1. Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel, Department of Physics, Queen Mary and Westfield College, University of London, London E1 4NS, U.K., Department of Materials, Eindhoven University of Technology, Eindhoven, The Netherlands, and Department of Materials Science and Engineering, Queen Mary and Westfield College, University of London, London E1 4NS, U.K.