Composite Spin-Triplet Superconductivity in an $SU(2)\otimes SU(2)$ Symmetric Lattice Model

Abstract

The two-channel Anderson lattice model which has $SU(2)\otimes SU(2)$ symmetry is of relevance to understanding of the magnetic, quadrupolar and superconducting phases in U$_{1-x}$Th$_x$Be$_{13}$ or Pr base skutterudite compounds such as PrFe$_4$P$_{12}$ or PrOs$_4$Sb$_{12}$. Possible unconventional superconducting phases of the model are explored. They are characterized by a composite order parameter comprising of a local magnetic or quadrupolar moment and a triplet conduction electron Cooper-pair. This binding of local degrees of freedom removes the entropy of the non Fermi-liquid normal state. We find superconducting transitions in the intermediate valence regime which are suppressed in the stable moment regime. The gap function is non analytic and odd in frequency: a pseudo-gap develops in the conduction electron density of states which vanishes as $|ω|$ close to $ω=0$. In the strong intermediate valent regime, the gap function acquires an additional $\k$-dependence.