1. Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
2. State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
3. Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
4. National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
5. Department of Mathematics and Physics, North Carolina Central University, Durham, North Carolina 27707, United States