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

TL;DR

This study reveals that layered honeycomb lattice Li$_2$RuO$_3$ exhibits a Mott insulating state driven by strong electron correlations, confirmed through photoemission spectroscopy and advanced band structure calculations.

Contribution

It demonstrates the presence of a Mott insulating state in Li$_2$RuO$_3$ using combined experimental and theoretical approaches, highlighting the role of electron correlations in this material.

Findings

Li$_2$RuO$_3$ is a Mott insulator with strong electron correlations.

Band structure calculations show a gap opening with hybrid functionals.

Surface and bulk electronic structures are similar.

Abstract

We investigate the role of electron correlation in the electronic structure of honeycomb lattice Li$_2$RuO$_3$ using photoemission spectroscopy and band structure calculations. Monoclinic Li$_2$RuO$_3$ having Ru network as honeycomb lattice undergoes magneto-structural transition at T$_c$ $\sim$ 540 K from high temperature phase $C2/m$ to low temperature dimerized phase $P2_1/m$. Room temperature valence band photoemission spectra reveal an insulating ground state with no intensity at Fermi level ($E_F$). Ru 4$d$ 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 $d_{zx}$/$d_{yz}$ orbital, whereas $d_{xy}$ orbital is already gapped. Ru 3$d$ core level spectra with prominent unscreened feature provides direct evidence of strong electron correlation among Ru 4$d$ electrons which is also manifested by $|E-E_F|^2$ dependence of spectral density of states (DOS) in the vicinity of $E_F$ in the high-resolution spectra, establishing Li$_2$RuO$_3$ as Mott insulator.