Ab initio downfolding study of the iron-based ladder superconductor BaFe2 S3

Motivated by the recent discovery of superconductivity in the iron-based ladder compound BaFe2S3 under high pressure, we derive low-energy effective Hamiltonians from first principles. We show that the complex band structure around the Fermi level is represented only by the Fe 3dxz (mixed with 3dxy) and 3dx2-y2 orbitals. The characteristic band degeneracy allows us to construct a four-band model with the band unfolding approach. We also estimate the interaction parameters and show that the system is more correlated than the 1111 family of iron-based superconductors. Provided the superconductivity is mediated by spin fluctuations, the 3dxz-like band plays an essential role, and the gap function changes its sign between the Fermi surface around the Γ point and that around the Brillouin-zone boundary.