QDev Seminar: Boris Spivak – University of Copenhagen

QDev Seminar: Boris Spivak

University of Washington

Macroscopic character of composite  superconductors

“d-wave” symmetry of the superconducting order in the cuprate high temperature superconductors is a well established fact, and one which identifies them as “unconventional.” However, in macroscopic contexts – including many potential applications (i.e. superconducting “wires”) – the material is a composite of randomly oriented superconducting grains in a metallic matrix, in which Josephson coupling between grains mediates the onset of long-range phase coherence. Here, we analyze the physics at length scales large compared to the size of such grains, and in particular the macroscopic character of the long-range order that emerges. While XY-glass order and macroscopic d-wave superconductivity may be possible, we show that under many circumstances – especially when the d-wave superconducting grains are embedded in a metallic matrix – the most likely order has global s-wave symmetry. I also show that magnetic field may enhance superfluid density in the wires. Finally I would like to discuss an anomalous metallic state which exists near a quantum superconductor-metal transition.