Correction to: Nature Physics https://doi.org/10.1038/s41567-018-0070-7, published online 19 March 2018.

In the version of this Letter originally published, the key to the green and open circles in Fig. 4 was reversed; it should have shown that the green circles correspond to ‘From χ versus T’ and the open circles correspond to ‘From χ versus Hy’. This has now been corrected in all versions of the Letter.