Determination of the symmetry of the pair function in superconducting YBCO

Although quite remarkable, the phenomenon of superconductivity is widespread, occurring in roughly half of the metallic elements, thousands of alloys, and even several purely organic charge transfer salts. It is characterized by a zero resistance state, hence its manifestation in a closed ring allows an electric current established in the ring to persist indefinitely. Within the framework of the prevailing theory, it was generally believed that the highest observable critical temperature was in the range of 30-40K. The recent (1987) discovery of superconductivity in a series of cuprate ceramics near 100K, has forced a reconsideration of this long-held idea. In this dissertation, the author describes experiments which he believes demonstrate a fundamental similarity between the conventional superconductor, lead, and one of the high Tc materials, YBa2Cu3O7, (YBCO). These experiments involve the study of a composite ring of the two materials. This ring consists of a single strip of lead attached to opposite sides of a sample of YBCO such that current carried in the ring must cross two junctions during each circuit of the loop. At liquid helium temperature, 4K, each material alone is superconducting, but to establish a persistent current it is necessary for the local regions around the junctions to be superconducting as well, hence the two materials must be, in some sense, compatible. He argues that establishment of a persistent current in this ring and the observation of no measurable suppression of Tc, below that of lead, implies that the pair functions, which govern the superconducting properties in each material, are of the same symmetry