1. Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 0FA, United Kingdom
2. Department of Physics, University of Konstanz, D-78457 Konstanz, Germany
3. School of Physics & Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom
4. Institute for Theoretical Physics, University of Regensburg, D-93040 Regensburg, Germany
5. Department of Materials Science and Engineering, University of Ioannina, Ioannina, 45110, Greece