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TT: Fachverband Tiefe Temperaturen

TT 19: CE: Metal-Insulator Transition 2

TT 19.9: Vortrag

Mittwoch, 24. März 2010, 12:00–12:15, H18

Absence of an electric potential gradient across the overlayer of LaAlO₃/SrTiO₃ heterostructures inferred from XPS — •Götz Berner¹, Michael Sing¹, Stefan Thiel², Jochen Mannhart², and Ralph Claessen¹ — ¹Experimentelle Physik 4, Universität Würzburg — ²Experimentelle Physik VI, Universität Augsburg

The origin of the interface electron gas (2DEG) in LaAlO₃/SrTiO₃ oxide heterostructures (LAO/STO) is a heavily discussed topic. Apart from possible influences of oxygen defects recent experimental and theoretical work suggests an electronic reconstruction as the driving mechanism for the 2DEG. In the most simple picture half an electron is transferred from the surface to the interface to neutralize the electric potential piling up across the polar LAO overlayers. Microscopically, density-functional (DFT) calculations find a potential gradient which shifts the LAO electronic states until the valence-band maximum crosses the chemical potential and electrons are transferred from the surface to the STO conduction-band minimum at the interface.

By x-ray photoelectron spectroscopy (XPS) this potential gradient should be observable as a significant broadening of the Al 1s core level. Moreover, probing the topmost LAO layer there should appear finite spectral weight at the chemical potential. However, in our XPS studies on several samples with different overlayer thicknesses none of these signatures is observed. In addition, we determined the band offsets of LAO and STO at the interface and arrived at values which suggest a flat band situation in contrast to the DFT calculations.