext has been studied by measuring the current-voltage characteristics \(E{(j)_{{H_{ext}} = const}}\) of YBa2Cu3O7 − δ (δ ≈ 0.05) ceramic samples. The measurements have been performed at T = 77.3 K in a range of very weak magnetic fields 0 < Hext ≲ 0.5Hc2J, where Hc2J is the upper critical field of the Josephson weak links. The results have been used to construct the field dependences of the magnetoresistance Δρ(Hext) of the superconducting ceramics. It has been established that the parameters of the power equation E = A(j − jcJ)ν and the magnetoresistance Δρ are nonmonotonic functions of the external magnetic field. The presence of extrema in the curves A(Hext), jcJ(Hext), ν(Hext), and Δρ(Hext) indicates that different systems of weak links between grain boundaries, which are capable of forming extended Josephson contacts, undergo sequential transitions to a resistive state with an increase in Hext.