Supported in part by the National Natural Science Foundation of China.No. 19562001 and the Science Foundation of China Academy of Engineering Physics under Grant No.9504050
1. Graduate School, China Academy of Engineering Physics P. O. Box 8009, Beijing 100088; 2. Department of Physics, Guang xi University, Nan ning 530004
Supported in part by the National Natural Science Foundation of China.No. 19562001 and the Science Foundation of China Academy of Engineering Physics under Grant No.9504050