Acknowledgements

This work is partly supported by National Natural Science Foundation of China (Grant No. 41775128, 41405034, 11204319), the External Cooperation Program of the Chinese Academy of Sciences (Grant No. GJHZ1726), the Special Fund for Basic Research on Scientific Instruments of the Chinese Academy of Science (Grant No. YZ201315), and the Chinese Academy of Science President's International Fellowship Initiative (PIFI, 2015VMA007). Finally, the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, is gratefully acknowledged.
