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Electrostatic, polarisation and redox effects in oxide nano-structures

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The interface between LaAlO3 (LAO) and SrTiO3 (STO) has received worldwide attention since the discovery of the interface quasi two-dimensional electron gas (2DEG) in 2004 [1]. It has since been shown that the 2DEG, which can be spin-polarised, superconducting or charge ordered, is reversibly switched on or off with an external electric field, promising potential applications in field-effect transistors with reduced feature sizes, and added functionality over conventional semiconductor systems. Despite this progress the origin of the 2DEG is still debated, which is not only important for basic scientific understanding, but also for the design and engineering of improved and novel devices. One of the debated mechanisms, called the ‘polar-catastrophe’ [2], is based on the result that LAO has a fixed spontaneous, but non-switchable, polarisation of precisely 0.5 electrons per unit cell [3,4], which requires screening charges at its surfaces via an electronic transfer from the LAO surface to the interface STO Ti 3d conduction band [5]. However several experimental findings have raised doubts to the validity of this explanation: missing interface electrons, lack of surface holes, no internal field in thin insulating films and an inconsistent LAO critical thickness for metallicity. We present a paradigm that plausibly explains several pieces to this puzzle. It is based on a surface redox reaction (e.g. surface oxygen vacancies) and a subsequent electronic transfer across the film that screens the LAO polarization charge [6]. The process is studied with a simple model describing the formation energy of such redox screening processes, supported by first principles calculations. The model predicts the growth of the density of related surface defects and hence STO Ti 3d population with LAO thickness, showing a minimum critical thickness, in agreement with spectroscopic data. We also show that the surface defects generate trapping potentials that will affect the mobility of the interface 2DEG. We build upon these models to include thin films of a ferroelectric material whose polarization can be switched, unlike LAO . The redox process in these systems may now not only be responsible for the interface carriers, but also the facilitation of ferroelectricity itself, which is again thickness dependent. Results from first principles calculations will be presented.

1. A. Ohotmo and H. Hwang, Nature 427, 423 (2004) 2. N. Nakagawa, H. Hwang and D. Muller, Nat. Mater. 5, 204 (2006) 3. M. Stengel and D. Vanderbilt, Phys. Rev. B 80 , 241103 (2009) 4. N.C. Bristowe, P.B. Littlewood and E. Artacho, J. Phys.: Condens. Matter 23, 081001 (2011) 5. N.C. Bristowe, E. Artacho and P.B. Littlewood, Phys. Rev. B 80 , 045425 (2009) 6. N.C. Bristowe, P.B. Littlewood and E. Artacho, arXiv:1008.1951 (2010)

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