1. Institute of Materials, China Academy of Engineering Physics, P.O. Box No. 9, Jiangyou, Sichuan 621907, P. R. China
2. Department of Physics, McGill University, 3600 University Street, Montreal, Quebec H3A 2T8, Canada
3. Science and Technology on Surface Physics and Chemistry Laboratory, P.O. Box No. 9-35, Huafengxincun, Jiangyou, Sichuan 621908, P. R. China