Effects of the crystal structure in the dynamical electron density-response of hcp transition metals

TL;DR

This study investigates how the crystal structure of hcp transition metals Sc and Ti influences their electron density response, revealing isotropic free-electron-like behavior at small wave vectors and complex directional effects on plasmon hybridization.

Contribution

It provides a detailed all-electron TDDFT analysis of the anisotropic dynamical response in hcp transition metals, highlighting the role of local-field and exchange-correlation effects.

Findings

Isotropic free-electron-like response at small wave vectors

Crystal local-field effects influence plasmon energy

Directional dependence causes plasmon hybridization with d-electron transitions

Abstract

We present an all-electron study of the dynamical density-response function of hexagonal close-packed transition metals Sc and Ti. We elucidate various aspects of the interplay between the crystal structure and the electron dynamics by investigating the loss function, and the associated dielectric function, for wave-vector transfers perpendicular and parallel to the hexagonal plane. As expected, but contrary to recent work, we find that the free-electron-like aspects of the dynamical response are rather isotropic for small wave vectors. The crystal local-field effects are found to have an impact on the plasmon energy for small wave vectors, which gives rise to an interplay with the exchange-correlation effects built into the many-body kernel. The loss function lineshape shows a significant dependence on propagation direction; in particular, for propagation on the hexagonal plane the plasmon hybridizes substantially with fine structure due to d-electron transitions, and its dispersion curve becomes difficult to establish, beyond the small wave vector limit. The response is calculated in the framework of time-dependent density functional theory (TDDFT), based on a full-potential linearized augmented-plane-wave (LAPW) ground-state, in which the exchange-correlation effects are treated in the local-density approximation.