Electronic Transport in Unconventional Superconductors

TL;DR

This paper studies how impurities affect electronic transport in unconventional superconductors at low temperatures, revealing universal behaviors and providing insights into the order parameter symmetry through transport measurements.

Contribution

It presents a theoretical analysis of impurity effects on low-temperature transport in unconventional superconductors, highlighting universal eigenvalues and the role of impurities as probes of the excitation spectrum.

Findings

Universal eigenvalues in impurity-induced states at certain temperature ranges

Recovery of Wiedemann-Franz law deep in the superconducting phase

Impurities serve as sensitive probes of the order parameter symmetry

Abstract

We investigate the electronic transport coefficients in unconventional superconductors at low temperatures, where charge and heat transport are dominated by electron scattering from random lattice defects. We discuss the features of the pairing symmetry, Fermi surface, and excitation spectrum which are reflected in the low temperature heat transport. For temperatures $k_B T \la \gamma \ll \Delta_0$, where $\gamma$ is the bandwidth of impurity induced Andreev states, certain eigenvalues become {\it universal}, i.e., independent of the impurity concentration and phase shift. Deep in the superconducting phase ($k_B T \la \gamma$) the Wiedemann-Franz law, with Sommerfeld's value of the Lorenz number, is recovered. We compare our results for theoretical models of unconventional superconductivity in high-T$_c$ and heavy fermion superconductors with experiment. Our findings show that impurities are a sensitive probe of the low-energy excitation spectrum, and that the zero-temperature limit of the transport coefficients provides an important test of the order parameter symmetry.