α − RuCl 3 : A spin-orbit assisted Mott insulator on a honeycomb lattice

We examine the role of spin-orbit coupling in the electronic structure of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$, in which Ru ions in $4{d}^{5}$ configuration form a honeycomb lattice. Our x-ray absorption spectroscopy measurements at the Ru $L$ edges exhibit distinct spectral features associated with the presence of substantial spin-orbit coupling, as well as an anomalously large branching ratio. Furthermore the measured optical spectra can be described very well with first-principles electronic structure calculations obtained by taking into account both spin-orbit coupling and electron correlations. We propose that $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ is a spin-orbit…