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M. Reticcioli, Z. Wang, M. Schmid, U. Diebold, M. Setvin, C. Franchini:
"Intrinsic excess charge on polar surfaces: charge density waves, polarons and bipolarons";
Poster: DPG-Frühjahrstagung (DPG Spring Meeting) of the Surface Science Division, SurfaceScience21, virtual; 02.03.2021.

Surfaces of oxide materials hosting excess charge show electronic and chemical properties completely different than the corresponding bulk crystal. Due to uncompensated broken bonds, bulk-terminated polar surfaces are known to intrinsically host excess charge, conventionally assumed to form a metallic two-dimensional electron gas. Here, by investigating the KTaO3(001) polar surface by density functional theory calculations and surface sensitive experiments, we show that excess charge is accommodated more effectively by in-homogeneous distributions, with different degree of charge localization. Our data provide evidence for the coexistence of standing density waves with long periodicity, eventually combined with minor structural distortions, and strongly localized charge in form of small electron polarons and bipolarons (quasi particles originating from the electron-phonon coupling, associated to sharp in-gap states and local distortions). These novel solutions emerging spontaneously on polar surfaces, dominate the electronic and chemical properties of the material, with large impact on the device functionalities (e.g., the effects of bipolarons on the CO adsorption are discussed in a separate session by M. Setvin).