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Two-dimensional electron gas with universal subbands at the surface of SrTiO3

Abstract

As silicon is the basis of conventional electronics, so strontium titanate (SrTiO3) is the foundation of the emerging field of oxide electronics1,2. SrTiO3 is the preferred template for the creation of exotic, two-dimensional (2D) phases of electron matter at oxide interfaces3,4,5 that have metal–insulator transitions6,7, superconductivity8,9 or large negative magnetoresistance10. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs), which is crucial to understanding their remarkable properties11,12, remains elusive. Here we show, using angle-resolved photoemission spectroscopy, that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO3 (including the non-doped insulating material) independently of bulk carrier densities over more than seven decades. This 2DEG is confined within a region of about five unit cells and has a sheet carrier density of 0.33 electrons per square lattice parameter. The electronic structure consists of multiple subbands of heavy and light electrons. The similarity of this 2DEG to those reported in SrTiO3-based heterostructures6,8,13 and field-effect transistors9,14 suggests that different forms of electron confinement at the surface of SrTiO3 lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO3-based devices and a novel means of generating 2DEGs at the surfaces of transition-metal oxides.

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Figure 1: Electronic structure of SrTiO 3 and effects of electron confinement.
Figure 2: Universal electronic structure at the surface of SrTiO3.
Figure 3: Summary of subbands for the 2DEG at the surface of SrTiO3.
Figure 4: Fermi surface of 2DEG at the surface of SrTiO3.

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Acknowledgements

We are grateful to N. Bontemps, R. Claessen, Y. Fagot-Revurat, M. Gabay, I. C. Infante, D. Malterre, A. J. Millis and F. Reinert for discussions, to E. Jacquet for help with the sample preparation and to R. Guerrero for help with the transport measurements. This work was supported by the ANR OXITRONICS and the CNRS-CSIC PICS ‘POSTIT’ project under grant number PICS2008FR1. The Synchrotron Radiation Center, University of Wisconsin-Madison, is supported by the National Science Foundation under award no. DMR-0537588. The Ames Laboratory is operated for the US DOE by Iowa State University under contract number W-7405-ENG-82. R.W. is a research fellow of CONICET-Argentina, supported by CONICET (grant PIP 112-200801-00047) and ANPCyT grant PICT 837/07. X.G.Q. is supported by the MOST and NSF of China, and G.H. is supported by the Spanish Government under project numbers MAT2008-06761-C03 and NANOSELECT CSD2007-00041.

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Contributions

A.F.S.-S. and O.C. contributed equally to this work, from project conception and ARPES measurements to data analysis, interpretation and writing of the manuscript. The contributions of other authors are as follows. Project conception: G.H., M.B., A.B., M.J.R.; ARPES measurements: T.K., F.F., S.P., F.B., A.N.; infrastructure for ARPES experiments at SOLEIL: F.B., A.T.-I., P.L.F.; samples: X.G.Q., G.H., M.B., Y.A., P.L., A.B.; transport measurements: N.R., Y.A.; data analysis, interpretation, slab-LDA calculations: R.W., M.J.R.; input to writing the manuscript: M.J.R. All authors extensively discussed the results and the manuscript.

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Correspondence to A. F. Santander-Syro.

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The file contains Supplementary Text, sections 1-7, Supplementary Table 1, Supplementary Figures 1-6 with legends and additional references. (PDF 1621 kb)

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Santander-Syro, A., Copie, O., Kondo, T. et al. Two-dimensional electron gas with universal subbands at the surface of SrTiO3 . Nature 469, 189–193 (2011). https://doi.org/10.1038/nature09720

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