1. Physics Department and IRIS Adlershof, Humboldt-Universität zu Berlin, 12489 Berlin, Germany
2. Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany
3. Institute of Physics, Carl-von-Ossietzy Universität Oldenburg, 26129 Oldenburg, Germany
4. Center for Nanoscale Dynamics (CeNaD), Carl-von-Ossietzy Universität Oldenburg, 26129 Oldenburg, Germany