fitting experimental data.

We thank Drs C.-L. Wang, A. A. Bokov and L. Bellaiche for fruitful discussion. This work is financially supported by the National Natural Science Foundation of China (NSFC), Grant No. 51390472, 11574246, U1537210, and National Basic Research Program of China, Grant No. 2015CB654903. F.L. acknowledges NSFC Grant No. 51572214. Z.J. acknowledges the support from China Scholarship Council (CSC No. 201506280055). We also acknowledge the “111 Project of China (Grant No. B14040), the Natural Science and Engineering Council of Canada (NSERC) and the United States Office of Naval Research (ONR Grants No. N00014-12-1-1045 and N00014-16-1-3106).

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