Spin-fluctuation mechanism of superconductivity in Ba(Fe1-xCox)2As2 ferropnictides

Abstract

Gap structure and spin-fluctuation pairing mechanism in the iron-based superconductors Ba(Fe1-xCox)2As2 is studied. We demonstrate that the overdoped compounds Ba(Fe1-xCox)2As2 have a three-gap structure with substantially different order parameters and a weak interband spin-fluctuation coupling. In the three-gap state of Ba(Fe1-xCox)2As2, the anisotropic electronic gap has an intermediate value between two isotropic hole gaps. During transition to the optimal regime, the electron gap strongly increases (by a factor of two) and gets close to the largest hole gap value, while the electron gap anisotropy sharply decreases pointing to a strong increase in the interband interaction, with spin fluctuations accompanying the antiferromagnetic transition. The same two-gap state with close values of the electron and largest hole gaps is preserved even with further decrease in doping. The performed study shows that the spin-fluctuation pairing mechanism has a significant effect on doping evolution of the electron gap only. The doping evolution of the dominant hole gap in Ba(Fe1-xCox)2As2 is not directly related to the evolution of the spin fluctuations.