Calorimetric evidence of multiband superconductivity in $\text{Ba}{\left({\text{Fe}}_{0.925}{\text{Co}}_{0.075}\right)}_2{\text{As}}_2$ single crystals

We report on the determination of the electronic heat capacity of a slightly overdoped $\left(x=0.075\right)$ $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ single crystal with a $T_c$ of 21.4 K. Our analysis of the temperature dependence of the superconducting-state specific heat provides strong evidence for a two-band $s$-wave order parameter with gap amplitudes $2{\Delta }_1\left(0\right)/{\text{k}}_B{\text{T}}_c=1.9$ and $2{\Delta }_2\left(0\right)/{\text{k}}_B{\text{T}}_c=4.4$.

Figure 1

(Color online) Temperature dependence of the specific heat $C/T$ of samples with $x=0.075$ and $x=0.153$. The dashed line represents the lattice contribution, $C_{lat}/T$, derived from the specific heat for $x=0.153$ (see text). The inset shows the low-temperature specific heat of both samples.

Figure 2

Temperature dependence of the electron specific heat, $C_e/T$, of the superconducting sample $\left(x=0.075\right)$. The dashed line represents the normal-state electron contribution, ${\gamma }_n=23.8\text{mJ}{\text{mol}}^{-1}{\text{K}}^{-2}$. The dotted line is a residual normal-state-like contribution, ${\gamma }_r=5.77\text{mJ}{\text{mol}}^{-1}{\text{K}}^{-2}$. The inset shows the normal- and superconducting-state electronic entropies.

Figure 3

(Color online) (a) The electron specific heat of the superconducting sample $\left(x=0.075\right)$, normalized to 1 mol of superconducting condensate, compared with the specific heat of single-band $s$-wave (blue line) and $d$-wave (green line) order parameters, in the weak-coupling limit. (b) The electron specific heat of the superconducting sample $\left(x=0.075\right)$, normalized to 1 mol of superconducting condensate. The red curve represents a two-gap fit. The blue and green curves are the partial specific-heat contributions of the two bands.