1. Department of Microtechnology and Nanoscience, Chalmers University of Technology, 41296 Gothenburg, Sweden
2. Institute of Solid State Physics, Technische Universität Berlin, 10623 Berlin, Germany
3. Department of Applied Physics, KTH Royal Institute of Technology, 16440 Kista, Sweden
4. Department of Physics, Chalmers University of Technology, 41296 Gothenburg, Sweden