Electronic Transport in Unconventional Superconductors

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.Comment: To appear in the Proceedings of the 1st International Conference on Quasiclassical Methods in Superconductivity, eds. D. Rainer and J.A. Sauls, Verditz, Austria (1998

Knight Shift in the FFLO State of a Two-Dimensional D-Wave Superconductor

We report on the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in two-dimensional d-wave superconductors with magnetic field parallel to the superconducting planes. This state occurs at high magnetic field near the Pauli-Clogston limit and is a consequence of the competition between the pair condensation and Zeeman energy. We use the quasiclassical theory to self-consistently compute the spatially nonuniform order parameter. Our self-consistent calculations show that the FFLO state of a d-wave order parameter breaks translational symmetry along preferred directions. The orientation of the nodes in real space is pinned by the nodes of the basis function in momentum space. Here, we present results for the Knight shift and discuss the implications for recent nuclear magnetic resonance measurements on CeCoIn5.Comment: 2 pages, 3 figures: LT-24 Conference, Orlando, Aug. 2005; to appear in AIP Conference Proceeding

The Debye-Waller factor of stabilized delta-Pu

The Debye-Waller factor has been calculated for stabilized delta-phase plutonium with 5% aluminum. A quasi-harmonic Born-von Karman force model with temperature dependent phonon frequencies was used to calculate the mean-square thermal atomic displacement from absolute zero to 800 K. Implementation of the observed anomalous softening of the long wavelength phonons with increasing temperature cannot account for the softening of the measured thermal parameter at high temperatures nor for its rather high value at low temperatures. The implications for diffraction measurements on delta-phase stabilized plutonium alloys are discussed.Comment: Presented at the conference Plutonium Futures - The Science 200

High-pressure Debye-Waller and Grueneisen parameters of Au and Cu

The lattice vibrations are determined in the quasi-harmonic approximation for elemental Au and Cu to twice their normal density by first-principles electronic band-structure calculations. It is found for these materials that the important moments of the phonon density of states can be obtained to high accuracy from short-ranged force constant models. We discuss the implications for the Grueneisen parameters on the basis of calculated phonon moments and their approximations by using bulk moduli and Debye-Waller factors.Comment: 4 pages, 2 figures to appear in the proceedings of the 13th APS Topical Conference on Shock Compression of Condensed Matter (scheduled for April 2004

The Resonance Peak in Sr2_2RuO4_4: Signature of Spin Triplet Pairing

We study the dynamical spin susceptibility, $\chi({\bf q}, \omega)$, in the normal and superconducting state of Sr$_2$RuO$_4$. In the normal state, we find a peak in the vicinity of ${\bf Q}_i\simeq (0.72\pi,0.72\pi)$ in agreement with recent inelastic neutron scattering (INS) experiments. We predict that for spin triplet pairing in the superconducting state a {\it resonance peak} appears in the out-of-plane component of $\chi$, but is absent in the in-plane component. In contrast, no resonance peak is expected for spin singlet pairing.Comment: 4 pages, 4 figures, final versio