1. Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada
2. Hebei Key Laboratory of Optic Electronic
Information Materials, College of Physics Science and Technology, Hebei University, Baoding 071002, P. R. China
3. Department of chemistry, Stanford University, Stanford, California 94305, United States
4. Beijing National Laboratory for Condensed
Matter Physics, and Institute of Physics, Chinese Academy of Sciences, 100190 Beijing, China