Coexistence of Both Localized Electronic States and Electron Gas at Rutile TiO 2 Surfaces.

Localized electron polarons formed by the coupling of excess electrons and ionic vibrations play a key role in the functionalities of materials. However, the mechanism of the coexistence of delocalized electrons and localized polarons remains underexplored. Here, we report the discovery of high-mobility two-dimensional electron gas at the rutile TiO 2 surface through argon ion irradiation-induced oxygen vacancies. Strikingly, the electron gas forms electron polarons at lower temperatures, resulting in an abrupt metal-insulator transition. Moreover, we find that the low-temperature conductivity in the insulating state is dominated by excess free electrons with a high mobility of ∼10 3 cm 2 V -1 s -1 , whereas the carrier density is dramatically suppressed with decreasing temperature. Remarkably, we reveal that the application of an electric field can lead to a collapse of the localized states, resulting in a metallic state. These results reveal the strongly correlated/coupled nature between the localized electrons and high mobility electrons and offer a new pathway to probe and harvest the exotic electron state at the complex oxide surfaces. This article is protected by copyright. All rights reserved.