Revelation of Mott insulating state in layered honeycomb lattice Li$_2$RuO$_3$

Abstract

We investigate the role of electron correlation in the electronic structure of honeycomb lattice Li2RuO3 using photoemission spectroscopy and band structure calculations. Monoclinic Li2RuO3 having Ru network as honeycomb lattice undergoes magneto-structural transition at Tc ∼ 540 K from high temperature phase C2/m to low temperature dimerized phase P 21/m. Room temperature valence band photoemission spectra reveal an insulating ground state with no intensity at Fermi level (EF ). Ru 4d band extracted from high and low photon energy valence band photoemission spectra reveal that the surface and bulk electronic structures are very similar in this system. Band structure calculations using generalized gradient approximation (GGA) leads to metallic ground state while screened hybrid (YS-PBE0) functional reveals opening up of a gap in almost degenerate dzx/dyz orbital, whereas dxy orbital is already gapped. Ru 3d core level spectra with prominent unscreened feature provides direct evidence of strong electron correlation among Ru 4d electrons which is also manifested by |E − EF | 2 dependence of spectral density of states (DOS) in the vicinity of EF in the high-resolution spectra, establishing Li2RuO3 as Mott insulator.