1. Department of Materials Science and Engineering, KTH Royal Institute of Technology, SE–100 44 Stockholm,
Sweden
2. Centre for Materials Science and Nanotechnology, University of Oslo, P.O. Box 1048 Blindern, NO-0316 Oslo,
Norway
3. Department of Physics, University of Oslo, P.O.
Box 1048 Blindern, NO–0316 Oslo, Norway