1. School of Materials Science and EngineeringUniversity of Science and Technology Beijing Beijing 100083 China
2. Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparation TechnologySchool of ScienceTianjin University Tianjin 300354 China
3. Centre for the Physics of Materials and Department of PhysicsMcGill University Montreal Quebec H3A2T8 Canada
4. Institute of Modern PhysicsFudan University Shanghai 200433 China