1. Key
Laboratory of Materials Physics, CAS Center for Excellence in Nanoscience,
and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute
of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China
2. University of Science and Technology of China, Hefei 230026, China
3. Department
of Mechanical and Aerospace Engineering, West Virginia University, P.O. Box 6106, Morgantown, West Virginia 26506, United States