1. Contribution from the Department of Applied Physics & Applied Mathematics, Columbia University, 200 SW Mudd Building, 500 W 120th Street, New York, New York 10027, University of Cambridge, Cavendish Lab, Cambridge CB3 0HE, UK, and Laboratoire TECSEN, UMR 6122 CNRS, Faculté des Sciences de Saint-Jérôme, case 262, Université Paul Cézanne-Aix Marseille III, 13397 Marseille cedex 20, France