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

Abstract

Doping dependence of the superconducting state structure and spin-fluctuation pairing mechanism in the Ba(Fe1−xCox)2As2 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)2As2 is characterized by three essentially different gaps. In the three-gap state an anisotropic (nodeless) electron gap Δe(x,φ) has an intermediate value between the dominant inner Δ2h(x) and outer Δ1h(x) hole gaps. Close to the AFM transition the electron gap Δe(x,φ) increases sharply and becomes closer in magnitude to the dominant inner hole gap Δ2h(x). The same two-gap state with close electron and inner hole gaps Δ2h(x)≈Δe(x,φ) is also preserved in the phase of coexisting antiferromagnetism and superconductivity. The doping dependence of the electron gap Δe(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 Δe(x,φ), the doping dependence of the hole gaps Δ1,2h(x) and the critical temperature Tc(x), both before and after the AFM transition, are associated with a change of the density of states γnh(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)2As2 family.

Source-reported materials — not catalogue approval

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