The Commensurate-Incommensurate Charge-Density-Wave Transition and Phonon Zone Folding in 1T-TaSe2 Thin Films

TL;DR

This study investigates the charge-density-wave transitions in 1T-TaSe2 thin films, revealing how transition temperatures decrease with reduced thickness and how phonon zone folding relates to lattice reconstruction.

Contribution

It provides experimental Raman evidence and ab initio calculations linking charge-density-wave phases to phonon zone folding and lattice reconstruction in thin 1T-TaSe2.

Findings

C-CDW transition temperature decreases from 473K to 413K as thickness reduces from 150 nm to 35 nm

Zone-folding of phonon modes observed in Raman spectra after lattice reconstruction

Suppression and broadening of Raman peaks in the I-CDW phase

Abstract

Bulk 1T-TaSe2 exhibits unusually high charge density wave (CDW) transition temperatures of 600 K and 473 K below which the material exists in the incommensurate (I-CDW) and the commensurate (C-CDW) charge-density-wave phases, respectively. The C-CDW reconstruction of the lattice coincides with new Raman peaks resulting from zone-folding of phonon modes from middle regions of the original Brillouin zone back to the Gamma point. The C-CDW transition temperatures as a function of film thickness are determined from the evolution of these new Raman peaks and they are found to decrease from 473K to 413K as the film thicknesses decrease from 150 nm to 35 nm. A comparison of the Raman data with ab initio calculations of both the normal and C-CDW phases gives a consistent picture of the zone-folding of the phonon modes following lattice reconstruction. In the I-CDW phase, the loss of translational symmetry coincides with a strong suppression and broadening of the Raman peaks. The observed change in the C-CDW transition temperature is consistent with total energy calculations of bulk and monolayer 1T-TaSe2.