Jump in specific heat at the superconducting transition temperature in $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ and $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Ni}}_x\right)}_2{\text{As}}_2$ single crystals

We present detailed heat capacity measurements for $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ and $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Ni}}_x\right)}_2{\text{As}}_2$ single crystals in the vicinity of the superconducting transitions. The specific-heat jump at the superconducting transition temperature $\left(T_c\right)$, $\Delta C_p/T_c$, changes by a factor of $\sim 10$ across these series. The $\Delta C_p/T_c$ vs $T_c$ data of this work [together with the literature data for $\text{Ba}{\left({\text{Fe}}_{0.939}{\text{Co}}_{0.061}\right)}_2{\text{As}}_2$, $\left({\text{Ba}}_{0.55}{\text{K}}_{0.45}\right){\text{Fe}}_2{\text{As}}_2$, and $\left({\text{Ba}}_{0.6}{\text{K}}_{0.4}\right){\text{Fe}}_2{\text{As}}_2$] scale well to a single log-log plot over two orders of magnitude in $\Delta C_p/T_c$ and over about an order of magnitude in $T_c$, giving $\Delta C_p/T_c\propto T_c^2$.

Figure 1

(Color online) Temperature-dependent specific heat of (a) $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$, (b) $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Ni}}_x\right)}_2{\text{As}}_2$ single crystals plotted as $C_p/T$ vs $T$. Inset to panel (b): enlarged $C_p/T$ vs $T$ plot near the superconducting transition for $\text{Ba}\left({\text{Fe}}_{0.954}{\text{Ni}}_{0.046}\right){\text{As}}_2$; lines show how $T_c$ and $\Delta C_p/T_c$ are estimated. Data in panels (a) and (b) are shifted by a multiple of $50\text{mJ}/\text{mol}{\text{K}}^2$ along the $y$ axis for clarity. Panel (c): data for both series plotted as $C_p/T$ vs $T^2$ without shifts.

Figure 2

(Color online) $\Delta C_p/T_c$ (circles, left axis) and $T_c$ (asterisks, right axis) as a function of concentration, $x$, $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ (upper panel) and $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Ni}}_x\right)}_2{\text{As}}_2$ (lower panel).

Figure 3

(Color online) $\Delta C_p/T_c$ vs $T_c$ for $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ (circles) and $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Ni}}_x\right)}_2{\text{As}}_2$ (stars) plotted together with literature data for $\text{Ba}{\left({\text{Fe}}_{0.939}{\text{Co}}_{0.061}\right)}_2{\text{As}}_2$ (Ref. 8), ${\text{Ba}}_{0.55}{\text{K}}_{0.45}{\text{Fe}}_2{\text{As}}_2$ (Ref. 18), ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_2{\text{As}}_2$ (Refs. 19, 20, 21) and $\text{Ba}{\left({\text{Fe}}_{0.943}{\text{Rh}}_{0.057}\right)}_2{\text{As}}_2$ and $\text{Ba}{\left({\text{Fe}}_{0.957}{\text{Pd}}_{0.043}\right)}_2{\text{As}}_2$ (Ref. 22). Dashed line has a slope $n=2$ and is a guide for the eyes.