Influence of the crystal field potential on the superconducting properties of PrRu4Sb12

We have determined the crystal field potential of the superconductor PrRu4Sb12 using inelastic neutron scattering. The ground state of the Pr+3 ions in Th point group symmetry is a Gamma1 singlet with a Gamma5 triplet at an energy of Delta5.6meV. Comparison with recent results on the PrOs4Sb12 compound, where and Delta0.6meV, supports the conjecture that inelastic quadrupolar fluctuations (or aspherical Coulomb scattering) are responsible for the strong enhancement of Tc. In PrRu4Sb12, the effect is reduced by the higher energy of the first excited level.