1. Department of Physics, Duke University, Durham, North Carolina 27708, United States
2. School of Sciences and Humanities, Wentworth Institute of Technology, Boston, Massachusetts 02115, United States
3. Department of Physics and Astronomy, Appalachian State University, Boone, North Carolina 28607, United States
4. Advanced Materials Laboratory, NIMS, Tsukuba 305-0044, Japan
5. Department of Physics, City University of Hong Kong, Kowloon, Hong Kong SAR
6. Department of Physics and Astronomy, Texas A&M University, College Station, Texas 77843, United States