1. School of Materials Science and EngineeringYunnan University Kunming 650091 P. R. China
2. Faculty of Materials Science and EngineeringKunming University of Science and Technology Kunming 650093 P. R. China
3. Department of PhysicsSouthern University of Science and Technology Shenzhen 518055 P. R. China
4. State Key Laboratory of Surface Physics and Department of PhysicsFudan University Shanghai 200433 P. R. China