1. Chimie de la Matière Condensée, UPMC−CNRS, 4 place Jussieu, 75005 Paris, France, Laboratoire de Physique des Solides, Université Paris-Sud, 91405 Orsay, France, Institute of Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Steyrergasse 17/VI, 8010 Graz, Austria, Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, Netherlands, and Laboratoire d’Optique des Solides, UPMC, 4 place Jussieu, 75 252 Paris, France