1. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371 Singapore
2. Center for Condensed Matter Science and Technology (CCMST), Department of Physics, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
3. International Center for Materials Physics, Academia Sinica, Shenyang 110015, People’s Republic of China