1. Institute of Condensed Matter Physics, Zhejiang Normal University, Jinhua 321004, China
2. School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, China
3. Department of Physics, Zhejiang University, Hangzhou 310058, China
4. Department of Fiber Science and Apparel Design, Cornell University, Ithaca, New York 14853-4401, USA