State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871;
Electron Microscopy Laboratory, School of Physics, Peking University, Beijing 100871;
Electron Microscopy Laboratory, School of Physics, Peking University, Beijing 100871;
State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871;
Electron Microscopy Laboratory, School of Physics, Peking University, Beijing 100871;