1. State Key Laboratory for Mesoscopic Physics, and Electron Microscopy Laboratory, School of Physics, Peking University, Beijing 100871, Peoples’s Republic of China, Laboratoire de Physique des Materiaux Divises et Interfaces (LPMDI), CNRS-UMR 8108, Universite Paris-Est, 77454 Marne la Vallee Cedex 2, France, and Physics Department, Hong Kong University of Science and Technology, Hong Kong