1. Institut für Festkörperphysik, Universität Hannover, Hannover 30167 Germany
2. Ioffe Institute, 194021 St. Petersburg, Russia
3. Micro and Nanoscience Laboratory, Aalto University, Tietotie 3, FIN-02150 Espoo, Finland
4. Faculty of Engineering and Natural Sciences, Sabanci University, 34956 Istanbul, Turkey