1. Department of Microtechnology and Nanoscience, Chalmers University of Technology, 412 96 Gothenburg, Sweden
2. RISE Research Institutes of Sweden, Box 857, S-50115 Borås, Sweden
3. Department of Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden
4. Department of Physics, Chemistry and Biology, Linköping University, 581 83 Linköping, Sweden
5. National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, U.K.