Elementary Excitations in Heavy-Fermion Superconductors.

A theoretical study is presented of the elementary excitations in heavy-fermion superconductors, under the hypothesis that the Cooper pairs are in a relative p-wave or d-wave. First, all p-wave and d-wave states are classified in the presence of strong spin-orbit coupling and crystal fields, which are important effects in heavy-fermions. It is found that p-wave states generally have either a gap on the entire Fermi surface or a gap with point nodes, as in the axial state, while d-wave states generally have a gap with lines of nodes, as in the polar state. Secondly, the specific heat, ultrasonic attenuation and thermal conductivity that result from the quasiparticle excitations out of such states are calculated. The gapless states of the axial and polar type produce low temperature power laws for these quantities that reflect the density of low-lying quasiparticle excitations. These results are then compared to the experimentally observed power laws in heavy-fermion superconductors. Finally, we study hydrodynamic collective modes associated with the rotational degrees of freedom of the p-wave or d-wave order parameter, called spin-orbit modes. These modes result in anomalous electron paramagnetic resonance and neutron scattering. In addition, they couple to sound propagation and produce anomalous absorption peaks. It is suggested that such a loss mechanism accounts for the sound attenuation peak observed in superconducting UBe _{13}.