1. School of Physics; Beijing Institute of Technology; Beijing 100081 P. R. China
2. KSU-Aramco Center; King Saud University; Riyadh 11451 Saudi Arabia
3. MINT Center; University of Alabama; Tuscaloosa AL 35487 USA
4. State Key Laboratory for Mesoscopic Physics; Department of Physics; Peking University; Beijing 100871 P. R. China
5. School of Physics and CRANN; Trinity College Dublin; Dublin 2 Ireland
6. Institute of Plasma Physics; Chinese Academy of Sciences; Hefei 230031 P. R. China
7. Department of Physics and Institute of Applied Physics; National Taiwan University; Taipei 10617 Taiwan