Andreev Spectroscopy of EuCsFe4As4 Stoichiometric Superconducting Pnictide

EuCsFe 4 As 4 pnictide (1144 family) single crystals with a critical temperature of T c ≈ 36.8 K and optimal superconducting properties in the stoichiometric composition are grown. Multiple-gap superconductivity determined by bulk order parameters $$\Delta _{{\text{L}}}^{{{\text{out}}}}$$ , $$\Delta _{{\text{L}}}^{{{\text{in}}}}$$ , and $${{\Delta }_{{\text{S}}}}$$ with characteristic ratios $$2\Delta _{{\text{L}}}^{{{\text{out}}}}(0){\text{/}}{{k}_{{\text{B}}}}{{T}_{{\text{c}}}} \approx 5.3$$ , $$2\Delta _{{\text{L}}}^{{{\text{in}}}}(0){\text{/}}{{k}_{{\text{B}}}}{{T}_{{\text{c}}}} \approx 3.2$$ , and $$2{{\Delta }_{{\text{S}}}}(0){\text{/}}{{k}_{{\text{B}}}}{{T}_{{\text{c}}}} \approx 1.3$$ , respectively, is found for the first time below T c using incoherent multiple Andreev reflection effect spectroscopy. It is shown that the gap structure of EuCsFe 4 As 4 is similar to that of BaFe 2 – x Ni x As 2 superconducting pnictides of the sister 122 family.