Polarized neutron scattering as a probe of spin nematicity in the iron-based superconductor BaFe$_{\mathrm{2-x}}$Ni$_{\mathrm{x}}$As$_{2}$*

We use polarized neutron scattering to demonstrate that in-plane spin excitations in electron doped superconductor BaFe1.904Ni0.096As2 [1] change from isotropic to anisotropic in the tetragonal phase well above the antiferromagnetic ordering and tetragonal-to-orthorhombic lattice distortion temperatures without an uniaxial pressure [2]. The anisotropic low-energy spin excitations at the same momentum transfer share similar features with the spin nematic phase probed in the detwinned samples with uniaxial pressure, and consistent with in-plane resistivity anisotropy [3]. These results indicate that the polarized neutron scattering is a good probe of the spin nematicity in the tetragonal phase of iron pnictides [4]. References [1] Yanchao Chen, et.al., Supercond. Sci. Technol. 24, 065004 (2011) [2] Huiqian Luo, et.al., Phys. Rev. Lett. 111, 107006 (2013). [3] Xingye Lu, et.al., Science 345, 657 (2014) [4] Huiqian Luo, et.al., unpublished manuscript (2015).