Impurity-induced quasiparticle transport and universal-limit Wiedemann-Franz violation in d-wave superconductors

Due to the node structure of the gap in a d-wave superconductor, the presence of impurities generates a finite density of quasiparticle excitations at zero temperature. Since these impurity-induced quasiparticles are both generated and scattered by impurities, prior calculations indicate a universal limit (Omega-->0, T-->0) where the transport coefficients obtain scattering-independent values, depending only on the velocity anisotropy nu(f)/nu(2). We improve upon prior results, including the contributions of vertex corrections and Fermi-liquid corrections in our calculations of universal-limit electrical, thermal. and spin conductivity. We find that while vertex corrections modify electrical conductivity and Fermi-liquid corrections renormalize both electrical and spin conductivity, only thermal conductivity maintains its universal value, independent of impurity scattering or Fermi-liquid interactions. Hence, low-temperature thermal conductivity measurements provide the most direct means of obtaining the velocity anisotropy for high-T-c cuprate superconductors.