Physical Review Letters on chiral phonons

R. Yang^1,2,*, Y.-Y. Zhu^1,*, M. Steigleder², Y.-C. Liu³, C.-C. Liu³, X.-G. Qiu⁴, Tiantian Zhang^5,†, and M. Dressel^2,‡

• ¹Key Laboratory of Quantum Materials and Devices of Ministry of Education, School of Physics, Southeast University, Nanjing 211189, China
• ²Physikalisches Institut Universität Stuttgart, 70569 Stuttgart, Germany
• ³Centre for Quantum Physics, Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement (MOE), School of Physics, Beijing Institute of Technology, Beijing 100081, China
• ⁴Institute of Physics, Chinese Academy of Science, 100190 Beijing, China
• ⁵Institute of Theoretical Physics, Chinese Academy of Science, 100190 Beijing, China
  
  ^*These authors contributed equally to this work.
  ^†Contact author: ttzhang@itp.ac.cn
  ^‡Contact author: dressel@pi1.physik.uni-stuttgart.de

  Abstract

  We investigated the infrared-active phonons in ferromagnetic Weyl semimetal Co3⁢Sn2⁢S2 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 Co3⁢Sn2⁢S2, the CD of phonons exhibits significant temperature dependence, hinting at a promising approach for manipulation.