1 School of Physics, Xidian University, Xi’an 710071, China
2 Institute of Applied Physics, Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany
3 Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany
4 Institute of Biological and Chemical Systems, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany
5 Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
6 e-mail: lium@xidian.edu.cn
7 e-mail: uli@uiuc.edu
8 e-mail: peng.gao@xidian.edu.cn