1. University of Exeter, Physics Building, Stocker Road, Exeter EX4 4QL, United Kingdom
2. Fritz Haber Institute of the Max Planck Society, 14195 Berlin, Germany
3. Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany
4. Institute of Physics, Johannes Gutenberg University Mainz, 55099 Mainz, Germany
5. Graduate School of Excellence Materials Science in Mainz, Staudingerweg 9, 55128 Mainz, Germany