Ab initio Low-Dimensional Physics Opened Up by Dimensional Downfolding: Application to LaFeAsO

An ab initio downfolding method is formulated to construct low-dimensional models for correlated electrons. In addition to the band downfolding by constrained random phase approximation formulated for 3D models, screening away from the target layer (chain) is further involved. Eliminating the off-target degrees of freedom, namely, dimensional downfolding yields ab initio low-dimensional models. The method is applied to derive a 2D model for a layered superconductor LaFeAsO, where the interlayer screening crucially makes the effective interaction short ranged and reduces the onsite Coulomb interactions by 10-20 \% from the 3D model for the 5 iron-3d orbitals.