Experimental Constraints on the Pairing State of the Cuprate Superconductors: an Emerging Consensus

TL;DR

This paper reviews experimental evidence on the pairing symmetry of cuprate superconductors, mainly YBa2Cu3O7, supporting a d-wave \\dx2y2\\\ pairing state, while also discussing conflicting data and alternative theories.

Contribution

It provides a comprehensive critical analysis of recent experiments confirming the d-wave pairing symmetry in cuprates and discusses inconsistencies and alternative explanations.

Findings

Penetration depth measurements indicate no additional transition near Tc.

Low-temperature data show sign change in the gap function.

Quantum interference experiments support d-wave pairing with nodes.

Abstract

We present a critical discussion of recent experimental probes of the pairing state of the high temperature superconductors, focusing primarily, but not exclusively, on \Yba, where the best data currently exist. Penetration depth measurements near \Tc\ give no indication of an extra transition, indicating that the pairing state is a one-dimensional representation of the crystal symmetry. Penetration depth measurements at low temperatures provide strong evidence for a change in sign of the gap function over the Fermi surface. Quantum mechanical phase interference experiments generally confirm this and in addition show that the nodal positions are consistent with a \dx2y2\ pairing state. This pairing state is consistent with photoemission measurements of the gap function, Raman scattering, the effect on \Tc\ of impurities, and many other data (reviewed by two of us previously) which indicate the presence of low lying excitations in the superconducting state. We also discuss evidence that apparently does not fit in with a \dx2y2\ pairing state, and we describe possible alternative scenarios.