Temperature dependent gap anisotropy in Bi₂Sr₂CaCu₂ O_(8+x) as evidence for a mixed-symmetry ground state

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$_c$.