Specific Heat Measurements of Ba0.68K0.32Fe2As2 Single Crystals: Evidence for a Multiband Strong-Coupling Superconducting State

The specific heat of high-purity Ba0.68K0.32Fe2As2 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 Delta(1) approximate to 11 meV and Delta(2) approximate to 3: 5 meV obtained from an alpha-model analysis. The reduced gap value, 2 Delta(max)/k(B)T(c) approximate to 6.6, the magnitude of the specific-heat jump, Delta C-p(T-c)/T-c, and its slope below T-c exhibit a strong-coupling character. We also show that an Eliashberg model with two hole and two electron bands gives the correct values of Tc, the superconducting gaps, and the free-energy difference.