Magnetic-Field Dependence of Chirality-Resolved Optical Phonon in Weyl semimetal

Weyl semimetal (WSM), the solid-state realization of chiral Weyl fermions, exhibits chiral zeroth Landau levels in the magnetic field contributing to novel transport and optical properties. While electronic transport has been a focus of studies, the optical response of WSM is relatively under-explored, yet it can also reflect the topological nature of WSM. For example, the chirality of the Weyl fermions can be determined by measuring the photocurrent using circularly polarized mid-infrared light. In this work, we study the chirality-resolved optical phonons in type-I WSM TaP under magnetic field, using Raman spectroscopy with excitations of both linear and circularly polarized lasers. Under magnetic field, the B₁¹ mode prohibited at zero field for 633 nm excitation can be evidently observed for linear polarization. With circularly polarized excitation, the measured phonon modes break the classical Raman tensor theory, and the symmetry-prohibited modes can be observed. Moreover, the phonon mode intensity exhibits non-reciprocity with polarizations of incident and scattered light (σ+/σ+ and σ+/σ-). Our observation is closely related to the chiral Landau levels and shows the capability of Raman spectroscopy in studying the magnetic field response of WSMs.