Correlation between the electric-field effect and weak-link type in YBa2Cu3−xOy and YBa2Cu3−xOy /Agx high-Tc superconducting ceramics

An experimental study is reported of the effect of an electric field E⩽120 MV/m and of temperature T on the critical current Ic and I-V characteristics of yttrium-based high-Tc superconducting ceramics. The materials studied were copper-deficient ceramics, YBa2Cu3−xOy (D samples), and YBa2Cu3−xOy/Agx ceramics [S samples with silver present in amounts equal to the copper deficiency (0⩽x⩽0.4)]. It has been established that in D samples at 77 K, the electric field increases Ic and reduces substantially R for I>Ic, whereas in S samples no field effect is observed. Measurements of the Ic(T) dependence near the critical temperature showed that they can be described for all samples by a relation of the type Ic=const(1−T/Tc)α, where α≈1 for the D samples, and α≈2 for the S samples. The results obtained suggest that the electric-field effect correlates with the existence in the ceramic of SIS-type weak links at grain boundaries.