1. School of Science, Hebei University of Technology, Tianjin 300401, China
2. Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
3. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
4. Department of Physics and Astronomy, Uppsala University, SE-75120 Uppsala, Sweden
5. Department of Physics and Astronomy, Key Laboratory of Quantum Information of Yunnan Province, Yunnan University, Kunming 650091, China