1. Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, No. 398 Ruoshui Road, SEID, Suzhou Industrial Park, Suzhou, Jiangsu 215123, P. R. China
2. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China