Distinct Electronic Structure of the Electrolyte Gate-Induced Conducting Phase in Vanadium Dioxide Revealed by High-Energy Photoelectron Spectroscopy †,^ CarlosE.ViolBarbosa, †,^ JanosKiss, † JaewooJeong, ‡ NagaphaniAetukuri, ‡ MaheshG.Samant, ‡

The development of new phases of matter at oxide interfaces and surfaces by extrinsic electric fields is of considerable significance both scientifically and technologically. Vanadium dioxide (VO2), a strongly correlated material, exhibits a temperature-driven metal-to-insulator transition, which is accompanied by a structural transformation from rutile (high-temperature metallic phase) to monoclinic (low-temperature insulator phase). Recently, it was discovered that a low-temperature conducting state emerges in VO2 thin films upon gating with a liquid electrolyte. Using photoemission spectroscopy measurements of the core and valence band states of electrolyte-gated VO2 thin films, we show that electronicfeaturesinthegate-inducedconductingphasearedistinctfromthoseofthetemperature-induced rutile metallic phase. Moreover, polarization-dependent measurements reveal that the V 3d orbital ordering, which is characteristic of the monoclinic insulating phase, is partially preserved in the gate-induced metallic phase, whereas the thermally induced metallic phase displays no such orbital ordering. Angle-dependent measurements show that the electronic structure of the gate-induced metallic phasepersiststo adepth ofat least ∼40A, theescape depthofthe high-energy photoexcited electronsusedhere. The distinctelectronic structuresof the gate-induced and thermally induced metallic phases in VO2 thin films reflect the distinct mechanisms by which these states originate. The electronic characteristics of the gate-induced metallic state are consistent with the formation of oxygen vacancies from electrolyte gating.