1. LURE, Bât. 209 D, Université Paris-Sud, 91405 Orsay Cedex, France, Department of Physics, North Carolina State University, Raleigh, North Carolina 27695-8202, Department of Physics, Washington University, Seattle, Washington 98195, and CRMC2, Campus de luminy, Case 913, 13288, Marseille Cedex 9, France