[1] Beijing Normal Univ, Dept Phys, Beijing 100875, Peoples R China
[2] Chinese Acad Sci, Inst Phys, Beijing 100190, Peoples R China
[3] Tsinghua Univ, Sch Mat Sci & Engn, Beijing 100084, Peoples R China
[4] Beijing Inst Technol, Res Ctr Mat Sci, Beijing 100081, Peoples R China
[5] Univ Calif Riverside, Dept Phys & Astron, Riverside, CA 92521 USA
[6] Univ Calif Berkeley, Dept Elect Engn & Comp Sci, Berkeley, CA 94720 USA