We have succeeded in establishing the crystal-field ground state of CeRu${}_2$Al${}_{10}$, an orthorhombic intermetallic compound recently identified as a Kondo insulator. Using polarization-dependent soft x-ray absorption spectroscopy at the Ce $M_{4,5}$ 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 CeRu${}_2$Al${}_{10}$, but demonstrate also the potential for soft x-ray absorption spectroscopy to obtain information not easily accessible by neutron techniques for the study of Kondo insulators in general.

• Received 13 June 2012

DOI:https://doi.org/10.1103/PhysRevB.86.081105