Anisotropic electron-phonon coupling and chiral phonons in van der Waals room temperature ferromagnet Fe$_{5}$GeTe$_{2}$

TL;DR

This work demonstrates anisotropic electron–phonon coupling and the existence of chiral phonons in the van der Waals ferromagnet Fe5GeTe2 at room temperature through a combination of temperature- and polarization-dependent Raman spectroscopy and ab initio calculations. The authors identify a tilt in angular Raman intensity patterns and Fano resonances that quantify anisotropic and resonant electron–phonon interactions, and provide direct spectroscopic evidence for chiral phonons via cross-circular polarization measurements supported by calculations of phonon circular polarization. A simplified single octuple-layer model with first-principles Raman tensors captures the observed behavior, linking lattice dynamics to magnetic order and electronic structure. Collectively, the results position Fe5GeTe2 as a versatile platform for exploring couplings among lattice, electronic, and magnetic degrees of freedom and for advancing understanding of chiral phonons in magnetic van der Waals materials with potential implications for phonon-based device concepts and thermal transport phenomena.

Abstract

The layered van der Waals Fe$_5$GeTe$_2$ (F5GT) compound exhibits room-temperature ferromagnetism, making it a promising candidate for technological applications. In our study, combined temperature- and polarization-dependent Raman measurements, along with modern {\it ab initio} calculations, reveal important aspects of the lattice dynamics. The angle dependence of Raman intensity under linear polarization configuration exhibits a strong tilt in the laboratory coordinate, indicating the existence of anisotropic electron-phonon coupling. The electron-phonon coupling was also examined via the Fano parameter of the asymmetric peak in the Raman spectra. Finally, the threefold rotational symmetry guarantees the existence of chiral phonons. We present direct spectroscopic evidence for these chiral vibrational modes through cross-circularly polarized Raman measurements, complemented by theoretical calculations of phonon circular polarization. Together, these results identify F5GT as an ideal platform for investigating emergent couplings among lattice, electronic, and magnetic degrees of freedom and for advancing the understanding of chiral phonons in magnetic van der Waals materials.

Figures (21)

• Figure 1: (a) Room temperature Fe$_{5}$GeTe$_{2}$ crystal structure, realized rhombohedral R3m symmetry. Conventional unit cell contains three octuple layers, separated by the van der Waals (vdW) gaps. (b) Such a structure is confirmed by the X-ray diffraction (XRD) pattern obtained at $300$ K. (c) The $M$--$T$ curve measured for in-plane and out-of-plane magnetic field.
• Figure 2: Raman spectrum measured at $80$ K in VV (inset: VH) polarization configuration. Dominant modes in VV configuration ---A$_{1}$(1), A$_{1}$(2), E(1), and E(2)--- are labeled. Theoretically obtained modes are marked by blue bars below.
• Figure 3: Polar plots of normalized Raman intensity for the discussed modes (as labeled) at $80$ K for the vertical polarization of the incident light on the sample. Red (brown) lines correspond to fit using Eq. (\ref{['eq.da1phi']}) [Eq. (\ref{['eq.dephi2']})] in the SM Note1.
• Figure 4: (a) Superposition of double-degenerated modes E results in right-handed and left-handed circular motions at the $\Gamma$ point. (b) Schematic of the helicity of the photon switched by the chiral phonon. (c,d) The helicity-resolved Raman spectra of F5GT at $80$ K (RL and LR configurations are presented by blue and red lines/points, respectively). Blue and red dashed lines correspond to the extracted E mode peaks. (e) Theoretically calculated the phonon circular polarization. Colored fat-bands correspond to the branches realizing the chiral phonons.
• Figure S1: Crystallographic axes orientation of the actual sample used for all Raman analysis.