基金项目: | The authors would like to thank Dr.J.L.Li for helpful discussions.The first and second authors gratefully acknowledge the financial support provided by NSFC;The research of the third author is supported by a Singapore MOE AcRF Tier 2 Grant;Singapore MOE AcRF Tier 1 Grant;The fourth author is partially supported by NSFC;the Construct Program of the Key Discipline in Hunan Province |