1. Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu, 610054, P. R. China
2. Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington
99352, United States
3. Materials
Science and Engineering, Northwestern University, Evanston, Illinois 60208,
United States
4. Department of Physics, Applied Physics, & Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, United States