Nematic spin correlations in the tetragonal state of uniaixial-strained BaFe$_{2-x}$Ni$_x$As$_2$

Understanding the microscopic origins of electronic phases in high-transition temperature (high-$T_c$) superconductors is important for elucidating the mechanism of superconductivity. In the paramagnetic tetragonal phase of BaFe$_{2-x}T_x$As$_2$ (where $T$ is Co or Ni) iron pnictides, an in-plane resistivity anisotropy has been observed. Here, we use inelastic neutron scattering to show that low-energy spin excitations in these materials change from fourfold symmetric to twofold symmetric at temperatures corresponding to the onset of the in-plane resistivity anisotropy. Because resistivity and spin excitation anisotropies both vanish near optimal superconductivity, we conclude that they are likely intimately connected.