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Thermoelectric probe of defect state induced by ionic liquid gating in vanadium dioxide

Thermoelectric measurements detect the asymmetry between the density of states above and below the chemical potential in a material. It provides insights into small variations in the density of states near the chemical potential, complementing electron transport measurements. Here, combined resistance and thermoelectric power measurements are performed on vanadium dioxide (VO2), a prototypical correlated electron material, under ionic-liquid (IL) gating. With IL gating, charge transport below the metal-to-insulator-transition (MIT) temperature remains in the thermally activated regime, while the Seebeck coefficient exhibits an apparent transition from semiconducting to metallic behavior. The contrasting behavior indicates changes in electronic structure upon IL gating, due to the formation of oxygen defect states. The experimental results are corroborated by numerical simulations based on a model density of states incorporating a gating induced defect band. This study reveals thermoelectric measurements to be a convenient and sensitive probe for the role of defect states induced by IL gating in suppressing the MIT in VO2, which remains benign in charge transport measurements, and possibly for studying defect sates in other materials.