Heat capacity of Ba1-xKxFe2As2, x=0 and 0.41

Heat-capacity measurements on exceptionally high-quality samples of Ba1-xKxFe2As2 give the values of a number of parameters relevant to the electronic structure, the structural/magnetic transition in the undoped parent compound, and the superconducting transition in the near optimally doped, x = 0.41 sample. In BaFe2As2 the changes in the lattice and magnetic structure appear as a single, sharp first-order transition at 140 K; the Sommerfeld coefficient is γ = 5.1 mJ K-2 mol-1. For the superconducting, x = 0.41, sample, the residual density of electron states in the superconducting states is essentially zero. The specific-heat anomaly at Tc suggests extreme strong coupling, with Δ(0)/kBTc ∼ 2.6, and a normal-state Sommerfeld coefficient γn ∼ 45 mJ K-2 mol-1.