Peculiarities of the current-voltage characteristics of a Josephson medium in a YBCO high-temperature superconductor

The influence of a weak magnetic field ($H<$ 150 Oe) on the current-voltage (I–U) characteristic of a YBa$_2$Cu$_3$O$_{7-x}$ (YBCO) high-temperature superconductor (HTSC) near the superconducting transition temperature has been studied. It is established that there exist narrow ($<$ 0.2 K) temperature regions where the I–U curve exhibits sharp bending for $H<$ 30 Oe and the ohmic behavior changes to a quadratic dependence of the voltage on current in a region of several milliamperes. At higher temperatures, the I–U curve bending exhibits smearing. This behavior is observed at a temperature below that corresponding to a zero critical current. Above a certain current, the temperature and magnetic field exhibit equivalent effects on the I–U curve of YBCO. Experimental results are explained by a sharp decrease in the critical currents of intergranular Josephson junctions under the action of magnetic field and by the current-induced formation of uncoupled (with respect to the order parameter) superconducting grains. Characteristic currents for the transition of the intergranular Josephson medium into an incoherent state are determined and the first critical fields in YBCO are evaluated.