[1] Fuyang Normal Univ, Key Lab Funct Mat & Devices Informat Anhui Educ I, Fuyang 236037, Peoples R China
[2] Tsinghua Univ, Dept Phys, Beijing 100084, Peoples R China
[3] Tsinghua Univ, State Key Lab Low Dimens Quantum Phys, Beijing 100084, Peoples R China