1. Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany
2. Department of Physics and Materials Science, University of Luxembourg, L-4422 Belvaux, Luxembourg
3. Institute of Nanotechnology, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany
4. Institute of Materials Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany
5. Institute of Quantum Materials and Technologies, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany