1. Institute of Physics, Swiss Federal Institute of Technology Lausanne (EPFL), CH-1015 Lausanne, Switzerland
2. Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 0FA, U.K.
3. School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287, United States
4. Institute for Solid State Physics, Friedrich-Schiller University Jena, 07743 Jena, Germany