1. Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China
2. Center for Basic Research on Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
3. Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China