Two-gap features in the specific heat of (M,K)Fe₂As₂ (M = Ba, Sr)

The specific heat coefficient C_p/T was investigated for the optimally doped Ba_0.6K_0.4Fe₂As₂ and Sr_0.55K_0.45Fe₂As₂. Previously, both the single- and the two-gap pairing have been suggested for Ba _0.6K_0.4Fe₂As₂ single crystals. Our analysis reveals that the controversy is mainly caused by the differences in the adopted phonon background. This is especially true in the (Ba,K)Fe ₂As₂ system, in which the phonon contribution below 20 K significantly deviates from the simplified Debye model and, in addition, strongly depends on the doping. The different pairing features reported previously can be reproduced from the same C_p/T data set if the respective phonon baselines are adopted. The soft-phonon shifts, therefore, were examined on the (Ba,K)Fe₂As₂ and (Sr,K)Fe ₂As₂ systems, as well as the (Ba_0.6K _0.4)(Fe,Co)₂As₂ system. The data show that although the K-Ba replacement may change the low-temperature slope β=dC_p/Td(T2) by 50%, the effects of both K-Sr and Co-Fe replacements are much weaker. The carrier part, C_e/T, is consequently extracted for both Sr_0.55K_0.45Fe₂As ₂ and Ba_0.6K_0.4Fe₂As₂. The two-gap features appear in both cases, but the coupling strength is much stronger for the Ba-based superconductors.