Phonon States in NbTe$_4$ and TaTe$_4$ Quasi-One-Dimensional van der Waals Crystals

We report the results of polarization-dependent Raman spectroscopy of phonon states in single-crystalline quasi-one-dimensional NbTe$_4$ and TaTe$_4$ van der Waals materials. The measurements were conducted in the wide temperature range from 80 K to 560 K. Our results show that although both materials have identical crystal structures and symmetries, there is a drastic difference in the intensity of their Raman spectra. While TaTe4 exhibits well-defined peaks through the examined frequency and temperature ranges, NbTe4 reveals extremely weak Raman signatures. The measured spectral positions of the phonon peaks agree with the phonon band structure calculated using the density-functional theory. We offer possible reasons for the in-tensity differences between the two van der Waals materials. Our results provide insights into the phonon properties of NbTe$_4$ and TaTe$_4$ van der Waals materials and indicate the potential of Raman spectroscopy for studying charge-density-wave quantum condensate phases.
Comment: 24 pages; 6 figures; 1 table