1. Chair in Hybrid Nanosystems, Nanoinstitute Munich, Faculty of Physics, Ludwig-Maximilians-Universität München, Königinstraße 10, 80539 München, Germany
2. Walter Schottky Institute and Physics Department, Technical University Munich, Am Coulombwall 4, 85748 Garching, Germany
3. School of Physics and Astronomy, Monash University Clayton Campus, Melbourne, Victoria 3800, Australia
4. The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom