1. Laboratoire Léon Brillouin and SBPM/DBCM CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France Department Biological Sciences University of Waikato, Hamilton, New Zealand Institut Laue-Langevin, 156X 38042 Grenoble Cedex, France Department of Physics and Astronomy University College London, Gower Street London WC1E 6BT, UK