Chiral phonons and the magneto-optical Raman effect in kagome lattice semiconductor Nb ₃ Cl ₈

Niobium chloride (Nb ₃ Cl ₈ ) is a typical kagome lattice semiconductor with a topological flat band in its band structure. It undergoes a magnetic phase transition from a paramagnetic state to a non-magnetic singlet state at around 94 K. We use polarization resolved Raman spectroscopy to study the chiral and non-chiral phonons in Nb ₃ Cl ₈ since phonon chirality plays a key role in studying topological state and quantum state, and experimentally observed nine chiral phonon modes (E _g ) and seven non-chiral phonon modes (A _1g ). Chiral phonons possess nonzero angular momentum, which can switch the helicity of incident photons in the Raman scattering process. The polarization plane of non-chiral phonons can be rotated by an out-of-plane magnetic field at 6 degrees per tesla, showing a giant magneto-optical Raman effect. We also observed that the frequency of two low-energy chiral modes (102 cm ⁻¹ and 120 cm ⁻¹ ) experiences a sudden shift at the magnetic phase transition temperature; the critical temperature is different in the warming and cooling processes, similar to the hysteresis curve in the magnetic susceptibility measurements. Our result sheds light on the potential applications of kagome semiconductors, such as magneto-optics.