1. School of Physical Science and Engineering, Beijing JiaoTong University, Beijing 100044, P. R. China
2. School of Science, Xihua University, Chengdu 610039, P. R. China
3. Railway Engineering Research Institute, China Academy of Railway Sciences Corporation Limited, Beijing 100081, P. R. China
4. Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada