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