Mott gapping in 3d ABO3 perovskites without Mott-Hubbard interelectronic U

TL;DR

This paper demonstrates that standard DFT without the empirical U parameter can accurately predict band gaps and structural features in 3d ABO3 perovskites, challenging the traditional view of strong electronic correlations as essential.

Contribution

It shows that ab-initio DFT with the SCAN functional can predict gapping and symmetry breaking in 3d perovskites without relying on the Hubbard U parameter, questioning the universality of strong correlations.

Findings

DFT without U predicts gapping trends in ABO3 perovskites.

Structural symmetry breaking is captured by DFT without U.

Strong dynamic correlations are not universally necessary for Mott insulating behavior.

Abstract

The existence of band gaps in Mott insulators such as perovskite oxides with partially filled 3d shells has been traditionally explained in terms of strong, dynamic inter-electronic repulsion codified by the on-site repulsion energy U in the Hubbard Hamiltonian. The success of the "DFT+U approach" where an empirical on-site potential term U is added to the exchange-and correlation Density Functional Theory (DFT) raised questions on whether U in DFT+U represents interelectronic correlation in the same way as it does in the Hubbard Hamiltonian, and if empiricism in selecting U can be avoided. Here we illustrate that ab-initio DFT without any U is able to predict gapping trends and structural symmetry breaking (octahedra rotations, Jahn-Teller modes, bond disproportionation) for all ABO3 3d perovskites from titanates to nickelates in both spin-ordered and spin disordered paramagnetic phases. We describe the paramagnetic phases as a supercell where individual sites can have different local environments thereby allowing DFT to develop finite moments on different sites as long as the total cell has zero moment. We use a recently developed exchange and correlation functional ("SCAN") that is sanctioned by the usual single-determinant, mean-field DFT paradigm with static correlations, but has a more precise rendering of self-interaction cancelation. Our results suggest that strong dynamic electronic correlations are not playing a universal role in gapping of 3d ABO3 Mott insulators, and opens the way for future applications of DFT for studying a plethora of complexity effects that depend on the existence of gaps, such as doping, defects, and band alignment in ABO3 oxides.