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 $T_c\approx 17K$ 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 $<8$%, $\approx 12$%, and $\approx 20$%, 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 $\Delta (T)$ 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 ${\lambda }_{\Gamma L}\approx {\lambda }_{L\Gamma }<<{\lambda }_{S\Gamma },{\lambda }_{SL}$.

arXiv comment20 pages, 7 figures (draft)