1. State Key Laboratory for Advanced Metals and Materials; University of Science and Technology Beijing; Beijing 100083 China
2. Department of Physics; University of Illinois at Urbana-Champaign; Urbana IL 61801-3080 USA
3. School of Materials Science and Engineering; Beijing Institute of Technology; Beijing 100081 China
4. Department of Materials Science and Engineering; University of Tennessee; Knoxville TN 37996 USA