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Similar to silicon that is the basis of conventional electronics, strontiumtitanate (SrTiO3) is the bedrock of the emerging field of oxide electronics.SrTiO3 is the preferred template to create exotic two-dimensional (2D) phasesof electron matter at oxide interfaces, exhibiting metal-insulator transitions,superconductivity, or large negative magnetoresistance. However, the physicalnature of the electronic structure underlying these 2D electron gases (2DEGs)remains elusive, although its determination is crucial to understand theirremarkable properties. Here we show, using angle-resolved photoemissionspectroscopy (ARPES), that there is a highly metallic universal 2DEG at thevacuum-cleaved surface of SrTiO3, independent of bulk carrier densities overmore than seven decades, including the undoped insulating material. This 2DEGis confined within a region of ~5 unit cells with a sheet carrier density of~0.35 electrons per a^2 (a is the cubic lattice parameter). We unveil aremarkable electronic structure consisting on multiple subbands of heavy andlight electrons. The similarity of this 2DEG with those reported inSrTiO3-based heterostructures and field-effect transistors suggests thatdifferent forms of electron confinement at the surface of SrTiO3 lead toessentially the same 2DEG. Our discovery provides a model system for the studyof the electronic structure of 2DEGs in SrTiO3-based devices, and a novel routeto generate 2DEGs at surfaces of transition-metal oxides.

Unveiling a two-dimensional electron gas with universal subbands at the surface of SrTiO3