Experimental and theoretical work at Princeton University was supported by the Gordon and Betty Moore Foundation (GBMF4547/Hasan) and the United States Department of Energy (US DOE) under the Basic Energy Sciences programme (grant number DOE/BES DE-FG-02-05ER46200). Work at the Institute of Physics of the Chinese Academy of Science (IOP CAS) was supported by the National Key R&D Program of China (grant number 2017YFA0206303). Work at Boston College is supported by US DOE grant DE-FG02-99ER45747. We also acknowledge the Natural Science Foundation of China (grant numbers 11790313, 11774422 and 11774424), National Key R&D Program of China (numbers 2016YFA0300403 and 2017YFA0302903), the Key Research Program of the Chinese Academy of Sciences (number XDPB08-1), Princeton Center for Theoretical Science (PCTS) and Princeton Institute for the Science and Technology of Materials (PRISM)’s Imaging and Analysis Center at Princeton University. T.-R.C. was supported by the Ministry of Science and Technology under a MOST Young Scholar Fellowship (MOST Grant for the Columbus Program number 107-2636-M-006-004-), the National Cheng Kung University, Taiwan, and the National Center for Theoretical Sciences (NCTS), Taiwan. M.Z.H. acknowledges support from Lawrence Berkeley National Laboratory and the Miller Institute of Basic Research in Science at the University of California, Berkeley in the form of a Visiting Miller Professorship. We thank D. Huse and T. Neupert for discussions.

Reviewer information

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