1. College
of Sciences, Huazhong Agricultural University, Wuhan 430070, P. R. China
2. Institute
of Applied Physics, Huazhong Agricultural University, Wuhan 430070, P. R. China
3. State
Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China