1. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain
2. Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA
3. Department of Condensed Matter Physics and Institute of Nanoscience and Nanotechnology (IN2UB), University of Barcelona, 08028 Barcelona, Spain
4. Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
5. ICREA-Catalan Institution for Research and Advanced Studies, Passeig Lluïs Companys, Barcelona, Spain