1. Department of Physics, University of Science and Technology of China, Hefei 230026, China
2. Ames Laboratory, US DOE, Ames, Iowa 50011, USA
3. Department of Physics, Iowa State University, Ames, Iowa 50011, USA
4. International Center for Quantum Design of Functional Materials (ICQD), Hefei National Laboratory for Physics Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China