Anisotropic Fano resonance in a Weyl semimetal candidate LaAlSi

Quantum interference effects play an important role in the optical properties of topological materials. One example is Fano resonance, featured by an asymmetric Breit-Wigner-Fano (BWF) line shape that describes interference between discrete and continuous scattering states. In this work, we present the anisotropic Fano resonance in a type-II Weyl semimetal candidate LaAlSi by polarized Raman spectroscopy using five excitation wavelengths from UV to near-infrared. The B₁ phonon mode for 532-nm laser excitation shows anisotropic BWF line shape whose frequency, linewidth, and asymmetry factor all depend on the laser polarization angle. Such anisotropic Fano resonance is unique and rarely observed in solid-state materials. Meanwhile, a scattering background occurs along with the asymmetry constituting the continuous states, which can be explained by double resonant Raman scattering due to flat phonon dispersions in the density functional theory (DFT) calculations. The experimental observations combined with DFT calculations provide valuable insights into the microscopic scattering pathways of LaAlSi, which is essential in understanding the optical scattering process and electron-phonon interactions in topological quantum materials.