1. Institutes of Physical Science and Information Technology, Anhui University, Hefei 230601, P.R. China
2. Key Laboratory of Materials Physics, 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
3. College of Physical Science and Technology, Yangzhou University, Yangzhou 225002, P. R. China