This paper is dedicated to the memory of N. B. Ming, who supported the Center for Microstructure of Quantum Materials at Nanjing University. This work was supported by the National Basic Research Program of China (grant 2015CB654901), the National Natural Science Foundation of China (grants 11574135, 51772143, 11474147, 51672125, 11774153 and 11874199), the Fundamental Research Funds for the Central Universities (grant 0213-14380058), and the National Natural Science Foundation of China/The Research Grants Council of Hong Kong (NSFC/RGC, grant 11861161004). Y.N. and P.W. are supported by the National Thousand-Young-Talents Program and the Program for High-Level Entrepreneurial and Innovative Talents Introduction, Jiangsu Province. S.C. is supported by Program A for Outstanding Ph.D. candidate of Nanjing University (grant 201801A013) and the Postgraduate Research and Practice Innovation Program of Jiangsu Province (grant KYCX18_0045). TEM work at the University of California, Irvine used the facilities of the Irvine Materials Research Institute (IMRI) and was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under grant DE-SC0014430. The research at the University of Nebraska–Lincoln is supported by the National Science Foundation (NSF) under the Nebraska MRSEC programme (grant DMR-1420645). Y.N. acknowledges discussions with H. Hwang, D. Li and S. S. Hong.

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Nature thanks Gertjan Koster and the other anonymous reviewer(s) for their contribution to the peer review of this work.