1. Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany.
2. Department of Applied Physics, Chalmers University of Technology, S-412 96 Göteborg, Sweden.
3. Centre de Recherche de la Matière Condensée et des Nanosciences–CNRS, Campus de Luminy, Case 913, F-13288 Marseille Cedex 9, France.