Acknowledgment: This research is supported by the National Natural Science Foundation of China (Grant No. 71704016,71331008, 71402010), the Natural Science Foundation of Hunan Province (Grant No. 2017JJ2267), the Educational Economy and Financial Research Base of Hunan Province (Grant No. 13JCJA2), and the Project of China Scholarship Council for Overseas Studies (201508430121, 201208430233).