1. School of Materials Science and Engineering; Beihang University; Beijing 100191 P. R. China
2. Frontier Institute of Science and Technology; State Key Laboratory for Mechanical Behavior of Materials; Xi'an Jiaotong University; Xi'an 710049 China
3. School of Physics; Trinity College; Dublin 2 Ireland
4. Department of Materials Science and Engineering; The Ohio State University; Columbus OH 43210 USA