Nonlinear electronic density response of the warm dense electron gas: multiple perturbations and mode coupling

TL;DR

This paper presents new ab initio PIMC results for the nonlinear density response of warm dense electron gases under multiple perturbations, highlighting mode-coupling effects and extending analytical theories for accurate modeling.

Contribution

It introduces comprehensive PIMC simulations of electron gases with multiple harmonic perturbations and extends existing analytical models to include mode-coupling effects.

Findings

Mode-coupling effects dominate nonlinear responses under multiple perturbations.

Generalized imaginary-time correlation functions can estimate mode-coupling effects.

Extended analytical theory accurately describes PIMC results with low computational cost.

Abstract

We present extensive new ab initio path integral Monte Carlo (PIMC) results for an electron gas at warm dense matter conditions that is subject to multiple harmonic perturbations. In addition to the previously investigated nonlinear effects at the original wave number [Dornheim \emph{et al.}, PRL \textbf{125}, 085001 (2020)] and the excitation of higher harmonics [Dornheim \emph{et al.}, PRR \textbf{3}, 033231 (2021)], the presence of multiple external potentials leads to mode-coupling effects, which constitute the dominant nonlinear effect and lead to a substantially more complicated density response compared to linear response theory. One possibility to estimate mode-coupling effects from a PIMC simulation of the unperturbed system is given in terms of generalized imaginary-time correlation functions that have been recently introduced by Dornheim \emph{et al.}~[JCP \textbf{155}, 054110 (2021)]. In addition, we extend our previous analytical theory of the nonlinear density response of the electron gas in terms of the static local field correction [Dornheim \emph{et al.}, PRL \textbf{125}, 235001 (2020)], which allows for a highly accurate description of the PIMC results with negligible computational cost.