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

Superconductivity in iron pnictides can be derived from electron or hole-doping to their antiferromagnetic (AF) ordered parent compounds. In the normal state above the superconducting transition temperature (Tc), an in-plane resistivity anisotropy has been observed in uniaxial strained iron pnictides BaFe$_{2-x}$T$_x$As$_2$ (T $=$ Co, Ni) near the AF ordered state. This anisotropy has been suggested as a signature of the spin Ising-nematic phase that breaks the in-plane four-fold rotational symmetry of the underlying tetragonal lattice, but direct evidence for a spin nematic state is lacking. Here we use inelastic neutron scattering (INS) to show that low-energy spin excitations in uniaxial strained BaFe$_{2-x}$Ni$_x$As$_2$ (x $=$ 0, 0085) change from four-fold symmetry to two-fold symmetry in the tetragonal phase at temperatures corresponding to the onset of the in-plane resistivity anisotropy. Our results thus indicate that spin excitation anisotropy plays a crucial role in the electronic nematic behavior of iron pnictides.