Temperature dependent gap anisotropy in Bi$_{2}$Sr$_{2}$CaCu$_{2}$ O$_{8+x}$ as evidence for a mixed-symmetry ground state

TL;DR

This paper explains the temperature-dependent gap anisotropy in Bi2Sr2CaCu2O8+x using Ginzburg-Landau theory, suggesting a mixed s- and d-wave symmetry of the superconducting order parameter, with implications for anisotropy and specific heat behavior.

Contribution

It introduces a theoretical model combining s- and d-wave components to explain experimental gap anisotropy and predicts associated anisotropic and thermodynamic properties.

Findings

Gap anisotropy varies with temperature and direction.

Orthorhombic anisotropy in the gap is predicted.

Anomalous specific heat behavior below T_c is suggested.

Abstract

In a recent experiment, Ma et al. measured the temperature dependence of the gap anisotropy of oxygen-annealed Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$. Their measurements were taken along the two directions $\Gamma - M$ and $\Gamma - X$. They found that the gap along both directions is nonzero at low temperatures and that the ratio is strongly temperature dependent. We show, using Ginzburg-Landau theory, that this behavior can be obtained if one assumes the existence of s-wave and d-wave components for the order parameter. Our theory predicts orthorhombic anisotropy in the gap and anomalous behavior for the electronic specific heat below T$_c$.