1. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16801, USA
2. Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA
3. Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
4. Department of Physics, University of North Florida, 1 UNF Drive, Jacksonville, Florida 32224, USA
5. Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China