Asymmetric behavior in Raman-Active Phonon Mode in TaAs Thin Films 

Topological materials not only display exotic electronic structure properties, but their vibrational properties have been investigated as well for phenomena such as chiral phonons and asymmetric phonon line shape. Recent studies indicating an asymmetric phonon mode in infrared spectroscopy in TaAs piqued our attention in exploring Raman-active modes. Both bulk crystal and thin film TaAs have been studied for vibrational properties. Raman spectroscopy studies on bulk crystal TaAs are in agreement with previous theory and experimental results, but thin film analysis is particularly interesting since it shows an asymmetric line shape in the B₁ phonon mode at a lower frequency. To further understand this behavior in thin film TaAs, first-principles calculations have been performed on strained bulk TaAs. These simulations show that thin film TaAs undergoes epitaxial strain caused by substrate GaAs, which is most likely responsible for the observed phonon softening. The asymmetric profile results from interaction between the phonon mode and electronic states at the Fermi level. Our findings are noteworthy in that they demonstrate how switching from a bulk crystal to a thin film sample can change the behavior of vibration characteristics, resulting in an asymmetric line shape in phonons, also known as the Fano line shape.