1. School of Materials Science and Engineering, Beihang University, Beijing 100191, P. R. China
2. Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore
3. Department of Physics, Southern University of Science and Technology, Shenzhen 518055, P. R. China
4. School of Physics, Henan Normal University, Xinxiang 453007, P. R. China
5. Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States