Effect of anisotropy on universal transport in unconventional superconductors

We investigate the universal electronic transport for a mixed $d_{x^2-y^2}+s$-wave superconductor in the presence of an anisotropic elliptical Fermi surface. Similar to the universal low-temperature transport predicted in a $d_{x^2-y^2}$-wave superconductor with a circular Fermi surface, anisotropic universal features are found in the low-temperature microwave conductivity and thermal conductivity in the anisotropic system. The effects of anisotropy on the penetration depth, impurity induced $T_c$ suppression, and the zero-frequency density of states are also considered. While a small amount of anisotropy can lead to a strong suppression of the effective scattering rate, and hence the density of states at zero frequency, experimental data suggest that large effects are restored by a negative $s$-component gap admixture.