1. School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031, China
2. Institute for Micro and Nanotechnologies MacroNano(R) and Institute for Materials Science and Engineering, Chair of Materials for Electrical Engineering and Electronics, Technische Universität Ilmenau, 98693 Ilmenau, Germany
3. College of Physics, Sichuan University, Chengdu 610065, China
4. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China