[1] Peking Univ, Sch Phys, State Key Lab Mesoscop Phys, Beijing 100871, Peoples R China
[2] Peking Univ, Sch Phys, Electron Microscopy Lab, Beijing 100871, Peoples R China
[3] Tsinghua Univ, Dept Phys, State Key Lab Low Dimens Quantum Phys, Beijing 100084, Peoples R China
[4] Collaborat Innovat Ctr Quantum Matter, Beijing 100084, Peoples R China