ИСТИНА

Remarkable features of alkali-metal iron pnictides of the 111 and 1144 families, such as absence of magnetism, superconductivity in stoichiometric state and nontrivial band structure [1-3] still challenge experimenters. Here we present direct spectroscopic study of the superconducting (SC) order parameter in single crystals of LiFeAs with Tc ≈ 17.5 K, and novel EuCsFe4As4 with Tc ≈ 34 K. In order to reveal the structure of the SC order parameter, here we used incoherent multiple Andreev reflection effect (IMARE) spectroscopy. The superconductor-thin normal metal-superconductor (SnS) junctions with semiballistic high-transparent barrier were produced by a planar break-junction technique [4]. In LiFeAs single crystals, we determined three bulk SC gaps: the small gap Δ_S(0) ≈ 1.3 meV (possibly shows ~35% anisotropy in the k-space) leads to corresponding characteristic ratio 2Δ_S(0) / kTc ≈ 1.7; the middle gap Δ_L with 2Δ_L(0) / kTc ≈ 3.5–5.0 (~30% anisotropy, the range corresponds to the minimum and maximum Cooper pair coupling energies in the bands with Δ_L); and the nodeless largest gap Δ_Г having ~10% k-space anisotropy and 2Δ_Г(0) / kTc ≈ 8. The moderate level of anisotropy for all order parameters supports their nodeless nature. The three SC gaps turn to zero at one and the same Tc ≈ 17.5 K, whereas their temperature trends are typical for a moderate interband coupling. The anisotropy degree of the Δ_Г and Δ_L gaps remains almost constant with temperature until Tc. Above Tc, in both SC compounds we reproducibly observe manifestation of an electron density of states peak in the vicinity of the Fermi level. The relation between superconducting and normal-state features in the studied compounds is also discussed. We compare the determined gap structure of alkali-metal LiFeAs and EuCsFe4As4 compounds with that of other iron pnictide families, such as Ba-122 and 1111. [1] S.V. Borisenko, et al., Symmetry 4, 251 (2012). [2] A. Iida, et al., J. Am. Chem. Soc. 138, 3410 (2016). [3] T.E. Kuzmicheva, S.A. Kuzmichev, JETP Lett. 114, 630 (2021). [mini-review] [4] S.A. Kuzmichev, T.E. Kuzmicheva, Low Temp. Phys. 42, 1008 (2016). [review]