1. School of Electrical Engineering, Southwest Jiaotong University, Chengdu 610031, China
2. School of Physical Science and Technology, Southwest Jiaotong University, Chengdu, China
3. College of Materials Science and Engineering, Hunan University, Changsha 410082, China
4. School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, China
5. School of Physics and Electronics, Hunan University, Changsha 410082, China