  Understanding the microscopic origins of electronic phases in high-transition
temperature (high-Tc) superconductors is important for elucidating the
mechanism of superconductivity. In the paramagnetic tetragonal phase of
BaFe2-xTxAs2 (where T is Co or Ni) iron pnictides, an in-plane resistivity
anisotropy has been observed. Here we use inelastic neutron scattering to show
that low-energy spin excitations in these materials change from four-fold
symmetric to two-fold symmetric at temperatures corresponding to the onset of
the in-plane resistivity anisotropy. Because resistivity and spin excitation
anisotropies both vanish near optimal superconductivity, we conclude that they
are likely intimately connected.
