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