1. Department of Applied Physics, Aalto University, P.O. Box 15100, FI-00076 Aalto, Espoo, Finland
2. Department of Physics, University of Helsinki, P.O. Box 43, FI-00014 Helsinki, Finland
3. Department of Physics, Centre for Materials Science and Nanotechnology, University of Oslo, Blindern P.O. Box 1048, Oslo, Norway
4. Helsinki Institute of Physics, P.O. Box 64, FI-00014 Helsinki, Finland