1. Laboratoire de Chimie Physique, Bâtiment 349, Université Paris-Sud, 91405 Orsay, France. Current address: Soft Matter group, IPkM, Heinrich Heine Universität, Düsseldorf, Germany
2. Laboratoire de Chimie Physique, Bâtiment 349, Université Paris-Sud, 91405 Orsay, France. Current address: Glass and Time, IMFUFA, Department of Sciences, Building 27 Roskilde University RUC Postbox 260 DK-4000 Roskilde, Denmark
3. Chemical Sciences Division at ORNL, Oak Ridge; and Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States
4. Institut Laue Langevin, Grenoble, France
5. ICREA-Departament de Física Aplicada, EPSC, Universitat Politècnica de Catalunya, Esteve Terradas 15, 08860 Castelldefels, Spain
6. Laboratoire de Chimie Physique, Bâtiment 349, Université Paris-Sud, 91405 Orsay, France. Current address: Laboratoire Léon Brillouin, CEA-CNRS, Saclay, France