1. University of Michigan − Shanghai Jiao Tong University Joint Institute, Shanghai Jiao Tong University, Shanghai 200240, China
2. Department of Physics, Center for Material Science and Nanotechnology, University of Oslo, Blindern, P.O. Box 1048, N-0316 Oslo, Norway
3. Institut Matériaux Microélectronique Nanosciences de Provence, UMR 6242 CNRS, Université Aix-Marseille, 13397 Marseille Cedex 20, France