Competing Spin and Charge Symmetry Breaking in the Nematic State of EuFe 2 As 2 Revealed by 57 Fe Nuclear Forward Scattering

The electronic and magnetic symmetries of EuFe 2 As 2 at ambient pressure were investigated by combining 57 Fe nuclear forward scattering (NFS) with magnetic-field-induced detwinning. While the magnetic hyperfine field ([Formula: see text]) is consistent with the conventional stripe-type antiferromagnetic (sAFM) configuration, the electric field gradient (EFG) tensor reveals a spontaneous symmetry lowering to monoclinic. Furthermore, this monoclinic distortion persists even in the pressure-induced paramagnetic phase at low temperatures, which is consistent with the asymmetry of the X-ray diffraction peaks. Our findings characterize a unique state arising from an intense competition between the spin-density ([Formula: see text]) and charge-density (EFG) channels. This monoclinic charge-density modulation, coexisting with collinear sAFM order, challenges the conventional orthorhombic picture of iron-based superconductors and provides a new perspective on multi-channel nematic instability.