1. Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Str. 3, 79104 Freiburg, Germany
2. Applied Physical Chemistry, KTH Royal Institute of Technology, SE 100 44 Stockholm, Sweden
3. Department of Applied Mathematics, Research School of Physics, ACT The Australian National University, Canberra, 2601 Australia
4. Centre for Materials Science and Nanotechnology, Department of Physics, University of Oslo, P.O.
Box 1048, Blindern, NO-0316 Oslo, Norway