Inherent Circular Dichroism of Phonons in Magnetic Weyl Semimetal Co3Sn2 S2

We investigated the infrared-active phonons in ferromagnetic Weyl semimetal Co3Sn2S2 using optical spectroscopy. Below the Curie temperature (TC≈175 K), we observed asymmetric Fano line shapes of phonons peaks in the optical conductivities, reflecting the presence of electron-phonon coupling. Additionally, the detected phonon signals by the polar Kerr rotation and the ellipticity spectroscopy indicate the circular dichroism (CD) of phonons. We attribute the CD of phonons to their distinct couplings with charge excitations on the tilted Weyl nodal rings in two circularly polarized channels. Our findings provide experimental evidence that, without external fields, phonons can also become circularly polarized by coupling with the electronic topology. Since the magnetic exchange splitting gradually shifts the topological bands in Co3Sn2S2, the CD of phonons exhibits significant temperature dependence, hinting at a promising approach for manipulation.