Pairing symmetry in cuprate superconductors

TLDR

Pairing symmetry in cuprate superconductors is a key, debated issue with significant fundamental and applied implications. The paper reviews order‑parameter concepts, symmetry breaking, and classification in cuprates. The authors survey non‑phase‑sensitive tests and extensively review phase‑sensitive methods, such as the half‑integer flux‑quantum effect, along with related symmetry‑sensitive experiments. Recent phase‑sensitive and other symmetry‑sensitive tests largely confirm predominantly d‑wave symmetry in optimally doped cuprates.

Abstract

Pairing symmetry in the cuprate superconductors is an important and controversial topic. The recent development of phase-sensitive tests, combined with the refinement of several other symmetry-sensitive techniques, has for the most part settled this controversy in favor of predominantly $d$-wave symmetry for a number of optimally hole- and electron-doped cuprates. This paper begins by reviewing the concepts of the order parameter, symmetry breaking, and symmetry classification in the context of the cuprates. After a brief survey of some of the key non-phase-sensitive tests of pairing symmetry, the authors extensively review the phase-sensitive methods, which use the half-integer flux-quantum effect as an unambiguous signature for $d$-wave pairing symmetry. A number of related symmetry-sensitive experiments are described. The paper concludes with a brief discussion of the implications, both fundamental and applied, of the predominantly $d$-wave pairing symmetry in the cuprates.