Decoupling of the superconducting and magnetic/structural phase transitions in electron-doped ${\text{BaFe}}_2{\text{As}}_2$

Study and comparison of over 30 examples of electron-doped ${\text{BaFe}}_2{\text{As}}_2$ for transition metal $\left(\text{TM}\right)=\text{Co}$, 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.

Figure 1

(Color) The temperature-dependent resistivity, normalized by room-temperature value, for electron-doped $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{TM}}_x\right)}_2{\text{As}}_2$ ($\text{TM}=\text{Co}$, Ni, Cu, and Co/Cu) series: (a) $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$.[3] Inset: low-temperature data for $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ (b) $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Ni}}_x\right)}_2{\text{As}}_2$. Inset: low-temperature data for $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Ni}}_x\right)}_2{\text{As}}_2$, (c) $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Cu}}_x\right)}_2{\text{As}}_2$, (d) $\text{Ba}{\left({\text{Fe}}_{1-x-y}{\text{Co}}_x{\text{Cu}}_y\right)}_2{\text{As}}_2$. Inset: low-temperature data for $\text{Ba}{\left({\text{Fe}}_{1-x-y}{\text{Co}}_x{\text{Cu}}_y\right)}_2{\text{As}}_2$.

Figure 2

(Color online) (a) Transition temperature as a function of the number of substitutional transition-metal ions per Fe site; (b) transition temperature as a function of extra electrons contributed by TM substitution per Fe site. For both plots the transition temperatures were determined in a manner similar to that described in Ref. 3 and the text.

Figure 3

(Color online) Normalized structural parameters measured at $\sim 300\text{K}$. (a) $a/a_0$, (b) $c/c_0$, (c) $V/V_0$, and (d) $\left(a/c\right)/\left(a_0/c_0\right)$ as a function of transition-metal doping, $x$, and (e) $\left(a/c\right)/\left(a_0/c_0\right)$ as a function of extra conduction electrons, $e$. ($a_0=3.9621\text{Å}$ and $c_0=13.0178\text{Å}$).