Electronically highly cubic conditions for Ru in α−RuCl3

We studied the local Ru 4d electronic structure of α−RuCl3 by means of polarization-dependent x-ray absorption spectroscopy at the Ru L2,3 edges. We observed a vanishingly small linear dichroism indicating that electronically the Ru 4d local symmetry is highly cubic. Using full multiplet cluster calculations we were able to reproduce the spectra excellently and to extract that the trigonal splitting of the t2g orbitals is −12±10 meV, i.e., negligible as compared to the Ru 4d spin-orbit coupling constant. Consistent with our magnetic circular dichroism measurements, we found that the ratio of the orbital and spin moments is 2.0, the value expected for a Jeff=12 ground state. We have thus shown that as far as the Ru 4d local properties are concerned, α−RuCl3 is an ideal candidate for the realization of Kitaev physics.