Direct observation of spin excitation anisotropy in the paramagnetic orthorhombic state of BaFe2-xNixAs2

We use transport and inelastic neutron-scattering measurements to investigate single crystals of iron pnictide BaFe2-xNix As-2 (x = 0,0.03), which exhibit a tetragonal-to-orthorhombic structural transition at T-s and stripe antiferromagnetic order at T-N (T-s >= T-N). Using a tunable uniaxial pressure device, we detwin the crystals and study their transport and spin excitation properties at antiferromagnetic wave-vector S-1(1,0) and its 90 degrees rotated wave-vector S-2(0,1) under different pressure conditions. We find that uniaxial pressure necessary to detwin and maintain the single domain orthorhombic antiferromagnetic phase of BaFe2-x Ni x As-2 induces resistivity and spin excitation anisotropy at temperatures above zero pressure T-s. In the uniaxial pressure-free detwinned sample, spin excitation anisotropy between S-1(1,0) and S-2(0,1) first appears in the paramagnetic orthorhombic phase below T-s. These results are consistent with predictions of spin nematic theory, suggesting the absence of structural or nematic phase transition above T-s in iron pnictides.