1. Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, Walther-Meißner-Straße 8, 85748 Garching, Germany
2. Physik-Department, Technische Universität München, 85748 Garching, Germany
3. Nanosystems Initiative Munich (NIM), 80799 Munich, Germany
4. Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba R3T2N2, Canada