Title: Ising-nematic spin correlations in the tetragonal state of uniaxial strained iron pnictides
Abstract: A determination of the microscopic origins of electronic phases in high-transition temperature (high-Tc) superconductors is important for understanding the mechanism of superconductivity. In the paramagnetic tetragonal phase, an in-plane resistivity anisotropy has been observed in the BaFe2−xTxAs2 (T = Co, Ni) iron pnictides, but its microscopic origin is unknown. Here we use inelastic neutron scattering to show that low-energy spin excitations in these materials change from four-fold symmetry to two-fold symmetry at temperatures corresponding to the onset of the in-plane resistivity anisotropy. Since both the resistitivy and spin excitation anisotropy vanishes near optimal superconductivity, we conclude that the spin Ising-nematic phase leads to the resistivity anisotropy and is also important for high-Tc superconductivity.
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