1. Institute of Physics; Martin Luther University Halle-Wittenberg; 06120 Halle Germany
2. Fraunhofer Institute for Microstructure of Materials and Systems IMWS; 06120 Halle Germany
3. Institute of Theoretical Solid State Physics; Karlsruhe Institute of Technology; 76131 Karlsruhe Germany
4. Institute of Nanotechnology; Karlsruhe Institute of Technology; 76021 Karlsruhe Germany