  We have measured the crystalline electric field (CEF) excitations of the
CeMIn5 (M = Co, Rh, Ir) series of heavy fermion superconductors by means of
inelastic neutron scattering. Fits to a CEF model reproduce the inelastic
neutron scattering spectra and the high temperature magnetic susceptibility.
The CEF parameters, energy level splittings, and wavefunctions are tabulated
for each member of the CeMIn5 series and compared to each other as well as to
the results of previous measurements. Our results indicate that the CEF level
splitting in all three materials is similar, and can be thought of as being
derived from the cubic parent compound CeIn3 in which an excited state quartet
at ~12 meV is split into two doublets by the lower symmetry of the tetragonal
environment of the CeMIn5 materials. In each case, the CEF excitations are
observed as broad lines in the inelastic neutron scattering spectrum. We
attribute this broadening to Kondo hybridization of the localized f moments
with the conduction electrons. The evolution of the superconducting transition
temperatures in the different members of CeMIn5 can then be understood as a
direct consequence of the strength of this hybridization. Due to the importance
of Kondo spin fluctuations in these materials, we also present calculations
within the non-crossing approximation (NCA) to the Anderson impurity model
including the effect of CEF level splitting for the inelastic neutron
scattering spectra and the magnetic susceptibility.
