Neutron spin resonance as a probe of Fermi surface nesting and superconducting gap symmetry in Ba\(_{0.67}\)K\(_{0.33}\)(Fe\(_{1-x}\)Co\(_{x}\))\(_{2}\)As\(_{2}\)

Abstract

We use inelastic neutron scattering to study energy and wave vector dependence of the superconductivity-induced resonance in hole-doped Ba\(_{0.67}\)K\(_{0.33}\)(Fe\(_{1-x}\)Co\(_{x}\))\(_{2}\)As\(_{2}\) (\(x=0,0.08\) with \(T_c\approx 37, 28\) K, respectively). In previous work on electron-doped Ba(Fe\(_{0.963}\)Ni\(_{0.037}\))\(_2\)As\(_2\) (\(T_N=26\) K and \(T_c=17\) K), the resonance is found to peak sharply at the antiferromagnetic (AF) ordering wave vector \({\bf Q}_{\rm AF}\) along the longitudinal direction, but disperses upwards away from \({\bf Q}_{\rm AF}\) along the transverse direction. For hole doped \(x=0, 0.08\) without AF order, we find that the resonance displays ring-like upward dispersion away from \({\bf Q}_{\rm AF}\) along both the longitudinal and transverse directions. By comparing these results with calculations using the random phase approximation, we conclude that the dispersive resonance is a direct signature of isotropic superconducting gaps arising from nested hole-electron Fermi surfaces.