Linear-polarized soft-x-ray absorption ͑XAS͒ and inelastic neutron scattering ͑INS͒ experiments have been performed on CeMIn 5 with M = Rh, Ir, and Co to determine the crystal-field scheme and characteristic Kondo temperatures T ‫ء‬ for the hybridization between 4f and conduction electrons. The ground-state wave functions are determined from the polarization-dependent soft-XAS data at the cerium M 4,5 edge and the crystal-field splittings from INS. The characteristic temperature T ‫ء‬ has been determined from the line widths of the neutron scattering data. We find that the quasielastic linewidths of the superconducting compounds CeCoIn 5 and CeIrIn 5 are comparable with the low-energy crystal-field splitting.

Neutron scattering measurements have been performed with time-of-flight spectrometers on an YbB 12 polycrystal. The spectra show clearly the presence of magnetic excitations and a gap at around 14 meV. At low temperature, two broad signals (FWHM ∼ 7 meV), one asymmetric, the other one of Lorentzian shape, are well defined. After subtracting the phonon contribution, the former signal is further decomposed into a sharp 15.5 meV peak and a broader 20 meV peak. The 15.5 meV peak behaves similarly to that of the exciton at 14.5 meV for SmB 6 as regards both the Q-and T -dependencies. The 20 meV peak was identified as manifesting a transition from a localized Kondo singlet with a gap to a magnetic state. With decreasing temperature, the gap disappears and the scattering becomes quasi-elastic, like for the metallic Kondo system. The second signal, centred at 37 meV, was assigned previously to a crystal-field excitation. However, the observed behaviour is anomalous, indicating a shift of the intensity from the lower peaks to the higher peak, suggesting a change in the T K -value due to lattice distortion.

We have succeeded in establishing the crystal-field ground state of CeRu2Al10, an orthorhombic intermetallic compound recently identified as a Kondo insulator. Using polarization dependent soft x-ray absorption spectroscopy at the Ce M4,5 edges, together with input from inelastic neutron and magnetic susceptibility experiments, we were able to determine unambiguously the orbital occupation of the 4f shell and to explain quantitatively both the measured magnetic moment along the easy a axis and the small ordered moment along the c-axis. The results provide not only a platform for a realistic modeling of the spin and charge gap of CeRu2Al10, but demonstrate also the potential of soft x-ray absorption spectroscopy to obtain information not easily accessible by neutron techniques for the study of Kondo insulators in general.Comment: submitted to Physical Review B Rapid Communication

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We infer that soft-x-ray absorption spectroscopy is a versatile method for the determination of the crystal-field ground state symmetry of rare earth heavy fermion systems, complementing neutron scattering. Using realistic and universal parameters, we provide a theoretical mapping between the polarization dependence of Ce M(4,5) spectra and the charge distribution of the Ce 4f states. The experimental resolution can be orders of magnitude larger than the 4f crystal-field splitting itself. To demonstrate the experimental feasibility of the method, we investigated CePd2Si2, thereby settling an existing disagreement about its crystal-field ground state.