Anomalous Resistive Transitions and New Phenomena in Hard Superconductors

Abstract

The value of the critical current-critical field of cold-worked Nb, Mo-Re and Nb-Zr wires at constant temperature is not a single-valued function of the field and current direction. The previous history of a specimen during an experiment plays an important role and a measurement of the critical field-critical current can influence the response of the specimen to subsequent measurements. When $I_c$ vs $H_c$ curves are determined by proceeding from low to high fields, anomalous resistive transitions are observed which may not occur in measurements proceeding from high to low fields. Further, it is possible to condition a specimen to enable it to reach maximum values of the critical field and critical current. This phenomenon is also encountered in the operation of superconducting solenoids. The polarity of the field and current during this treatment is seen to be significant. For a given field, the resistive transition depends on the rate of increase of the current. Quenching of the superconductivity on reducing the field has been observed. Low critical current-critical field curves are observed on rotation of a transverse field in the plane perpendicular to the axis of the wire. The influence of current density and temperature on these phenomena has been investigated.