Effect of anisotropy on universal transport in unconventional superconductors

TL;DR

This paper studies how anisotropy affects universal electronic transport properties in unconventional superconductors with mixed d-wave and s-wave pairing, revealing that anisotropy influences low-temperature conductivity and density of states.

Contribution

It demonstrates that anisotropic Fermi surfaces modify universal transport features in mixed pairing superconductors, and explores the impact on various physical properties.

Findings

Anisotropy alters low-temperature microwave and thermal conductivities.

Small anisotropy suppresses effective scattering rate and zero-frequency density of states.

Large effects are mitigated by negative s-wave gap admixture, aligning with experimental observations.

Abstract

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 suggests that large effects are restored by a negative $s$-component gap admixture.