Increased transition temperatures in YBa₂Cu₃O_y superconducting ceramics by exposure to nitrogen

Abstract

The discovery of metallic oxide ceramics with superconducting transition temperatures in excess of 77 K (refs 1, 2) has led to an explosive growth of research on these materials. These solids are based on the oxygen-deficient orthorhombic perovskite^3,4 YBa₂Cu₃O_7–δ, in which changes in the value of δ significantly affect the superconducting properties. These changes can be produced by annealing in various environments or by the detailed processing technique during sample manufacture (see, for example, ref. 5). In addition to the effects of oxygen concentration, it is known that the materials are susceptible to atmospheric attack through the combined effects of H₂O and CO₂ absorption. These effects lead to the development of a carbon-rich, insulating surface layer⁶, and subsequent loss of superconducting behaviour. Such degradation effects require that materials be kept in inert conditions at all times. During a recent series of observations of the transport properties of these materials, we discovered that exposure to helium gas at low temperatures causes dramatic changes in the nature of the transition⁷. Here we report the results of a parallel study using nitrogen gas, in which we observed increases of almost 40 K in the transition temperature in a nitrogen environment.