1. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
2. Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan
3. Department of Physical Science, University of Chinese Academy of Sciences, Beijing 100049, China
4. Department of Physics, Faculty of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan