The characterization of superconducting materials - conflicts and correlations

A superconducting material can be said to be characterized when sufficient of its properties have been measured for its response to be predicted under any conceivable experimental constraints. A reasonable and straightforward proposition at first sight, but not one that stands up to close inspection. The experimental constraints constitute a moving target, because of increasing demands made on materials and because our conception of possible experimental situations is expanding. Furthermore there are problems in deciding which properties constitute a sufficient set for the description of a material, particularly when the material is inhomogeneous or has one dimension smaller than the characteristic length associated with some material property. The problems and pitfalls in making superconducting measurements on inhomogeneous superconducting materials will be reviewed by reference to recent work on A15 and Nb-Ti conductors. The breadth of information available in comparative studies of inductive transitions and various types of resistive transition will be assessed. The problem of the critical current criterion and its relation to a time dependent voltage-current characteristic will be discussed as will also the measurement and definition of the upper critical field in inhomogeneous superconductors.