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Two-gap features in the specific heat of (M,K)Fe_{2}As_{2} (M = Ba, Sr)

The specific heat coefficient / p C T was investigated for the optimally doped Ba 0.6 K 0.4 Fe 2 As 2 and Sr 0.55 K 0.45 Fe 2 As 2 . Previously, both the single and the two-gap pairing have been suggested for Ba 0.6 K 0.4 Fe 2 As 2 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 2 As 2 system, where 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 T data set if the respective phonon baselines are adopted. The soft-phonon shifts, therefore, were examined on the (Ba,K)Fe 2 As 2 and (Sr,K)Fe 2 As 2 systems, as well as the (Ba 0.6 K 0.4 )(Fe,Co) 2 As 2 system. The data show that although the K-Ba replacement may change the low-temperature slope by 50%, the effects of both K-Sr and Co-Fe replacements are much weaker. The carrier part, T , is consequently extracted for both Sr 0.55 K 0.45 Fe 2 As 2 and Ba 0.6 K 0.4 Fe 2 As 2 . Two-gap features appear in both cases, but the coupling strength is much stronger for the Ba-based superconductors.