Effect of the electronic charge gap on LO bond-stretching phonons in undoped La2CuO4 calculated using LDA plus U

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-PU calculations with antiferromagnetic supercells of La2CuO4 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-PU is a promising framework to model phonons in undoped cuprates and other perovskite oxides with complex, interrelated structural and electronic degrees of freedom.

Automated summary

By adjusting the strength of on-site repulsion in DFT-PU calculations, phonon energies of cuprates can reach experimental values, establishing a correlation between them. Using distorted supercells within DFT-PU proves to be a promising framework for modeling phonons in undoped cuprates and other perovskite oxides.