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 Ba(Fe1-xCox)(2)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 Q(1) = Q(AF) = (1, 0) below the Neel temperature T-N, and superconductivity below T-c (T-s > T-N > T-c). In the high-temperature paramagnetic tetragonal phase (T >> Ts), spin excitations centered at Q(1) and 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 Q(2) but still are gapless at Q(1). Upon entering into the superconducting state, a neutron spin resonance appears at Q(1) with no magnetic scattering at 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 Ba(Fe1-xCox)(2)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.