Experimental Discovery of a Correlated Oxide Topological Insulator.

Topology has introduced a new paradigm in solid state physics. Materials with non trivial, symmetry-protected topological order form the class of topological insulators, exhibiting robust conductivity at the surface while remaining insulating within the bulk. The entry of scanning probe microscopy and spectroscopy added further insight regarding the expansion and localization of the related conducing surface states, promoting the pursuit of technological applicability of topological phenomena. However, up to now all confirmed topological insulators are either metallic alloys with small band gaps or exhibit small domain sizes and therefore have limited use for real life applications. Here, we present experimental results on the surface of the transition metal oxide Na2IrO3, bridging the fields of topology and strongly correlated materials. Standard description of the electronic structure of Na2IrO3 fails, since spin orbit coupling, band-width, on-site Coulomb repulsion and Hund's coupling are all equally important and typical approximations are invalid. On the one hand, we report on a scanning tunneling spectroscopy study revealing a V-shaped density of states within a bulk band gap comparable to silicon at 300 K. On the other hand, we discuss the up to now not considered general issue of addressability of topologically protected surface states. It offers a natural explanation for the discrepancies in former experimental results regarding this material, providing a coherent picture of topologically protected and trivial conductivity channels in Na2IrO3. Correlated transition metal oxides constitute a promising new class of large band gap topological materials. Avoiding the chemical instability of Na2IrO3 at ambient atmosphere by suitable coating opens up the perspective to realistic technological operability of this 3D topological insulator.