Crystal fields in PrOs4Sb12: an inelastic neutron scattering study

We have performed inelastic neutron scattering measurements of the crystal field excitations in the heavy fermion superconductor PrOs4Sb12 using the time-of-flight direct geometry spectrometer LRMECS (IPNS). Following an analysis of the temperature dependence, from 1.8 K to 30 K, of the crystal field excitation at 11 meV, we conclude that the ground state is a Gamma1 singlet. In cubic symmetry, this ground state is normally inconsistent with the observation of two ground state transitions at 0.7 and 11 meV. However, extra crystal field terms required by the Th point group symmetry resolve this discrepancy and give good agreement with our data. We discuss aspherical Coulomb scattering as a possible cause for the enhancement of the superconducting transition temperature in PrOs4Sb12.