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.