Spin waves with source-free time-dependent spin density functional theory

TL;DR

This paper investigates a source-free approach within time-dependent spin density functional theory for modeling spin waves, revealing limitations like violations of Larmor's theorem and incorrect magnon behavior in certain regimes.

Contribution

It analyzes the source-free construction in TD-SDFT for spin waves, highlighting its limitations and practical implications in magnetic systems.

Findings

Violation of Larmor's theorem in paramagnetic case

Incorrect long-wavelength behavior of ferromagnetic magnons

Discussion of practical implications of these violations

Abstract

Time-dependent spin density functional theory (TD-SDFT) allows the theoretical description of spin and magnetization dynamics in electronic systems from first quantum mechanical principles. TD-SDFT accounts for electronic interaction effects via exchange-correlation scalar potentials and magnetic fields, which have to be suitably approximated in practice. We consider here an approach that was recently proposed for the ground state by Sharma et al. [J. Chem. Theor. Comput. 14, 1247 (2018)], which enforces the so-called "source-free" condition by eliminating monopole contributions contained within a given approximate exchange-correlation magnetic field. This procedure was shown to give good results for the structure of magnetic materials. We analyze the source-free construction in the linear-response regime, considering spin waves in paramagnetic and ferromagnetic electron gases. We observe a violation of Larmor's theorem in the paramagnetic case and a wrong long-wavelength behavior of ferromagnetic magnons. The practical implications of these violations are discussed.