1. National Laboratory of Solid State Microstructures and School of Physics Nanjing University Nanjing 210093 China
2. School of Mechanics and Photoelectric Physics Anhui University of Science and Technology Huainan 232001 China
3. School of Materials Science and Physics China University of Mining and Technology Xuzhou 221116 China
4. Physics Department and Solid State Institute, Technion Haifa 32000 Israel