1. School of Resources and Environmental Engineering, Anhui University, Hefei, 230601, Anhui Province, P. R. China
2. Institute of Intelligent Machines, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, P. R. China
3. Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, P. R. China