1. Institut für PhysikTechnische Universität Ilmenau D‐98693 Ilmenau Germany
2. Bremen Center for Computational Materials ScienceUniversity Bremen D‐28359 Bremen Germany
3. Institute for Theoretical PhysicsUniversity Bremen D‐28359 Bremen Germany
4. Center for Nanostructured GrapheneDepartment of PhysicsTechnical University of Denmark DK‐2800 Kongens Lyngby Denmark