Universality in transport processes of unconventional superconductors

Abstract

We show that some of the low temperature transport coefficients (e.g., electrical and thermal conductivities, viscosity and sound attenuation) areunivcrsal, i.e., independent of the impurity concentration and phase shift for specific classes of unconventional superconductors. The existence of a universal limit depends on the symmetry of the order parameter and is achieved at low temperatures κ _B T ≪ γ ⊦ Δ₀, where γ is the bandwidth of the impurity induced Andreev bound states. The density of states is finite at zero energy and leads to the re-appearance of the Wiedemann-Franz law deep in the superconducting phase fork _B T≪γ. Our findings also show that impurity concentration studies at low temperatures can distinguish between different order parameter symmetries.