Effects of Dissolved Gas on the Superconducting Characteristics of Tantalum

The adding of a metallic impurity to a monatomic superconductor is a standard technique employed in experiments where it is desirable to vary systematically some of the material properties of the specimen, e.g., the mean free path or the penetration depth. While it has long been surmized that the addition of a gaseous impurity would produce comparable effects, there has, to our knowledge, been no systematic work carried out to verify this assumption or to measure the magnitude of the effects produced.We here give a preliminary report on some recent work in which the concentration of a nitrogen contaminant in tantalum has been varied systematically. One effect of the nitrogen contaminant is striking. Very small amounts of gas allow, by a mechanism as yet unknown, for the formation of a "substructure" which permits superconducting inclusions to exist at fields considerably higher than the so-called critical field associated with flux exclusion in the bulk of the specimen. The general behavior of the contaminated specimens is basically the same as that observed in inhomogeneous alloys containing relatively large metallic impurity concentrations. Work is currently in progress to illuminate the basic mechanisms involved in the formation of a substructure in tantalum.