Temperature Dependence of the Superconducting Order Parameter in Stoichiometric Alkali Metal-Based Pnictide EuCsFe _4 As _4

We present the study of the superconducting order parameter in single crystals of alkali metal EuCsFe _4 As _4 pnictide belonging to the novel 1144 family, turning to the superconducting state in the stoichiometric composition at critical temperature T_c ≈ 37 K, and showing an antiferromagnetic ordering below T_c . In the superconducting state, using incoherent multiple Andreev reflection effect spectroscopy of superconductor-thin normal metal-superconductor (SnS) junctions, we show a multiple-gap superconductivity with possibly anisotropic large SC gap with an extended s -wave symmetry, and isotropic small SC gap. We directly determined the magnitudes, characteristic ratios, and temperature evolution of the superconducting gaps. The latter are found to be typical for a superconductor with rather weak interband interaction between SC condensates in the k -space.