1. Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden
2. Institut des Nanosciences de
Paris, Université Pierre et Marie Curie, 4 place Jussieu, boîte courrier 840, 75252 Paris Cedex 05,
France
3. Materials Physics, Royal Institute of Technology, Electrum 229, SE 164
40 Kista, Sweden