1. Institute of Materials Science, Vietnam Academy of Science and Technology 1 , Hanoi 100000, Vietnam
2. Department of Materials Science and Engineering, University of Virginia 2 , Charlottesville, Virginia 22903
3. Department of Physics, Kyung Hee University 3 , Seoul 02447, Korea
4. Department of Physics, North Carolina State University 4 , Raleigh, North Carolina 27695-8202