Revealing quasi-excitations in the low-density homogeneous electron gas with model exchange-correlation kernels

TL;DR

This paper investigates low-density homogeneous electron gases using time-dependent density functional theory with model exchange-correlation kernels, revealing quasi-excitations and collective phenomena relevant to metallic systems.

Contribution

It introduces a detailed analysis of excitations in low-density electron gases using model xc kernels, emphasizing the importance of exact constraints and dielectric function insights.

Findings

Identification of ghost excitons and collective plasmon modes

Revealing static charge-density waves at high r_s values

Enhanced understanding of low-density electron gas excitations

Abstract

Time-dependent density functional theory (TDDFT) within the linear response regime provides a solid mathematical framework to capture excitations. The accuracy of the theory, however, largely depends on the approximations for the exchange-correlation (xc) kernels. Away from the long-wavelength (or $q=0$ short wave-vector) and zero-frequency ($\omega=0$) limit, the correlation contribution to the kernel becomes more relevant and dominant over exchange. The dielectric function in principle can encompass xc effects relevant to describe low-density physics. Furthermore, besides collective plasmon excitations, the dielectric function can reveal collective electron-hole excitations, often dubbed ``ghost excitons.'' Beside collective excitons, the physics in the low-density regime is rich, as exemplified by a static charge-density wave that was recently found for $r_\mathrm{s} > 69$, and was shown to be associated with softening of the plasmon mode. These excitations are seen to be present in much higher density 2D HEGs, of $r_\mathrm{s} \geq 4$. In this work we perform a thorough analysis with xc model kernels for excitations of various nature. The uniform electron gas as a useful model of real metallic systems is used as a platform of our analysis. We highlight the relevance of exact constraints as we display and explain screening and excitations in the low-density region.