基金This paper is dedicated to Professor lan R. Petersen on the occasion of his 60th birthday. This work was supported by the Anhui Provincial Natural Science Foundation （No. 1708085MF144） and the National Natural Science Foundation of China （No. 61573330）.Acknowledgements We thank Dr. Daoyi Dong for helpful discussion.
基金This research was supported by the project （51425804） of the National Science Fund for Distinguished Young Scholars of China and the National Natural Science Foundation of China （NSFC） under grants U1234201, U1334203, and 51378439.