Observation of metallic surface states in the strongly correlated Kitaev-Heisenberg candidate Na2IrO3

TL;DR

This study uses high-resolution photoemission spectroscopy to discover metallic surface states in Na2IrO3, a strongly correlated insulator, revealing unexpected surface metallicity and challenging existing theoretical predictions.

Contribution

It provides the first experimental evidence of metallic surface states in Na2IrO3, contradicting theoretical expectations of an insulating surface and absence of Dirac cones.

Findings

Metallic surface band crossing the Fermi level

No Dirac cones at the expected surface Brillouin zone points

Surface metallicity observed despite bulk insulating behavior

Abstract

We report high-resolution angle-resolved photoemission spectroscopy measurements on the honeycomb iridate Na2IrO3. Our measurements reveal the existence of a metallic surface band feature crossing the Fermi level with nearly linear dispersion and an estimated surface carrier density of 3.2 $\times$ 10$^{13}$ cm$^{-2}$, which has not been theoretically predicted or experimentally observed, and provides the first evidence for metallic behavior on the boundary of this material, whereas the bulk bands exhibit a robust insulating gap. We further show the lack of theoretically predicted Dirac cones at the $\overline{M}$ points of the surface Brillouin zone, which confirms the absence of a stacked quantum spin Hall phase in this material. Our data indicates that the surface ground state of this material is exotic and metallic, unlike as predicted in theory, and establishes Na2IrO3 as a rare example of a strongly correlated spin-orbit insulator with surface metallicity.