Adiabatic and non-adiabatic phonon dispersion in a Wannier function approach

TL;DR

This paper introduces a first-principles method to accurately compute adiabatic and non-adiabatic phonon dispersions across the entire Brillouin zone, addressing convergence issues and revealing new Kohn anomalies in materials like MgB₂ and CaC₆.

Contribution

The authors develop a Wannier function-based approach for phonon calculations that overcomes convergence problems and captures non-adiabatic effects throughout the Brillouin zone.

Findings

Identification of previously unobserved Kohn anomalies in MgB₂ and CaC₆.

Improved agreement between calculated and experimental phonon spectra in MgB₂.

Non-adiabatic effects are significant across the entire Brillouin zone in CaC₆.

Abstract

We develop a first-principles scheme to calculate adiabatic and non-adiabatic phonon frequencies in the full Brillouin zone. The method relies on the variational properties of a force-constants functional with respect to the first-order perturbation of the electronic charge density and on the localization of the deformation potential in the Wannier function basis. This allows for calculation of phonon dispersion curves free from convergence issues related to Brillouin zone sampling. In addition our approach justify the use of the static screened potential in the calculation of the phonon linewidth due to decay in electron-hole pairs. We apply the method to the calculation of the phonon dispersion and electron-phonon coupling in MgB$_2$ and CaC$_6$. In both compounds we demonstrate the occurrence of several Kohn anomalies, absent in previous calculations, that are manifest only after careful electron and phonon momentum integration. In MgB$_2$, the presence of Kohn anomalies on the E$_{2g}$ branches improves the agreement with measured phonon spectra and affects the position of the main peak in the Eliashberg function. In CaC$_6$ we show that the non-adiabatic effects on in-plane carbon vibrations are not localized at zone center but are sizable throughout the full Brillouin zone. Our method opens new perspectives in large-scale first-principles calculations of dynamical properties and electron-phonon interaction.