Fig. 1: NMR evidence for sample detwinning under uniaxial stress.

a, b ⁷⁵As-NMR central line in LaFeAsO with (b) and without (a) uniaxial stress at T = 141 K and H₀ = 15 T. Upon application of stress, the integrated intensity of the higher-frequency peak increases at the expense of the low-frequency peak. c, d ⁷⁷Se-NMR spectra of FeSe with (d) and without (c) stress at T = 10 K and H₀ = 9 T, respectively. At variance with LaFeAsO, the remaining intensity of the second peak in FeSe indicates that the crystal is not fully detwinned. This is because the maximum stress applicable on FeSe crystals is about half of that for LaFeAsO. FeSe distorts and exfoliates above ~5 MPa, while LaFeAsO does not break up to at least 15 MPa. This thus limits the maximal stress that can be applied on FeSe and explains that we could not fully detwin it. LaFeAsO spectra are fitted by three Lorentzian functions (the small peak at ~109.77 MHz is present at any temperature and is attributed to an impurity—see Supplementary Fig. 6). FeSe spectra are fitted by two Lorentzian functions. The 1:1 area ratio of the peaks in a, c shows that the twin domains are equally populated in the unstrained samples. The different widths for the two peaks are consistent with other reports (e.g., ref. ⁶¹) but not presently understood. Notice that the quadrupolar interaction contributes to the peak splitting in LaFeAsO. As explained in Supplementary Note 2, this effect has been taken into account for extracting the Knight-shift anisotropy in Fig. 2. e Schematic drawing of the uniaxial stress device with the NMR coil.