Revisiting orbital-fluctuation-mediated superconductivity in LiFeAs: Nontrivial spin-orbit interaction effects on the band structure and superconducting gap function

The precise gap structure in LiFeAs (Tc=18K) given by ARPES studies offers significant information that helps us understand the pairing mechanism in iron-based superconductors. The most remarkable characteristic in the LiFeAs gap structure would be that "the largest gap emerges on the tiny hole-pockets around the Z point." This result has been naturally explained in terms of the orbital-fluctuation scenario [T. Saito, Phys. Rev. B 90, 035104 (2014)]PRBMDO1098-012110.1103/PhysRevB.90.035104, whereas the opposite result is obtained by the spin-fluctuation scenario. In this paper, we study the gap structure in LiFeAs by taking the spin-orbit interaction (SOI) into account, motivated by the recent ARPES studies that revealed a significant SOI-induced modification of the Fermi surface topology. For this purpose, we construct two possible tight-binding models with finite SOI by referring the band structures given by different ARPES groups. In addition, we extend the gap equation for multiorbital systems with finite SOI, and calculate the gap functions by applying the orbital-spin fluctuation theory. On the basis of both SOI-induced band structures, the main characteristics of the gap structure in LiFeAs are naturally reproduced only in the presence of strong interorbital interactions between (dxz/yz-dxy) orbitals. Thus the experimental gap structure in LiFeAs is a strong evidence for the orbital-fluctuation pairing mechanism.