Field-independent relationship between critical current and voltage drop for bulk YBCO superconducting sample in a magnetic field

We present results of experimental measurements of critical current and voltage drop at constant injected current versus increasing and decreasing magnetic field, for a paralellipipedic YBa2Cu3O7-δ sample. The measurements were carried out at 77 K, cycling the field from 0-76 mT and back. The irreversible dependence on the applied field and the analogy between the two types of experimental curves are discussed. The existence of a biunivocal relationship between critical current and voltage drop, irrespective of the increasing or decreasing character of the applied magnetic field, is pointed out. The modified Ambegaokar-Halperin (AH) model for a Josephson junction turns out to be an appropriate theoretical approach for explaining this behaviour. A unitary treatment mode is proposed that reduces the hysteretical dependences on magnetic field of both critical current and voltage to the irreversible behaviour of the noise rounding parameter characterizing the Josephson junction. The accord between experimental curves and theoretical results supports the idea of representing the superconducting sample as an array of AH-modelled Josephson junctions and also gives the possibility of making predictions for different parameter values characterizing the weak links, such as the normal resistance of a Josephson junction and the critical current density in the absence of a magnetic field and thermal fluctuations. (author)