1. Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, USA
2. Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA
3. Institute of Chemistry, Chinese Academy of Science, Beijing 10019, China
4. Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
5. Department of Physics, Oakland University, Rochester, Michigan 48309, USA