Multiple Andreev Reflections Spectroscopy of Superconducting LiFeAs Single Crystals: Anisotropy and Temperature Behavior of the Order Parameters

The superconducting state of LiFeAs single crystals with the maximum critical temperature $T_c\,\approx\,17\,K$ 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, $\Delta_{\Gamma}\,= (5.1-6.5)\,meV$, $\Delta_L\,= (3.8-4.8)\,meV$, and $\Delta_S\,= (0.9-1.9)\,meV$ (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 $<\,8$%, $\approx12$%, and $\approx20$%, 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 ${\Delta}(T)$ dependences by the system of the two-band Moskalenko and Suhl equations. It has been shown that the densities of states in bands forming ${\Delta}_{\Gamma}$ and ${\Delta}_L$ are approximately the same, intraband pairing dominates in this case, and the interband coupling constants are related as ${\lambda}_{{\Gamma}L}\,\approx\,{\lambda}_{L{\Gamma}}\,<< {\lambda}_{S{\Gamma}},\,{\lambda}_{SL}$.