Effects of Anisotropic Charge on Transverse Optical Phonons in NiO

TL;DR

This study investigates how anisotropic charge distribution affects transverse optical phonons in NiO, revealing polarization-dependent energy shifts explained by first-principles calculations, challenging previous Heisenberg-based interpretations.

Contribution

It provides a novel explanation for phonon energy differences in NiO based on anisotropic charge polarization, supported by experimental and theoretical analysis.

Findings

Transverse optical phonon energies depend on polarization relative to magnetic order.

Anisotropic charge polarization explains phonon energy shifts.

First-principles calculations confirm experimental observations.

Abstract

Phonon dispersion of detwinned NiO is measured using inelastic x-ray scattering. It is found that, near the zone center, the energy of the transverse optical phonon mode polarized parallel to the antiferromagnetic order is ~1 meV lower than that of the mode polarized perpendicular to the order, at room temperature. This is explained via anisotropic polarization of the Ni and O atoms, as confirmed using a Berry's phase approach with first-principles calculations. Our explanation avoids an apparent contradiction in previous discussions focusing on Heisenberg interaction.