1. School of Materials Science and EngineeringBeihang University Beijing 100191 P. R. China
2. Advanced Research Institute of Multidisciplinary ScienceBeijing Institute of Technology Beijing 100081 P. R. China
3. School of PhysicsTrinity College Dublin 2 CT 06106 Ireland
4. Department of Materials Science and EngineeringThe Pennsylvania State UniversityUniversity Park PA 16802 USA