1. State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400044, P. R. China
2. School of Materials Science and Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China
3. School of Physics and Electronic Information, Gannan Normal University, Ganzhou, Jiangxi 341000, P. R. China
4. Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China