Acknowledgments
This work was supported by funding from the 973 National Fundamental Research Program of China (2015CB553904 to Q.Z.), the National Natural Science Foundation of China (81021061 and 81230047 to Q.Z., 81330063 and 81272189 to Y.C., 81272694 to X. Cheng, 81201956 to J.L., and 81402342 to L.Z.), the Key Project of Chinese Ministry of Education (NO213005A to Y.C.), the Specialized Research Fund for the Doctoral Program of Higher Education (20121417110001 to Y.C.), a research project supported by the Shanxi Scholarship Council of China (2013-053 to Y.C.), the Innovative Team in Science & Technology of Shanxi (2013131023 to Y.C.), and the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi (to Y.C.).