Universal observation of multiple order parameters in cuprate superconductors

TL;DR

This study reveals that cuprate superconductors exhibit multiple order parameters with mixed s+d-wave symmetry in-plane and pure s-wave out-of-plane, indicating a universal intrinsic property.

Contribution

It provides the first direct experimental evidence of multiple order parameters with different symmetries in cuprate superconductors.

Findings

In-plane mbda_a and mbda_b show an inflection point in temperature dependence.

Out-of-plane mbda_c exhibits no inflection point, indicating different symmetry.

Coupled s+d-order parameters are universal in cuprates.

Abstract

The temperature dependence of the London penetration depth \lambda was measured for an untwined single crystal of YBa_2Cu_3O_{7-\delta} along the three principal crystallographic directions (a, b, and c). Both in-plane components (\lambda_a and \lambda_b) show an inflection point in their temperature dependence which is absent in the component along the c-direction (\lambda_c). The data provide convincing evidence that the in-plane superconducting order parameter is a mixture of s+d-wave symmetry whereas it is exclusively s-wave along the c-direction. In conjunction with previous results it is concluded that coupled s+d-order parameters are universal and intrinsic to cuprate superconductors.