1. Institut für Physik, Carl von Ossietzky Universität Oldenburg, 26129 Oldenburg, Germany
2. Cambridge Graphene Centre, University of Cambridge, CB3 0FA Cambridge, United Kingdom
3. Institute for Theoretical Physics and Bremen Center for Computational Materials Science, University of Bremen, P.O. Box 330 440, 28334 Bremen, Germany
4. Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano, Italy
5. Istituto di Fotonica e Nanotecnologie-CNR, Piazza L. da Vinci 32, 20133 Milano, Italy
6. Center for Nanoscale Dynamics (CENAD), Carl von Ossietzky Universität Oldenburg, Institut für Physik, 26129 Oldenburg, Germany