Probing charge density wave phases and the Mott transition in $1T$-TaS$_2$ by inelastic light scattering

TL;DR

This study uses polarization-resolved Raman scattering and ab initio calculations to investigate charge density wave phases and the Mott transition in 1T-TaS2, revealing symmetry details, phase coexistence, and the Mott gap evolution.

Contribution

It provides new insights into the symmetry of the low-temperature phase, phase coexistence in the nearly-commensurate regime, and quantifies the Mott gap in 1T-TaS2.

Findings

P3 symmetry in the low-temperature phase excludes triclinic stacking.

Coexistence of IC-CDW and C-CDW phases in the NC-CDW regime.

Mott gap size determined to be approximately 170-190 meV.

Abstract

We present a polarization-resolved, high-resolution Raman scattering study of the three consecutive charge density wave (CDW) regimes in $1T$-TaS$_2$ single crystals, supported by \textit{ab initio} calculations. Our analysis of the spectra within the low-temperature commensurate (C-CDW) regime shows $\mathrm{P3}$ symmetry of the system, thus excluding the previously proposed triclinic stacking of the "star-of-David" structure, and promoting trigonal or hexagonal stacking instead. The spectra of the high-temperature incommensurate (IC-CDW) phase directly project the phonon density of states due to the breaking of the translational invariance, supplemented by sizeable electron-phonon coupling. Between 200 and 352\,K, our Raman spectra show contributions from both the IC-CDW and the C-CDW phase, indicating their coexistence in the so-called nearly-commensurate (NC-CDW) phase. The temperature-dependence of the symmetry-resolved Raman conductivity indicates the stepwise reduction of the density of states in the CDW phases, followed by a Mott transition within the C-CDW phase. We determine the size of the Mott gap to be $\Omega_{\rm gap}\approx 170-190$ meV, and track its temperature dependence.