1. Department of Physics, Centre for Material Science and Nanotechnology, University of Oslo, P.O. Box 1048, Blindern, N-0316 Oslo, Norway
2. National Centre for Nuclear Research, A. Soltana 7, 05-400 Otwock-Swierk, Poland
3. Royal Institute of Technology, KTH-ICT, Electrum 229, SE-164 40 Stockholm, Sweden