1. State Key Laboratory of Surface Physics, Key Laboratory for Computational Physical Sciences (MOE), and Department of Physics, Fudan University, Shanghai 200433, China
2. College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, China
3. Department of Physics and Electronic Engineering, Hanshan Normal University, Chaozhou 521041, China
4. State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China
5. Department of Physics and Astronomy, Center for Strongly Correlated Materials Research, Seoul National University, Seoul 151-747, South Korea