1. Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 0FA, U.K.
2. Institut für Festkörperphysik, Friedrich-Schiller-Universität, Max-Wien-Platz 1, 07743 Jena, Germany
3. Nordic Institute for Theoretical Physics, Roslagstullsbacken 23, SE-106 91 Stockholm, Sweden
4. Graphene Laboratories, Istituto Italiano di Tecnologia, Via Morego 30, I-16163 Genova, Italy
5. IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, P.zza L. da Vinci 32, 20133 Milano, Italy
6. Department of Materials Science and Engineering, University of Ioannina, Ioannina 45110, Greece