Zone-center phonons of bulk, few-layer, and monolayer 1T-TaS$_2$: Detection of the commensurate charge density wave phase through Raman scattering

TL;DR

This study combines first-principles calculations and Raman spectroscopy to identify the commensurate charge density wave phase in 1T-TaS$_2$ across various thicknesses, revealing its persistence down to monolayer and providing insights into its vibrational and orbital properties.

Contribution

It provides the first combined theoretical and experimental evidence that the commensurate CDW phase persists in monolayer 1T-TaS$_2$, challenging previous assumptions about phase suppression in thin layers.

Findings

Identification of low-frequency folded-back acoustic modes as CDW signatures

Experimental confirmation of the commensurate phase in monolayer samples

Probing of c-axis orbital texture related to the metal-insulator transition

Abstract

We present first-principles calculations of the vibrational properties of the transition metal dichalcogenide 1T-TaS$_2$ for various thicknesses in the high-temperature (undistorted) phase and the low-temperature commensurate charge density wave (CDW) phase. We also present measurements of the Raman spectra for bulk, few-layer, and monolayer samples at temperatures well below that of the bulk transition to the commensurate phase. Through our calculations, we identify the low-frequency folded-back acoustic modes as a convenient signature of the commensurate CDW wave structure in vibrational spectra. In our measured Raman spectra, this signature is clearly evident in all of the samples, indicating that the commensurate phase remains the ground state as the material is thinned, even down to a single layer. This is in contrast to some previous studies which suggest a suppression of the commensurate CDW transition in thin flakes. We also use polarized Raman spectroscopy to probe c-axis orbital texture in the low-T phase, which has recently been suggested as playing a role in the metal-insulator transition that accompanies the structural transition to the commensurate CDW phase.