1. Walter Schottky Institut und Physik Department, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany
2. Munich Center of Quantum Science and Technology (MCQST), Schellingstr. 4, 80799 Munich, Germany
3. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
4. KTH Royal Institute of Technology, Department of Applied Physics, Albanova University Centre, Roslagstullsbacken 21, 106 91 Stockholm, Sweden
5. Institut für Theoretische Physik, Universität Bremen, P.O. Box 330
440, 28334, Bremen, Germany
6. National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
7. Institute of Physics, University of Münster, 48149 Münster, Germany