Raman scattering fingerprints of the charge density wave state in one-dimensional NbTe4

Charge density waves (CDWs) are ordered quantum states of conduction electrons accompanied by periodic lattice distortions. Raman scattering (RS) spectroscopy is, therefore, well suited for probing CDW-induced structural modulations. The CDW state in quasi-one-dimensional NbTe4 is investigated using RS spectroscopy. At T = 5 K, the resonantly enhanced Raman spectrum exhibits 25 phonon modes. Polarization-dependent measurements reveal a strong coupling between phonon-mode symmetry and crystallographic symmetry, with modes polarized parallel or perpendicular to the crystallographic c axis, along which the one-dimensional structure is elongated. Temperature-dependent RS measurements identify a transition between commensurate and incommensurate CDW phases, accompanied by pronounced thermal hysteresis, with transition temperatures of approximately 45 K upon cooling and 90 K upon warming. The hysteresis width depends on the warming rate, indicating a finite nucleation rate of CDW domains and suggesting potential relevance for memory-device applications.