Observation of the Granular Josephson Mechanism and the Vortex-Glass Transition in the Polycrystalline GdBa2Cu3O7- δ Superconductor

The resistive transition between the normal and superconducting states in high critical temperature superconducting materials is characterized by the occurrence of two well-defined stages: a transition in the higher temperature thermal region called pairing transition and another known as coherence transition. In the presence of low magnetic fields, in the first transition, it is possible to identify a genuinely critical fluctuation regime, with Gaussian regimes. The Aslamazov-Larkin model in the 3D regime allows to analyze the amplitude of the order parameter, leading to the determination of parameters such as coherence length and critical field. In the coherence transition, the phase of the order parameter is the relevant quantity that varies between the grains of the system, leading to Josephson-type effects in the intergranular barriers due to the polycrystalline feature of this family of materials. The coherence transition for the sample GdBa₂Cu₃O_7-δ under the application of seven different electric transport currents was studied in this work. The sample was produced by the solid-state reaction and the resistive behavior was determined to perform the paraconductivity analysis and characterizes granularity effects on the superconducting order parameter. A genuinely critical region characterized by dynamic critical exponent z = 4.5 was identified, similar to the value reported for the transition vortex glass – fluid. This paper proposes to identify three-dimensional Gaussian fluctuations in the paracoherent to get a coherence length associated with Josephson effects in the granular barrier and to find a factor related with the Josephson current in function of the seven applied currents.