Figure 1: photographs of the three studied samples with different bulk-carriers doping levels (n3D) and corresponding energy-momentum intensity maps recorded around Γ point.
The experiments originally aimed at studying the bulk electronic structure of SrTiO3 for several dopings. The different bulk carrier concentrations are evidenced on the photographs (Fig. 1a,b,c) by transparency differences, the most insulating sample being the most transparent. The studies unveiled instead a striking fundamental discovery: the bare surface of this material spontaneously develops a highly-metallic two-dimensional electron gas even in the case of bulk-insulating samples. This is revealed by the band structures measured by high-resolution angle-resolved photoemission (Fig. 1d,e,f), where some bands crossing the Fermi energy EF (i. e. being metallic) are clearly evidenced. Moreover, for each band, the Fermi wave vector kF (i. e. the wave vector at which the band crosses EF) is independent of the bulk doping varying for over 7 decades, and its carrier concentration (directly extracted from kF) of about 0.35 electrons per surface unitary cell is similar to the one found in heterostructures of SrTiO3. This means that this two-dimensional electron gas is a universal characteristic of SrTiO3-based interfaces, irrespective of the different mechanisms that can form it for each type of surface/interface.
The observed bands correspond indeed to quantum-well states from the electrons confined in a nanometric slab beneath the surface. Such a confinement is caused by a strong electric field arising at the surface from the oxygen vacancies created by the sample cleaving under ultra-high vacuum. From the energy separation between the observed bands and elementary quantum mechanics, the spatial extension of the confinement region is deduced to be up to 4-5 atomic layers, again in agreement with the two-dimensional electron gases in other SrTiO3-based interfaces.
Reference:
Two-dimensional electron gas with universal subbands at the surface of SrTiO3,
A.F. Santander-Syro, O. Copie, T. Kondo, F. Fortuna, S. Pailhès, R. Weht, X.G. Qiu, F. Bertran, A. Nicolaou, A. Taleb-Ibrahimi, P. Le Fèvre, G. Herranz, M. Bibes, N. Reyren, Y. Apertet, P. Lecœur, A. Barthélémy and M.J. Rozenberg, Nature 469, 189 (2011).
