1. Department of Materials and Interfaces, The Weizmann Institute of Science, 76100 Rehovot, Israel, Department of Physics, University of California, Berkeley, California 94720, Condensed Matter Physics and Chemistry Department, RisØ National Laboratory, DK 4000 Roskilde, Denmark, and H. C. Ørsted Laboratory, Niels Bohr Institute, DK 2100 Copenhagen, Denmark