Decoupling of the superconducting and magnetic/structural phase transitions in electron-doped BaFe₂As₂

Study and comparison of over 30 examples of electron-doped BaFe₂As₂ for transition metal (TM)=Co, Ni, Cu, and (Co/Cu mixtures) have led to an understanding that the suppression of the structural/antiferromagnetic phase transition to low-enough temperature in these compounds is a necessary condition for superconductivity but not a sufficient one. Whereas the structural/antiferromagnetic transitions are suppressed by the number of TM dopant ions (or changes in the c axis) the superconducting dome exists over a limited range of values of the number of valence electrons added by doping (or values of the a/c ratio). By choosing which combination of dopants is used we can change the relative positions of the upper phase lines and the superconducting dome, even to the extreme limit of suppressing the upper structural and magnetic phase transitions without the stabilization of a lower-temperature superconducting dome.