1. State
Key Laboratory of Superlattices and Microstructures, Institute of
Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China
2. College
of Science, National University of Defense Technology, Changsha 410073, Hunan Province, P. R. China
3. Department
of Physics, New Jersey Institute of Technology, 322 King Boulevard, Newark, New Jersey 07102, United States