Multiple-Gap Superconductivity of Nonmagnetic Stoichiometric CaKFe4As4 Pnictide

Abstract

Using incoherent multiple Andreev reflection effect spectroscopy of planar break-junctions, we have studied the superconducting gap structure of CaKFe₄As₄ pnictides (the 1144 family), those turning to the superconducting state in the stoichiometric composition with critical temperature \({{T}_{{\text{c}}}} \approx 36.1\) K. We have shown a coexistence of three bulk superconducting order parameters \({{\Delta }_{i}}\), and directly determined their magnitudes and corresponding characteristic ratios \(2{{\Delta }_{i}}(0){\text{/}}{{k}_{{\text{B}}}}{{T}_{{\text{c}}}} \approx 5.6, 3.9\), and 1.3 at \(T \ll {{T}_{{\text{c}}}}\). We compare the gap structure of CaKFe₄As₄ and sister pnictides of the 1144 and 122 families studied by us earlier.