Derivation of Static Low-Energy Effective Models by ab initio Downfolding Method without Double Counting of Coulomb Correlations: Application to SrVO3, FeSe and FeTe

TL;DR

This paper presents an improved ab initio downfolding method for deriving low-energy effective models that avoids double counting of electron correlations, applied to SrVO3, FeSe, and FeTe, revealing insights into their electronic structures.

Contribution

An improved formalism for downfolding that eliminates double counting of correlations, incorporating constrained self-energy corrections from high-energy degrees of freedom.

Findings

Bandwidths of effective models are similar to previous methods due to cancellations.

In SrVO3, the effective bandwidth is 2.56 eV, close to LDA results.

In FeSe and FeTe, self-energy effects significantly modify band structures and orbital levels.

Abstract

Derivation of low-energy effective models by a partial trace summation of the electronic degrees of freedom far away from the Fermi level, called downfolding, is reexamined. We propose an improved formalism free from the double-counting of electron correlation in the low-energy degrees of freedom. In this approach, the exchange-correlation energy in the local density approximation (LDA) is replaced with the constrained self-energy corrections defined by the sum of the contribution from eliminated high-energy degrees of freedom, Sigma H and that from the frequency-dependent part of the partially screened interaction, Delta Sigma L. We apply the formalism to SrVO3 as well as to two iron-based superconductors, FeSe and FeTe. The resultant bandwidths of the effective models are nearly the same as those of the previous downfolding formalism because of striking cancellations of Sigma H and Delta Sigma L. In SrVO3, the resultant bandwidth of the effective low-energy model is 2.56 eV (in comparison to LDA: 2.58 eV, full GW approximation: 2.19 eV). However, in the non-degenerate multi-band materials such as FeSe and FeTe, the momentum dependent self-energy effects yield substantial modifications of the band structures and relative shifts of orbital-energy levels of the effective models. The FeSe model indicates a substantial downward shift of the x2-y2 orbital level, which may lead to an increase in the filling of the x2-y2 orbital above half filling. It suggests a destruction of the orbital selective Mott phase of the x2-y2 orbital in agreement with the experimental absence of the antiferromagnetic phase.