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

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, wavelength, and polarization-dependent Raman measurements, along with {\it ab initio} calculations reveal important aspects of lattice dynamics and electron-phonon interactions. The angle-resolved Raman intensity under linear polarization configurations exhibits a strong tilt in the laboratory coordinate system, indicating the existence of anisotropic electron-phonon coupling. The temperature evolution of this anisotropy is discussed by extracting the phase factor of the Raman tensor elements from the angle-resolved intensity measured at different temperatures, also uncovering a spin-orbit coupling-mediated electron-phonon response in F5GT. The thermal evolution of electron-phonon coupling is also examined by measuring the temperature dependence of the Fano parameter of the asymmetric peak in the Raman spectra, while wavelength-dependent measurements establish the role of optical resonance in enhancing the anisotropic interaction. 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.