Ab initiodownfolding study of the iron-based ladder superconductorBaFe2S3

Abstract

Motivated by the recent discovery of superconductivity in the iron-based ladder compound ${\mathrm{BaFe}}_{2}{\mathrm{S}}_{3}$ 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 $3{d}_{xz}$ (mixed with $3{d}_{xy}$) and $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ 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 $3{d}_{xz}$-like band plays an essential role, and the gap function changes its sign between the Fermi surface around the $\mathrm{\ensuremath{\Gamma}}$ point and that around the Brillouin-zone boundary.