Acknowledgements

The authors thank Dr. X. Chen, K. Shi and X. J. Ding for helpful discussions. This work was financially supported by Major State Basic Research Development Program of China (Grant Nos. 2013CB922300), Natural Science Foundation of China (Grant Nos. 61674057, 11374097, 61376129, 61504156, and 61227902), Projects of Science and Technology Commission of Shanghai Municipality (Grant Nos. 15JC1401600 and 14XD1401500), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.
