1. Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany
2. Department of Physics and Materials Science, University of Luxembourg, Rue du Brill 41, L-4422 Belvaux, Luxembourg
3. Department of Mechanical and Materials Engineering, University of Turku, Vesilinnantie 5, 20500 Turku, Finland
4. Turku Collegium for Science, Medicine and Technology, University of Turku, FI-20520 Turku, Finland
5. Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais 30123-970, Brazil
6. Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany