1. School of Physics, Central South University, Changsha 410083, Hunan, China
2. Department of Physics and Astronomy, Uppsala University, Box 516, S-75120 Uppsala, Sweden
3. Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
4. Temple University, Philadelphia, Pennsylvania 19122, USA
5. Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA
6. Idaho National Laboratory, Idaho Falls, Idaho 83415 USA