Temperature Dependence of the Superconducting Order Parameter in Stoichiometric Alkali Metal-Based Pnictide EuCsFe4As₄

We present the study of the superconducting order parameter in single crystals of alkali metal EuCsFe4As4 pnictide belonging to the novel 1144 family, turning to the superconducting state in the stoichiometric composition at critical temperature T-c approximate to 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.