1. Department of Applied Physics and Center for Research on Interface Structures and Phenomena (CRISP), Yale University, New Haven, Connecticut 06520, USA
2. Brookhaven National Laboratory, Center for Functional Nanomaterials, Upton, New York 11973, USA
3. National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, China