1. Materials Physics, MAP, ICT, Royal Institute of Technology, Electrum 229, SE-164 40 Stockholm, Sweden, Department of Physics, Uppsala University, Box 530, SE-75121 Uppsala, Sweden, MAX-Lab, University of Lund, Box 118, SE-22100 Lund, Sweden, Physique des Interactions Ioniques et Moléculaires, UMR CNRS-Université de Provence 6633, F-13397 Marseille, France, and CRMCN-CNRS, Campus de Luminy, case 913, F-13288 Marseille, France