Jump in specific heat at the superconducting transition in iron pnictides*

Experiments reveal that in iron-based superconductors the jump $\Delta C$ of the specific heat at the superconducting transition is not proportional to the transition temperature $T_c$, as expected in the BCS theory. Rather, the ratio $\Delta C/T_c$ varies with $T_c$, and has a peak near optimal doping and decreases at smaller and larger dopings. We show that this behavior can be naturally explained by the interplay between superconductivity and antiferromagnetism in the underdoped regime. We demonstrate that $\Delta C/T_c$ is indeed peaked at the doping where the coexistence phase with antiferromagnetism develops, and decreases at deviations from this doping in both directions.