1. Department of Physics, State Key Laboratory for Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P. R. China, Instrumental Analysis Center, Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai 200030, P. R. China, and Institute of Physics ASCR, Na Slovance 2, CZ-182 21 Praha 8, Czech Republic