1. Institute of Physics, Martin Luther University Halle-Wittenberg, 06120 Halle, Germany
2. Institute of Theoretical Solid State Physics, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany
3. Meyer Burger Research AG, Hauterive 2068, Switzerland
4. Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany