1. Department of Physics, State Key Laboratory for Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P. R. China, Department of Physics, Zhoukou Normal University, 21 Qiyi Road, Zhoukou 466000, P. R. China, and Instrumental Analysis Center, Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai 200030, P. R. China