Effect of the electronic charge gap on LO bond-stretching phonons in undoped La$_2$CuO$_4$ calculated using LDA+U

TL;DR

This study shows that tuning the on-site Coulomb repulsion in DFT+U calculations accurately predicts phonon energies in undoped La$_2$CuO$_4$, aligning theoretical results with experimental observations.

Contribution

It introduces a DFT+U approach with antiferromagnetic supercells to model phonons in undoped cuprates, improving upon standard DFT predictions.

Findings

Phonon energies match experimental values when U is tuned correctly.

Using distorted supercells within DFT+U effectively models complex oxides.

Correlation between U, optical gap, and magnetic moments established.

Abstract

Typical density-functional theory calculations that wrongly predict undoped cuprates to be metallic also predict Cu-O half- and full-breathing phonon energies that are significantly softer than observed, presumably because of weak on-site Coulomb repulsion on the Cu 3d orbitals. We used DFT+U calculations with antiferromagnetic supercells of La$_2$CuO$_4$ to establish correlation between the on-site repulsion strength, tuned via adjusting the value of U, and phonon dispersions. We find that breathing and half-breathing phonons reach experimental values when U is tuned to obtain the correct optical gap and magnetic moments. We demonstrate that using distorted supercells within DFT+U is a promising framework to model phonons in undoped cuprates and other perovskite oxides with complex, interrelated structural and electronic degrees of freedom.