Dependence of carrier doping on the impurity potential in transition-metal-substituted FeAs-based superconductors

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In order to examine to what extent the rigid-band-like electron doping scenario is applicable to the transition metal-substituted Fe-based superconductors, we have performed angle-resolved photoemission spectroscopy studies of Ba(Fe₁₋ₓNiₓ)₂As₂ (Ni-122) and Ba(Fe₁₋ₓCuₓ)₂As₂ (Cu-122), and compared the results with Ba(Fe₁₋ₓCoₓ)₂As₂ (Co-122). We find that Ni 3d-derived features are formed below the Fe 3d band and that Cu 3d-derived ones further below it. The electron and hole Fermi surface (FS) volumes are found to increase and decrease with substitution, respectively, qualitatively consistent with the rigid-band model. However, the total extra electron number estimated from the FS volumes (the total electron FS volume minus the total hole FS volume) is found to decrease in going from Co-, Ni-, to Cu-122 for a fixed nominal extra electron number, that is, the number of electrons that participate in the formation of FS decreases with increasing impurity potential. We find that the Neel temperature TN and the critical temperature Tc maximum are determined by the FS volumes rather than the nominal extra electron concentration nor the substituted atom concentration.