Raman scattering fingerprints of the charge density wave state in one-dimensional NbTe$_4$

TL;DR

This study uses Raman scattering spectroscopy to analyze the charge density wave states in NbTe$_4$, revealing phase transitions, phonon mode behaviors, and hysteresis effects relevant for memory devices.

Contribution

First detailed Raman spectroscopy analysis of CDW phases in NbTe$_4$, highlighting phase transition hysteresis and phonon-symmetry coupling.

Findings

25 phonon modes at 5 K in NbTe$_4$

Temperature-dependent transition between commensurate and incommensurate CDWs

Hysteresis width varies with warming rate, indicating domain nucleation effects

Abstract

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. We investigate the CDW state in quasi-one-dimensional NbTe$_4$ 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.