Spin excitations in metallic kagome lattice FeSn and CoSn

Yaofeng Xie,Huibo Cao,Zhiping Yin,Luke L Daemen,Tong Chen,Qi Wang,Allan H Macdonald,Pengcheng Dai,Lebing Chen,Hechang Lei,J Ross Stewart,Qiangwei Yin,Matthew B Stone,Erxi Feng

doi:10.1038/s42005-021-00736-8

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. Our measurements on FeSn and CoSn reveal well-defined spin waves extending above 140 meV and correlated paramagnetic scattering around Γ point below 90 meV, respectively. In addition, we observed non-dispersive excitations at ~170 meV and ~360 meV arising mostly from hydrocarbon scattering of the CYTOP-M used to glue the samples to aluminum holder. Therefore, our results established the evolution of spin excitations in FeSn and CoSn, and identified anomalous flat modes overlooked by the neutron scattering community for many years.

Highlights

In 2D metallic kagome lattice materials (Fig. 1d–g), destructive interference of electronic hopping pathways around the kagome bracket can produce nearly localized electrons, and electronic bands that are flat in momentum space (Fig. 1h–j)[14,15,16,17,18]
There is no splitting of the degenerate electronic bands, a continuum of electronhole pair (Stoner continuum) excitations can still occur if the system has electron and hole pockets below and above the Fermi level
To account for electronic itineracy, we calculate the electronic structure of FeSn in the paramagnetic and AF ordered states using a combination of density functional theory (DFT) and dynamical mean field theory (DFT+DMFT)[46]

Summary

Introduction

In two-dimensional (2D) metallic kagome lattice materials, destructive interference of electronic hopping pathways around the kagome bracket can produce nearly localized electrons, and electronic bands that are flat in momentum space. 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. Momentum below the Fermi level have recently been identified in the antiferromagnetic (AF) kagome metal FeSn25, paramagnetic kagome metal CoSn26,27, as well as in other materials[28,29,30], their influence on spin-wave and Stoner excitations is unknown. A spin-polarized flat electronic band has been identified in the AF kagome metal FeSn at an energy E 1⁄4 230 ± 50 meV below the Fermi level by angle-resolved photoemission spectroscopy (ARPES) experiments[25]. Since neutron scattering measures electron-hole-pair excitations, having a flat spin-up electronic band below the Fermi level is a necessary, but not a sufficient condition to observe a flat Stoner continuum band. ARPES measurements have identified flat band at an energy E 1⁄4 270 ± 50 meV below the Fermi level in CoSn26,27, one would not expect to observe a flat Stoner continuum band due to degenerate electronic bands nature of the system[35]

Methods

Results

Conclusion