The Crystal Field Potential of PrOs4Sb12: Consequences for   Superconductivity
  The results of inelastic neutron scattering provide a solution for the crystal field level scheme in PrOs4Sb12 in which the ground state in the cubic crystal field potential of Th symmetry is a Gamma_1 singlet. The conduction electron mass enhancement is consistent with inelastic exchange scattering, and we propose that inelastic quadrupolar, or aspherical Coulomb, scattering is responsible for enhancing the superconducting transition temperature. PrOs4Sb12 appears to be the first compound in which aspherical Coulomb scattering is strong enough to overcome magnetic pair-breaking and increase Tc. 
E. A. Goremychkin, R. Osborn, E. D. Bauer, M. B. Maple, N. A.   Frederick, W. M. Yuhasz, F. M. Woodward, and J. W. Lynn