1. Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, P. R. China
2. University of Science and Technology of China, Hefei 230026, P. R. China
3. Institute of Physical Science and Information Technology, Anhui University, Hefei 230601, P. R. China