Doping evolution of the gap structure and spin-fluctuation pairing in $Ba(F{e}_{1-x}C{o}_x{)}_{2}A{s}_2$ superconductors

Abstract

Doping dependence of the superconducting state structure and spin-fluctuation pairing mechanism in the $Ba(F{e}_{1-x}C{o}_x{)}_{2}A{s}_2$ family is studied. BCS-like analysis of experimental data shows that in the overdoped regime, away from the AFM transition, the spin-fluctuation interaction between the electron and hole gaps is weak, and $Ba(F{e}_{1-x}C{o}_x{)}_{2}A{s}_2$ is characterized by three essentially different gaps. In the three-gap state an anisotropic (nodeless) electron gap ${\Delta }_e(x,\varphi )$ 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 ${\Delta }_e(x,\varphi )$ 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 ${\Delta }_{2h}(x)\approx {\Delta }_e(x,\varphi )$ is also preserved in the phase of coexisting antiferromagnetism and superconductivity. The doping dependence of the electron gap ${\Delta }_e(x,\varphi )$ is associated with the strong doping dependence of the spin-fluctuation interaction in the AFM transition region. In contrast to the electron gap ${\Delta }_e(x,\varphi )$, 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 non-phonon 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 $Ba(F{e}_{1-x}C{o}_x{)}_{2}A{s}_2$ family.

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