Department of Physics, Center of Atomic and Molecular Nanosciences, Tsinghua University, Beijing 100084;
Department of Physics, Beijing Institute of Technology, Beijing 100084;
Department of Physics, Center of Atomic and Molecular Nanosciences, Tsinghua University, Beijing 100084;
Department of Physics, Shanghai Jiao Tong University, Shanghai 200230;
Institute of Physics, Chinese Academy of Sciences, Beijing 100080;
