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 an d voltage drop at constant injected current versus increasing and decr easing magnetic field, for a paralellipipedic YBa2Cu3O7-delta 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. T he 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 appro priate theoretical approach for explaining this behaviour. A unitary t reatment mode is proposed that reduces the hysteretical dependences on magnetic field of both critical current and voltage to the irreversib le behaviour of the noise rounding parameter characterizing the Joseph son junction. The accord between experimental curves and theoretical r esults supports the idea of representing the superconducting sample as an array of AH-modelled Josephson junctions and also gives the possib ility of making predictions for different parameter values characteriz ing the weak links, such as the normal resistance of a Josephson junct ion and the critical current density in the absence of a magnetic fiel d and thermal fluctuations.