1. Department of Physics, Gothenburg University, 41296 Gothenburg, Sweden
2. University of Aix-Marseille, CNRS, IUSTI UMR 7343, 13 013 Marseille Cedex 13, France
3. KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden
4. Department of Mathematics and Mechanics, LUSAC-ESIX, University of Caen, 50130 Cherbourg, France