Excess charge on polar surfaces of ionic compounds is commonly described by the two-dimensional electron gas (2DEG) model, a homogeneous distribution of charge, spatially-confined in a few atomic layers. Here, by combining scanning probe microscopy with density functional theory calculations, we show that excess charge on the polar TaO2 termination of KTaO3(001) forms more complex electronic states with different degrees of spatial and electronic localization: charge density waves (CDW) coexist with strongly-localized electron polarons and bipolarons. These surface electronic reconstructions, originating from the combined action of electron-lattice interaction and electronic correlation, are energetically more favorable than the 2DEG solution. They exhibit distinct spectroscopy signals and impact on the surface properties, as manifested by a local suppression of ferroelectric distortions.

Department of Surface and Plasma Science Faculty of Mathematics and Physics Charles University 180 00 Prague 8 Czech Republic

Dipartimento di Fisica e Astronomia Università di Bologna 40127 Bologna Italy

Institute of Applied Physics Technische Universität Wien Vienna Austria

Materials Science and Technology Division Oak Ridge National Laboratory Oak Ridge TN USA

State Key Laboratory for Physical Chemistry of Solid Surfaces Collaborative Innovation Center of Chemistry for Energy Materials and Department of Chemistry College of Chemistry and Chemical Engineering Xiamen University Xiamen China

University of Vienna Faculty of Physics Center for Computational Materials Science Vienna Austria

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