Pressure dependence of the nematic spin correlations in detwinned BaFe$_{1.97}$Ni$_{0.03}$As$_{2}$*

In the paramagnetic tetragonal phase of BaFe$_{\mathrm{2-x}}$Ni$_{\mathrm{x}}$As$_{2}$, inelastic neutron scattering shows a spin excitation anisotropy emerges at a temperature well above the structure transition temperature ($T_{\mathrm{s}})$ [1], consistent with the onset of in-plane resistivity anisotropy [2]. However, how the applied uniaxial strain, which artificially breaks the four-fold symmetry, influences the observed anisotropy is still unclear. Here we studied the pressure dependence of the nematic spin correlations in detwinned BaFe$_{1.97}$Ni$_{0.03}$As$_{2}$. We find that the spin excitation anisotropy temperature ($T^{\ast})$ is largely enhanced when changing the pressure from a mediate level ($\sim$ 8MPa) to a high level ($\sim$ 15MPa) [3]. Our results suggest the $T^{\ast}$ may be not a characteristic temperature where the system transit to a nematic phase, but a temperature the nematic fluctuations can reach under a uniaxial stress.\\[4pt] [1] Xingye Lu et al., Science 345, 657-660 (2014)\\[0pt] [2] J. H. Chu et al., Science 329, 824-826 (2010)\\[0pt] [3] Wenliang Zhang et al., unpublished manuscript (2015).