Multiple andreev reflections spectroscopy of superconducting LiFeAs single crystals: Anisotropy and temperature behavior of the order parameters

Abstract

The superconducting state of LiFeAs single crystals with the maximum critical temperature T c ≈ 17 K in the 111 family has been studied in detail by multiple Andreev reflections (MAR) spectroscopy implemented by the break-junction technique. The three superconducting gaps, ΔΓ = 5.1–6.5 meV, ΔL = 3.8–4.8 meV, and ΔS = 0.9–1.9 meV (at T ≪ T c), as well as their temperature dependences, have been directly determined in a tunneling experiment with these samples. The anisotropy degrees of the order parameters in the k space have been estimated as <8, ∼12, and ∼20%, respectively. Andreev spectra have been fitted within the extended Kummel-Gunsenheimer-Nikolsky model with allowance for anisotropy. The relative electron-boson coupling constants in LiFeAs have been determined by approximating the Δ(T) dependences by the system of the two-band Moskalenko and Suhl equations. It has been shown that the densities of states in bands forming ΔΓ and ΔL are approximately the same, intraband pairing dominates in this case, and the interband coupling constants are related as λΓL ≈ λLΓ ≪ λSΓ, λSL.