Nematicity and superconductivity in FeSe_1-xS_x

The rich variety of the phase diagrams in FeSe systems demonstrates the strong interplay between the nematicity, magnetism, and superconductivity in Fe-based superconductors. Here, we investigate the electronic state in FeSe_1-xS_x focusing on the relation between the nematicity and superconductivity based on a multi-orbital Hubbard model by including both the spin-orbit interaction and the higher-order many-body effects called the vertex correction. We find that, in the tetragonal phase, the strong ferro-orbital fluctuations on xz and yz orbitals develop equally, and its divergence corresponds to the nematic transition. In the orthorhombic phase with nematicity, however, the orbital fluctuation on the yz orbital becomes dominant even if the induced orbital polarization is small, and the superconducting gap becomes large on the FSs with large yz-orbital component. The obtained abrupt change of the gap structure is consistent with the resent specific heat and thermal conductivity measurements on FeSe_1-xS_x. We also investigate the development of the orbital susceptibilities for FeSe_1-xS_x, and discuss the relation between the share modulus C₆₆ and the Raman nematic susceptibility in detail.