1. Center for NanoScience & Faculty of Physics, Ludwig-Maximilians-Universität München, Geschwister-Scholl-Platz 1, 80539 München, Germany
2. Fraunhofer Institute for Silicate Research (ISC), Neunerplatz 2, 97082 Würzburg, Germany
3. Leiden Institute of Physics, Leiden University, 2333 Leiden, The Netherlands
4. Department of Physics, Universität Konstanz, 78457 Konstanz, Germany