Correction to: Nature Physics https://doi.org/10.1038/s41567-019-0692-4, published online 28 October 2019.

In the version of this Article originally published, in the paragraph beginning ‘The bias and temperature dependence’, the units for two of the energy ranges of the spin gap were incorrect. ‘ΔE ≈ 150–300 eV’ should have been ‘ΔE ≈ 150–300 µeV’ and ‘eVdc ≈ 200–400 eV’ should have been ‘eVdc ≈ 200–400 µeV’. The online versions of the Article have been corrected.