1. Department of Condensed Matter Physics, Weizmann Institute of Science 1 , Rehovot 7610001, Israel
2. Departments of Physics and Electrical Engineering, University of Colorado Denver 2 , Denver, Colorado 80204, USA
3. Department of Physics, National University of Singapore 3 , Singapore
4. School of Physics and Astronomy, University of Manchester 4 , Manchester M13 9PL, United Kingdom
5. National Graphene Institute, University of Manchester 5 , Manchester M13 9PL, United Kingdom
6. Department of Physics Core Facilities, Weizmann Institute of Science 6 , Rehovot 7610001, Israel