[1] Chinese Acad Sci, Beijing Natl Lab Condensed Matter Phys, Inst Phys, Beijing 100190, Peoples R China
[2] Univ Chinese Acad Sci, Sch Phys, Beijing 100049, Peoples R China
[3] Beijing Normal Univ, Dept Phys, Beijing 100875, Peoples R China
[4] Max Planck Inst Chem Phys Solids, D-01187 Dresden, Germany
[5] Natl Synchrotron Radiat Res Ctr, Hsinchu 30076, Taiwan
[6] Songshan Lake Mat Lab, Dongguan 523808, Guangdong, Peoples R China
[7] Tsinghua Univ, State Key Lab Low Dimens Quantum Phys, Beijing 100084, Peoples R China