Theory of electron nematic order in LaFeAsO. NMR search for the spin nematic state in a LaFeAsO single crystal. Crystal structure of the new FeSe1− x superconductor. Tetragonal-to-orthorhombic structural phase transition at 90 K in the superconductor Fe1.01Se. Nematic susceptibility of hole-doped and electron-doped BaFe2As2 iron-based superconductors from shear modulus measurements. Contrasts in electron correlations and inelastic scattering between LiFeP and LiFeAs revealed by charge transport. Divergent nematic susceptibility in an iron arsenide superconductor. Symmetry-breaking orbital anisotropy observed for detwinned Ba(Fe1− xCo x)2As2 above the spin density wave transition. We demonstrate that superconductivity competes with the emerging nematicity. Spin–lattice relaxation rates are not affected at T nem, which unequivocally establishes orbital degrees of freedom as driving the nematic order. The splitting occurs for magnetic fields perpendicular to the Fe planes and has the temperature dependence of a Landau-type order parameter. Here, we report a very clear splitting of NMR resonance lines in FeSe at T nem = 91 K, far above the superconducting T c of 9.3 K. Setting the stage for superconductivity, it is heavily debated whether the nematic symmetry breaking is driven by lattice, orbital or spin degrees of freedom. In addition to a tendency towards magnetic order, these Fe-based systems have a propensity for nematic ordering: a lowering of the rotational symmetry while time-reversal invariance is preserved. A fundamental and unconventional characteristic of superconductivity in iron-based materials is that it occurs in the vicinity of two other instabilities.
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