Orientation of the ground-state orbital in CeCoIn5 and CeRhIn5

We present core level nonresonant inelastic x-ray scattering (NIXS) data of the heavy-fermion compounds CeCoIn5 and CeRhIn5 measured at the Ce N-4,N-5 edges. The higher than dipole transitions in NIXS allow determining the orientation of the Gamma(7) crystal-field ground-state orbital within the unit cell. The crystal-field parameters of the CeMIn5 compounds and related substitution phase diagrams have been investigated in great detail in the past; however, whether the ground-state wave function is the Gamma(+)(7) ((x2) - y(2)) or Gamma(-)(7) (xy orientation) remained undetermined. We show that the Gamma(-)(7) doublet with lobes along the (110) direction forms the ground state in CeCoIn5 and CeRhIn5. For CeCoIn5, however, we find also some contribution of the first excited state crystal-field state in the ground state due to the stronger hybridization of 4 f and conduction electrons, suggesting a smaller alpha(2) value than originally anticipated from x-ray absorption. A comparison is made to the results of existing density functional theory plus dynamical mean-field theory calculations.