We thank M. Gajda for discussions and M. McDonald for a careful reading of the manuscript. This work was supported by National Science Foundation (NSF) grant no. PHY-1511696 and the University of Chicago Materials Research Science and Engineering Center, which is funded by the NSF under grant no. DMR-1420709.

Reviewer information

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