1. School of Physics and Material Science, Anhui University, Hefei 230039, Anhui, P. R. China
2. College of Physics and Electronics
Engineering, Shanxi University, Taiyuan
030006, Shanxi, P. R. China
3. National Laboratory of Solid State Microstructures and Department
of Physics, Nanjing University, Nanjing
210093, Jiangsu, P. R. China