1. Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory; Southwest University of Science and Technology; Mianyang 621010 P. R. China
2. Physikalisches Institut; Karlsruhe Institute of Technology; P.O. Box 6980 Karlsruhe 76049 Germany
3. Department of Physics; Center for Condensed Matter and Materials Physics; Beihang University; Beijing 100191 P. R. China