[1] Shandong Univ, Sch Phys, Jinan 250100, Peoples R China
[2] Chinese Acad Sci, Beijing Natl Lab Condensed Matter Phys, Inst Phys, Beijing 100190, Peoples R China
[3] Chinese Acad Sci, Lab Semicond Mat Sci, Inst Semicond, Beijing 100083, Peoples R China