Nematic spin fluid in the tetragonal phase of BaFe2As2

We use inelastic neutron scattering to study spin waves below and above T-N in iron-arsenide BaFe2As2. In the low-temperature orthorhombic phase, we find highly anisotropic spin waves with a large damping along the antiferromagnetic a-axis direction. On warming the system to the paramagnetic tetragonal phase, the low-energy spin waves evolve into quasi-elastic excitations, while the anisotropic spin excitations near the zone boundary persist. These results strongly suggest the presence of a spin nematic fluid in the tetragonal phase of BaFe2As2, which may cause the electronic and orbital anisotropy observed in these materials.