1. School of Physics Beihang University Beijing 100191 P. R. China
2. College of Physics and Optoelectronic Engineering Shenzhen University Shenzhen 518060 P. R. China
3. School of Energy and Power Engineering Beihang University Beijing 100191 P. R. China
4. Shen Zhen Institute of Beihang University Shenzhen 518063 P. R. China
5. Physics Department The Ohio State University Columbus OH 43210 USA