Doping evolution of the gap structure and spin-fluctuation pairing in Ba (Fe1-xC ox ) 2A s2 superconductors

Doping dependence of the superconducting state structure and spin-fluctuation pairing mechanism in the Ba (F_{e1-xC ox ) 2}A s₂ family is studied. BCS-like analysis of experimental data shows that in the overdoped regime, away from the antiferromagnetic (AFM) transition, the spin-fluctuation interaction between the electron and hole gaps is weak, and Ba (F_{e1-xC ox ) 2}A s₂ is characterized by three essentially different gaps. In the three-gap state an anisotropic (nodeless) electron gap Δ_e(x ,φ ) has an intermediate value between the dominant inner Δ_{2 h}(x ) and outer Δ_{1 h}(x ) hole gaps. Close to the AFM transition the electron gap Δ_e(x ,φ ) increases sharply and becomes closer in magnitude to the dominant inner hole gap Δ_{2 h}(x ) . The same two-gap state with close electron and inner hole gaps Δ_{2 h}(x ) ≈Δ_e(x ,φ ) is also preserved in the phase of coexisting antiferromagnetism and superconductivity. The doping dependence of the electron gap Δ_e(x ,φ ) is associated with the strong doping dependence of the spin-fluctuation interaction in the AFM transition region. In contrast to the electron gap Δ_e(x ,φ ) , the doping dependence of the hole gaps Δ_{1 ,2 h}(x ) and the critical temperature T_c(x ) , both before and after the AFM transition, are associated with a change of the density of states γ_{n h}(x ) and the intraband electron-phonon interaction in the hole bands. The nonphonon spin-fluctuation interaction in the hole bands in the entire Co concentration range is small compared with the intraband electron-phonon interaction and is not dominant in the Ba (F_{e1-xC ox ) 2}A s₂ family.