Pentcheva, Rossitza; Pickett, Warren E. (2010): Electronic phenomena at complex oxide interfaces: insights from first principles. In: Journal of Physics Condensed Matter, Vol. 22, Nr. 4
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Oxide interfaces have attracted considerable attention in recent yearsdue to the emerging novel behavior which does not exist in thecorresponding bulk parent compounds. This opens possibilities for futureapplications in oxide-based electronics and spintronics devices. Amongthe different materials combinations, heterostructures containing thetwo simple band insulators LaAlO(3) and SrTiO(3) have advanced to amodel system exhibiting unanticipated properties ranging fromconductivity, to magnetism, even to superconductivity. Electronicstructure calculations have contributed significantly towardsunderstanding these phenomena and we review here the progress achievedin the past few years, also showing some future directions andperspectives. A central issue in understanding the novel behavior inthese oxide heterostructures is to discover the way (or ways) that theseheterostructures deal with the polar discontinuity at the interface.Despite analogies to polar semiconductor interfaces, transition metaloxides offer much richer possibilities to compensate the valencemismatch, including, for example, an electronic reconstruction.Moreover, electronic correlations can lead to additional complexbehavior like charge disproportionation and order, magnetism and orbitalorder. We discuss in some detail the role of finite size effects inultrathin polar films on a nonpolar substrate leading to anotherintriguing feature-the thickness-dependent insulator-to-metal transitionin thin LaAlO(3) films on a SrTiO(3)(001) substrate, driven by theimpending polar catastrophe. The strong and uniform lattice polarizationthat emerges as a response to the potential build-up enables the systemto remain insulating up to a few layers. However, beyond a criticalthickness there is a crossover from an ionic relaxation to an electronicreconstruction. At this point two bands of electron and hole character,separated both in real and in reciprocal space, have been shiftedsufficiently by the internal field in LaAlO3 to impose the closing ofthe bandgap. We discuss briefly further parameters that allow one tomanipulate this behavior, e. g. via vacancies, adsorbates or an oxidecapping layer.