Macroscopic character of composite high temperature superconducting wires

TL;DR

This paper investigates the macroscopic properties of composite high-temperature superconducting wires, revealing that despite the d-wave symmetry at the microscopic level, the effective large-scale order often exhibits s-wave symmetry due to the composite structure.

Contribution

It provides a detailed analysis of how macroscopic superconducting order emerges in composite wires, highlighting the conditions favoring s-wave symmetry over d-wave.

Findings

Long-range order in composite wires often has s-wave symmetry.

Josephson coupling mediates phase coherence among grains.

Macroscopic order differs from microscopic pairing symmetry.

Abstract

The "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 ({\it 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.