Spin excitations in metallic kagome lattice FeSn and CoSn

TL;DR

This study investigates spin excitations in metallic kagome lattice FeSn and CoSn, revealing well-defined spin waves in FeSn and clarifying the origin of a previously misinterpreted flat mode in CoSn.

Contribution

The paper provides the first detailed analysis of spin excitations in FeSn and CoSn, clarifying the nature of flat modes and correcting past misinterpretations due to experimental artifacts.

Findings

FeSn exhibits well-defined spin waves above 140 meV and a flat excitation at 170 meV.

The flat mode in CoSn originates from hydrocarbon scattering, not intrinsic magnetic excitations.

The study clarifies the evolution of spin excitations in metallic kagome lattice materials.

Abstract

In two-dimensional (2D) metallic kagome lattice materials, destructive interference of electronic hopping pathways around the kagome bracket can produce nearly localized electrons, and thus electronic bands that are flat in momentum space. When ferromagnetic order breaks the degeneracy of the electronic bands and splits them into the spin-up majority and spin-down minority electronic bands, quasiparticle excitations between the spin-up and spin-down flat bands should form a narrow localized spin-excitation Stoner continuum coexisting with well-defined spin waves in the long wavelengths. Here we report inelastic neutron scattering studies of spin excitations in 2D metallic Kagome lattice antiferromagnetic FeSn and paramagnetic CoSn, where angle resolved photoemission spectroscopy experiments found spin-polarized and nonpolarized flat bands, respectively, below the Fermi level. Although our initial measurements on FeSn indeed reveal well-defined spin waves extending well above 140 meV coexisting with a flat excitation at 170 meV, subsequent experiments on CoSn indicate that the flat mode actually arises mostly from hydrocarbon scattering of the CYTOP-M commonly used to glue the samples to aluminum holder. Therefore, our results established the evolution of spin excitations in FeSn and CoSn, and identified an anomalous flat mode that has been overlooked by the neutron scattering community for the past 20 years.