1. MOE Key Laboratory for Non-Equilibrium Synthesis and Modulation of Condensed Matter, School of Physics, Xi´an Jiaotong University , Shaanxi, Xi´an 710049 , China
2. Department of Physics and Astronomy, University of Missouri , Columbia, MO 65211 , USA
3. Xinjiang Astronomical Observatory, Chinese Academy of Sciences , Xinjiang, Urumqi 830011 , China