1. Department of Physics and Astronomy, University of Georgia, Athens, Georgia 30602, United States
2. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
3. Department of Physics and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
4. Department of Physics and Institute of Materials Science & Engineering, Washington University in St Louis, St. Louis, Missouri 63130, United States
5. Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States