1. Department of Physics, Linköping University, SE-581 83 Linköping, Sweden
2. Institute for Experimental and Applied Physics, Christian-Albrechts-Universität zu Kiel, Leibnizstraße 17, 24118 Kiel, Germany
3. Nanotechnology & Integrated Bioengineering Centre (NIBEC), Ulster University, BT37 0QB, Northern Ireland, United Kingdom