1. Department of Mechanical and Structural Engineering and Materials Science, University of Stavanger, N-4036 Stavanger, Norway
2. Physics Department, Institute of Energy Technology, P.O. Box 40, N-2027 Kjeller, Norway
3. Department of Physics, Centre for Materials Science and Nanotechnology, University of Oslo, P.O. Box 1048 Blindern, N-0316 Oslo, Norway