Effect of annealing on the specific heat of Ba(Fe_1-xCo _x)₂As₂

We report on the effect of annealing on the temperature and field dependencies of the low-temperature specific heat of the electron-doped Ba(Fe_1-xCo_x)₂As₂ for under- (x = 0.045), optimally (x = 0.08), and over- (x = 0.105 and 0.14) doped regimes. We observed that annealing significantly improves some superconducting characteristics in Ba(Fe_1-xCo_x)₂As₂. It considerably increases T_c, decreases γ₀ in the superconducting state, and suppresses the Schottky-like contribution at very low temperatures. The improved sample quality allows for a better identification of the superconducting gap structure of these materials. We examine the effects of doping and annealing within a self-consistent framework for an extended s-wave pairing scenario. At optimal doping our data indicate that the sample is fully gapped, while for both under- and overdoped samples significant low-energy excitations remain, possibly consistent with a nodal structure. The difference of sample quality offers a natural explanation for the variation in low-temperature power laws observed by many techniques.