1. Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA
2. Institute of Physics, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Beijing 100190, China
3. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
4. Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, USA
5. Department of Electrical and Computer Engineering, Vanderbilt University, Nashville, Tennessee 37235, USA