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Specific heat of Ba0.68K0.32Fe2As2: evidence for multiband strong-coupling superconductivity

The specific heat of high-purity $\rm Ba_{0.68}K_{0.32}Fe_2As_2$ single crystals with the highest reported superconducting $T_c$ = 38.5 K was studied. The electronic specific heat, $C_p$, below $T_c$ shows two gap features, with $Δ_1 \approx 11$ meV and $Δ_2 \approx 3.5$ meV obtained from an $α$-model analysis. The reduced gap value, $2Δ^{\rm max} / k_B T_c \approx 6.6$, the magnitude of the specific heat jump, $ΔC_p(T_c)/T_c$, and its slope below $T_c$ exhibit strong-coupling character. We also show that an Eliashberg model with two hole and two electron bands gives the correct values of $T_c$, the superconducting gaps, and the temperature dependence of the free-energy difference.