1. Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, HFIPS, Hefei 230031, P. R. China
2. Chinese University of Science and Technology, Hefei 230026, P. R. China
3. School of Physics and Astronomy and Yunnan Key Laboratory for Quantum Information, Yunnan University, Kunming 650091, P. R. China
4. Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium