Jump in specific heat in the presence of a spin-density wave at the superconducting transition in iron pnictides

Many experiments reveal that in iron-based superconductors the jump of the specific heat ΔC at the superconducting T_c is not proportional to T_c, as expected in BCS theory. Rather, ΔC/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. We demonstrate on general grounds that ΔC/T_c peaks at the doping where the coexistence phase with antiferromagnetism develops, and decreases at deviations from this doping in both directions. Our results are in quantitative agreement with the experiments.