[1] Tsinghua Univ, State Key Lab Low Dimens Quantum Phys, Beijing 100084, Peoples R China
[2] Tsinghua Univ, Dept Phys, Beijing 100084, Peoples R China
[3] Chinese Acad Sci, Suzhou Inst Nanotech & Nanobion, I Lab, Suzhou 215123, Jiangsu, Peoples R China