1. College of
Physics, Chongqing University, Chongqing
401331, P. R. China
2. State
Key Laboratory of Superlattices
and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, P.
R. China
3. School of Science, Chongqing Jiaotong University, Chongqing 400074, P.
R. China
4. College of Physics
and Electronic
Engineering, Xinjiang Normal University, Urumchi 830054, P. R. China