Acknowledgments
The author is grateful to Dr. Xunchang (John) ZHANG for constructive comments on the manuscript. This research was jointly supported by the National Key R&D Program of China (No. 2016YFE0202900), the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2020JM-163), the Natural Science Foundation of China (No. 41761144060), the International Partnership Program of Chinese Academy of Sciences (No. 161461KYSB20170013).