1. Department of Physics, Lund University, Box 118, 22100 Lund, Sweden
2. Nano Lund, Lund University, Box 118, 22100 Lund, Sweden
3. School of Physics and Technology and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan 430072, China
4. Institute for Advanced Studies, Wuhan University, Wuhan 430072, China