Correction to: Nature Nanotechnology https://doi.org/10.1038/s41565-018-0259-z, published online 24 September 2018.

In the version of this Letter originally published, the right-hand arrow in Fig. 3b was incorrectly labelled \([\bar 10]_{\mathrm{o}}\); it should have been \([\bar 101]_{\mathrm{o}}\); this has now been corrected. Also, ref. 29 was incorrectly included in the reference list; it has now been removed.