1. College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China
2. Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
3. Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China
4. School of Astronomy and Space Science, Nanjing University, Nanjing 210023, China