Lattice dynamics of layered kagome lattice material Nb₃Br₈ investigated via Raman spectroscopy and DFT

Niobium bromide (Nb₃Br₈) is a quantum material that crystallizes in a layered breathing kagome lattice with topologically flat bands, serving as a potential platform for correlated electronic states. Nb₃Br₈ also undergoes a structural phase transition when heated above 387 K that changes the stacking order of the lattice and induces a magnetic phase transition. Despite being a promising quantum material for the study of exotic electronic and magnetic states, an understanding of the lattice dynamics of Nb₃Br₈ remains elusive. In this study, exfoliated bulk and few layer Nb₃Br₈ flakes are investigated via linearly polarized Raman spectroscopy and density-functional theory (DFT) to study the crystal’s phonon modes and their symmetrical representations. 18 modes are resolved in exfoliated bulk, 10 are Eg modes and 8 are A1g modes, in close agreement with DFT analysis. Linearly polarized Raman spectroscopy performed above 387 K showed no changes in the Raman selection rules of the phonon modes, suggesting their insensitivity to the structural phase transition. Finally, layer-dependent Raman studies reveal that the intensity of bands in the Raman spectra decrease and experience a slight shift in frequency in thin layers, however the Raman selection rules remain consistent with bulk. This study serves to establish a foundation for future studies of Nb₃Br₈ andutilizing its characteristic Raman-active phonon modes across different structural phases and thicknesses down to bilayer.