Comparison of the Superconducting Properties of Potassium Iron Selenides with Isovalent Substitution

Using incoherent multiple Andreev reflection effect spectroscopy, we have studied the properties of the superconducting phase of (K,Na) x Fe 2– y Se 2 and K z Fe 2 – y (Se,S) 2 iron selenides with critical temperatures $${{T}_{c}} \approx 27{-} 31$$ K. In both compositions, we directly show a presence of a single superconducting order parameter, and directly determine its magnitude and temperature dependence. The obtained characteristic ratio $$2\Delta (0){\text{/}}{{k}_{B}}{{T}_{c}} \approx 4.1{-} 4.6$$ results from a strong coupling in the electron bands. The similarity between the superconducting gap structure and the observed $$\Delta (0)$$ scaling with critical temperature points to a uniform Cooper pairing mechanism in (K,Na) x Fe 2– y Se 2 and K z Fe 2– y (Se,S) 2 iron selenides.