Doping evolution of the gap structure and spin-fluctuation pairing in Ba(Fe1-xCox)2As2 superconductors

Doping dependence of the superconducting state structure and spin-fluctuation pairing mechanism in the Ba(Fe1-xCox)(2)As-2 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(Fe1-xCox)(2)As-2 is characterized by three essentially different gaps. In the three-gap state an anisotropic (nodeless) electron gap Delta(e)(x, phi) has an intermediate value between the dominant inner Delta(2h)(x) and outer Delta(1h)(x) hole gaps. Close to the AFM transition the electron gap Delta(e)(x, phi) increases sharply and becomes closer in magnitude to the dominant inner hole gap Delta(2h)(x). The same two-gap state with close electron and inner hole gaps Delta(2h)(x) approximate to Delta(e)(x, phi) is also preserved in the phase of coexisting antiferromagnetism and superconductivity. The doping dependence of the electron gap Delta(e)(x, phi) is associated with the strong doping dependence of the spin-fluctuation interaction in the AFM transition region. In contrast to the electron gap Delta(e)(x, phi), the doping dependence of the hole gaps Delta(1,2h)(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 gamma(nh)(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(Fe1-xCox)(2)As-2 family.