We use inelastic neutron scattering to study spin excitation anisotropy in mechanically detwinned $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ with $x=0.048$ and 0.054. Both samples exhibit a tetragonal-to-orthorhombic structural transition at $T_s$, a collinear static antiferromagnetic order at wave vector ${\mathbf{Q}}_1={\mathbf{Q}}_{\mathrm{AF}}=(1,0)$ below the $\mathrm{N}\acute{\mathrm{e}}\mathrm{el}$ temperature $T_N$, and superconductivity below $T_c$ ($T_s>T_N>T_c$). In the high-temperature paramagnetic tetragonal phase ($T\gg T_s$), spin excitations centered at ${\mathbf{Q}}_1$ and ${\mathbf{Q}}_2=(0,1)$ are gapless and have fourfold ($C_4$) rotational symmetry. On cooling to below $T_N$ but above $T_c$, spin excitations become highly anisotropic, developing a gap at ${\mathbf{Q}}_2$ but still are gapless at ${\mathbf{Q}}_1$. Upon entering into the superconducting state, a neutron spin resonance appears at ${\mathbf{Q}}_1$ with no magnetic scattering at ${\mathbf{Q}}_2$. By comparing these results with those from angle-resolved photoemission spectroscopy experiments, we conclude that the anisotropic shift of the $d_{yz}$ and $d_{xz}$ bands in detwinned $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ below $T_s$ is associated with the spin excitation anisotropy, and the superconductivity-induced resonance arises from the electron-hole Fermi surface nesting of quasiparticles with the $d_{yz}$ orbital characters.

• Received 27 July 2019
• Revised 14 September 2019

DOI:https://doi.org/10.1103/PhysRevB.100.134509