1. Center for NanoScience and Department of Physics; Ludwig-Maximilians-Universität München; Geschwister-Scholl-Platz 1, 80539 München Germany
2. Arnold Sommerfeld Center for Theoretical Physics; Ludwig-Maximilians-Universität München; Theresienstrasse 37, 80333 München Germany
3. Laboratory for Solid State Physics; ETH Zurich; 8093 Zurich Switzerland