Direct observation of the Mottness and p-d orbital hybridization in epitaxial monolayer α-RuCl3

Abstract α-RuCl3, with abundant studies in its bulk phase, has shown the promising potential to approach the two-dimensional Kitaev honeycomb model and to realize the consequential quantum spin liquids. In this material, some ingredients spark off the hunting of quantum spin liquid states: the localized magnetic moments on each Ru3+ ion guaranteed by the Mottness, the Kitaev-type interaction originating from the superexchange path over the p-d bonds, and the nearly two-dimensional nature of the van der Waals coupled honeycomb layers. Here, we worked out the growth art of α-RuCl3 monolayer on highly oriented pyrolytic graphite substrate for the first time, and then studied its electronic structure, particularly the delicate orbital occupations. Through scanning tunneling microscopy and spectroscopy study, the bonding configurations are justified by the features of pronounced t2g-pπ and eg-pσ hybridization, and the Mott nature is unveiled by an ~ 0.6 eV full gap at the Fermi level located in the t2g-pπ level. Our experimental results agree well with the density functional theory calculations of the monolayer system. In accordance with previous theoretical research, the epitaxial monolayer α-RuCl3 system holds high tunability comparing to its bulk phase and provides a novel platform to explore the Kitaev physics.