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Transition metal oxides and in particular perovskites offer a rich variety of properties such as superconductivity, ferroelectricity, magnetism and colossal magnetoresistance to name only a few. The combination of such properties in a layered material is an appealing prospect. Recent progress in growth techniques has enabled the synthesis of oxide heterostructures with atomic precision that show new, unexpected functionalities. The aim of the current project is to advance our understanding on how the disruption of charge neutrality at the interface can be utilized to generate new magnetic, charge and orbital order for future electronics and spintronics applications. To accomplish this, density functional theory calculations will be performed on a series of perovskite superlattices, taking into account electronic correlations within the LDA+U approach.
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Final report:
A major focus of the BaCaTeC project funded between July 2007 and December 2008 and subsequent research was to uncover the underlying mechanisms that lead to the emergence of novel electronic phases at oxide interfaces based on density functional theory (DFT) calculations.
One central aspect that was explored was the role of charge neutrality violation at the interface. Our results within the GGA+U approach demonstrate that complex oxides with multivalent cations offer additional degrees of freedom to compensate the valence discontinuity that do not exist for example in semiconductor heterostructures and can lead to charge and orbitally ordered phases e.g. at LaTiO3/SrTiO3 and the n-type LaAlO3/SrTiO3 interfaces [Phys. Rev. Lett. 99, 016802 (2007); Phys. Rev. B 78, 205106 (2008); J. Phys.: Cond. Matter 22, 043001 (2010)].
In contrast to these superlattices (SLs), the compensation mechanism identified in thin LaAlO3 films on a SrTiO3(001) substrate is qualitatively different. DFT results show that a strong lattice polarization counteracts the internal electric field in LaAlO3 films on SrTiO3(001) [Phys. Rev. Lett. 102, 107602 (2009)], leading to the experimentally observed thickness-dependent insulator-to-metal transition. The electronic reconstruction can be further tuned by an oxide capping layer where an electron-hole bilayer is formed [Phys. Rev. Lett., 104, 166804 (2010)]. A recent systematic study of a series of metallic contacts on LaAlO3/SrTiO3(001) provides guidelines how to modify the carrier density and band alignment at the interface through the choice of the metallic contact [Phys. Rev. B 85, 125404 (2012)].
Among the scientists supported by the funding was Katrin Otte (PhD student, LMU, visit August-September 2008). The achieved results built also the basis for a project within SFB/TR80 funded by the DFG.
[1] R. Pentcheva and W.E. Pickett Correlation-Driven Charge Order at the Interface of a Mott and a Band Insulator, Phys. Rev. Lett. 99, 016802 (2007).
[2] R. Pentcheva and W.E.Pickett Ionic relaxation contribution to the electronic reconstruction at the n-type LaAlO3/SrTiO3 interface, Phys. Rev. B 78, 205106 (2008).
[3] R. Pentcheva and W.E. Pickett, Electronic Phenomena at Complex Oxide Interfaces: Insights from First Principles (invited topical review), J. Phys.: Cond. Matter 22, 043001 (2010).
[4] R. Pentcheva and W.E.Pickett Avoiding the polarization catastrophe in LaAlO3 overlayers on SrTiO3 (001) through a polar distortion, Phys. Rev. Lett. 102, 107602 (2009).
[5] R. Pentcheva, M. Huijben, K. Otte, W.E. Pickett, J.E. Kleibeuker, J. Huijben, H. Boschker, D. Kockmann, W. Siemons, G. Koster, H.J.W. Zandvliet, G. Rijnders, D.H.A. Blank, H. Hilgenkamp, and A. Brinkman, Parallel electron-hole bilayer conductivity from electronic interface reconstruction, Phys. Rev. Lett., 104, 166804 (2010).
[6] R. Arras, V. G. Ruiz, W. E. Pickett and R. Pentcheva, Tuning the two-dimensional electron gas at the LaAlO3/SrTiO3(001) interface by metallic contacts, Phys. Rev. B 85, 125404 (2012).
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