1. School of Materials Science and Engineering, Anhui University, Hefei, Anhui 230601, P.R. China
2. Institute of Energy, Hefei Comprehensive Nation Science Center, Hefei, Anhui 230031, P.R. China
3. Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China
4. School of Physics and Electronics, Shandong Normal University, Jinan 250014, P. R. China