Spin fluctuation anisotropy as a probe of orbital-selective hole-electron quasiparticle excitations in detwinned Ba(Fe1−xCox)2As2

We use inelastic neutron scattering to study spin excitation anisotropy in mechanically detwinned $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_x)}₂{\mathrm{As}}₂$ 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 $\mathrm{N}\stackrel{\ifmmode \acute{}\else \'{}\fi{}}{\mathrm{e}}\mathrm{el}$ temperature ${T}_N$, and superconductivity below ${T}_c$ (${T}_s>{T}_N>{T}_c$). In the high-temperature paramagnetic tetragonal phase ($T\ensuremath{\gg}{T}_s$), 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 $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_x)}₂{\mathrm{As}}₂$ 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.