1. Department of Physics, Chalmers University of Technology, 41296 Gothenburg, Sweden
2. Department of Physics, Philipps-Universität Marburg, 35037 Marburg, Germany
3. Institute of Electrical and Microengineering, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
4. Institute of Materials Science and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland