A Superconducting Instability in the Infinite-U Anderson Lattice in the Presence of Crystal Electric Fields
We report evidence of a superconducting instability (of $T_{1g}$ symmetry) in the infinite-U Anderson lattice in the presence of crystal fields of cubic symmetry. We assume a lattice of $4f$ sites, each with a total angular momentum of $J=5/2$ that is split by crystal fields into a low-lying doublet of $\Gamma_7$ symmetry and an excited quartet of $\Gamma_8$ symmetry. Slave Bosons on the $4f$ sites create and destroy $4f^0$ configurations and Lagrange multipliers at each $4f$ site enforce the occupancy constraint due to the infinite Coulomb repulsion. Quasiparticle interactions are due to exchange of $4f$ density fluctuations, which are represented by fluctuations in the slave Bosons and Lagrange multipliers. We use the so-called analytic tetrahedron method to calculate the dressed (to order 1/N) Boson Green functions. In weak couping, the exchange of the dressed Bosons gives rise to a superconducting instability of $T_{1g}$, $xy(x^2-y^2)$, symmetry. The $A_{1g}$, ``s-wave'', channel has strongly repulsive interactions and hence no pairing instability. The $T_{2g}$ channel exhibits weakly repulsive interactions. Average quasiparticle interactions in the $E_g$, $x^2-y^2$, $3z^2-r^2$, channel fluctuate strongly as a function of the number of tetrahedra used to calculate the Bosonic Green functions,