Anomalous electron-phonon coupling in kagome ferromagnetic Weyl semimetal Co$_3$Sn$_2$S$_2$

Abstract

We present results of a Raman scattering study of the Kagome ferromagnet Co$_3$Sn$_2$S$_2$, with a focus on electronic and phononic excitations and their interplay. In addition, the electronic band structure is analyzed theoretically, enabling a semi-quantitative explanation of the spectra. A prominent feature in the electronic spectra is a redistribution of spectral weight from low to high energies starting at the Curie temperature Tc. The Raman intensity is suppressed below approximately 1000cm$^{-1}$ and increases above to a peak at 2000 cm$^{-1}$ in all symmetries. Two Raman active phonon modes are identified in A$_{1g}$ and E$_g$ symmetry. The A$_{1g}$ phonon couples strongly to the electronic continuum as indicated by the asymmetric Fano-type line shape. The asymmetry depends non-monotonically on temperature and is maximal close to the magnetic transition. In the limit $T\to 0$ the phonon is nearly symmetric. The evolution of the coupling strength and the electronic continuum as a function of temperature is attributed to a band splitting induced by the ferromagnetic phase transition which substantially reduces the DOS towards $T=0$. The $3d_{z^2}$ electrons of the Co atoms in the crystal field modulated by the A$_{1g}$ phonon are implied to be a critical component contributing to the strong electron-phonon coupling of that phonon. These results allow a comprehensive understanding of the bulk band structure evolution as a function of temperature in Co$_3$Sn$_2$S$_2$, offering key insights for further studies of the driving force behind the long-range magnetic order and novel topological states in this compound.Comment: 9 pages, 4 figure