Spin-charge-lattice coupling across the charge density wave transition in a Kagome lattice antiferromagnet

TL;DR

This study investigates how spin, charge, and lattice excitations interact across the charge density wave transition in a Kagome lattice antiferromagnet FeGe, revealing complex coupling effects and electronic structure insights.

Contribution

It provides the first detailed neutron scattering analysis of spin and lattice excitations across the CDW transition in FeGe, highlighting spin-charge-lattice coupling and electronic correlations.

Findings

Spin excitations below 100 meV are well described by spin waves.

High-energy spin excitations extend up to 180 meV, indicating quasiparticle behavior.

Spin wave dispersion and phonon modes harden below T_CDW without phonon Kohn anomaly.

Abstract

Understanding spin and lattice excitations in a metallic magnetic ordered system form the basis to unveil the magnetic and lattice exchange couplings and their interactions with itinerant electrons. Kagome lattice antiferromagnet FeGe is interesting because it displays rare charge density wave (CDW) deep inside the antiferromagnetic ordered phase that interacts with the magnetic order. We use neutron scattering to study the evolution of spin and lattice excitations across the CDW transition $T_{\rm CDW}$ in FeGe. While spin excitations below $\sim$100 meV can be well described by spin waves of a spin-1 Heisenberg Hamiltonian, spin excitations at higher energies are centered around the Brillouin zone boundary and extend up to $\sim180$ meV consistent with quasiparticle excitations across spin-polarized electron-hole Fermi surfaces. Furthermore, $c$-axis spin wave dispersion and Fe-Ge optical phonon modes show a clear hardening below $T_{\rm CDW}$ due to spin-charge-lattice coupling but with no evidence for a phonon Kohn anomaly. By comparing our experimental results with density functional theory calculations in absolute units, we conclude that FeGe is a Hund's metal in the intermediate correlated regime where magnetism has contributions from both itinerant and localized electrons arising from spin polarized electronic bands near the Fermi level.