Orientation of ground-state orbital in CeCoIn${}_5$ and CeRhIn${}_5$

Abstract

We present core level non-resonant inelastic x-ray scattering (NIXS) data of the heavy fermion compounds CeCoIn${}_5$ and CeRhIn${}_5$ measured at the Ce $N_{4,5}$-edges. The higher than dipole transitions in NIXS allow determining the orientation of the $\hat{I}{“}_7$ crystal-field ground-state orbital within the unit cell. The crystal-field parameters of the Ce$M$In${}_5$ compounds and related substitution phase diagrams have been investigated in great detail in the past; however, whether the ground-state wavefunction is the $\hat{I}{“}_7^{+}$ ($x^2-y^2$) or $\hat{I}{“}_7^{-}$ ($xy$ orientation) remained undetermined. We show that the $\hat{I}{“}_7^{-}$ doublet with lobes along the (110) direction forms the ground state in CeCoIn${}_5$ and CeRhIn${}_5$. For CeCoIn${}_5$, 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 $\hat{I}{\pm }^2$ 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.

6 pages, 4 figures