Revisit of the Orbital-Fluctuation-Mediated Superconductivity in LiFeAs: Nontrivial Spin-Orbit Interaction Effects on the Bandstructure and Superconducting Gap Function

摘要

Precise gap structure in LiFeAs (Tc = 18 K) given by ARPES studies offers ussignificant information to understand the pairing mechanism in iron-basedsuperconductors. The most remarkable characteristics in LiFeAs gap structurewould be that "the largest gap emerges on the tiny hole-pockets around Zpoint". This result had been naturally explained in terms of theorbital-fluctuation scenario (T. Saito et al., Phys. Rev. B 90, 035104 (2014)),whereas an opposite result is obtained by the spin-fluctuation scenario. Inthis paper, we study the gap structure in LiFeAs by taking the spin-orbitinteraction (SOI) into account, motivated by the recent ARPES studies thatrevealed the significant SOI-induced modification of the Fermi surfacetopology. For this purpose, we construct the two possible tight-binding modelswith finite SOI by referring the bandstructures given by different ARPESgroups. In addition, we extend the gap equation for multiorbital systems withfinite SOI, and calculate the gap functions by applying the orbital-spinfluctuation theory. On the basis of both SOI-induced band structures, maincharacteristics of the gap structure in LiFeAs are naturally reproduced only inthe presence of strong inter-orbital interactions between (xz/yz - xy)orbitals. Thus, the experimental gap structure in LiFeAs is a strong evidencefor the orbital-fluctuation pairing mechanism.