1. School of Physics and Electronics Hunan Key Laboratory of Nanophotonics and Devices Central South University 932 South Lushan Road Changsha Hunan 410083 P. R. China
2. Powder Metallurgy Research Institute and State Key Laboratory of Powder Metallurgy Central South University Changsha 410083 P. R. China
3. Department of Basic Sciences Air Force Engineering University Xi'an 710051 P. R. China