1. School of Materials Science and EngineeringHarbin Institute of TechnologyHarbin 150001P. R. China
2. Department of PhysicsUniversity of South FloridaTampa, Florida 33620USA
3. School of Materials Science and EngineeringInner Mongolia University of TechnologyNo. 49 Aimin StreetHohhot 010051P. R. China
4. Department of physicsQiqihar UniversityQiqihar 161006P. R. China