Temperature dependence of crystal field transitions in PrOs$_4$Sb$_{12}$

PrOs$_4$Sb$_{12}$ has attracted considerable attention as a heavy fermion superconductor, in which quadrupolar fluctuations play an important role in the pairing mechanism. Recently, we determined the crystal field potential using inelastic neutron scattering [Goremychkin \textit{et al} Phys. Rev. Lett. \textbf{93}, 157003 (2004)], and determined that the ground state is a a $\Gamma_1$ singlet. Although this does not favor quadrupolar Kondo models of the superconducting pairing, we have proposed that inelastic quadrupolar, or aspherical Coulomb, scattering of the conduction electrons by the 4f CF levels plays a significant role in enhancing T$_c$ compared to the isoelectronic lanthanum compound. Comparison with the crystal field level scheme in PrRu$_4$Sb$_{12}$ [Goremychkin \textit{et al} Physica B, in press], where T$_c$ has been reduced by praseodymium substitution, is consistent with this model. We report on recent high resolution neutron measurements of the temperature dependence of the transition to the $\Gamma_5$ triplet at 0.63 meV, whose linewidth provides a probe of \textit{s-f} interactions.