Andreev Spectroscopy of EuCsFe4As4 Stoichiometric Superconducting Pnictide

Abstract

EuCsFe4As4 pnictide (1144 family) single crystals with a critical temperature of Tc ≈ 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 Tc using incoherent multiple Andreev reflection effect spectroscopy. It is shown that the gap structure of EuCsFe4As4 is similar to that of BaFe2 – xNixAs2 superconducting pnictides of the sister 122 family.