Probing electron-phonon and phonon-phonon coupling in type-II Dirac semi-metal NiTe₂via temperature-dependent Raman spectroscopy

We report the temperature-dependent structural characterization of type-II Dirac semimetal NiTe₂in the form of a bulk single crystal and a nanoflake (200 nm thick). Detailed x-ray diffraction study along with Rietveld refinement analysis reveals superior crystallinity and linear thermal expansion coefficient (α_T) of 5.56 × 10^-6and 22.5 × 10^-6K^-1along a or b and c lattice directions, respectively. Temperature evolution of Raman spectra shows non-linear variations in the phonon frequency and full-width half maxima of the out-of-plane A1gand in-plane E_gmodes. Raman mode E2g1, corresponding to an in-plane vibration, disappears on decreasing the thickness from bulk to nanoflake. Quantitative analysis with anharmonic model yields dominating electron-phonon interaction over phonon-phonon interaction mediated by three- and four-phonon processes.