Acknowledgments: This work is supported in part by the National Natural Science Foundation of China under Grant No. 61309027, 61772553, 61702562, and 61702561; the Scientific Research Fund of Hunan Provincial Education Department under No. 13B148; the China Postdoctoral Science Foundation under No. 2013M542136; and the Key Research and Development Project of Hunan Science and Technology Plan Project under No. 2016SK2028.