Observation of the charge density wave collective mode in the infrared optical response of VSe$_2$

TL;DR

This study investigates the electronic and lattice changes in VSe₂ across its charge density wave transition using optical spectroscopy, revealing spectral weight transfer, collective modes, and phonon emergence.

Contribution

It provides detailed insights into the charge density wave phase transition in VSe₂, highlighting spectral weight transfer and lattice reconstruction effects.

Findings

Spectral weight shifts from Drude response to collective mode below 112 K

Emergence of infrared active phonons associated with lattice reconstruction

Charge dynamics become strongly damped with a mass enhancement factor of about 3

Abstract

We present a detailed study of the bulk electronic structure of high quality VSe$_{2}$ single crystals using optical spectroscopy. Upon entering the charge density wave phase below the critical temperature of 112 K, the optical conductivity of VSe$_2$ undergoes a significant rearrangement. A Drude response present above the critical temperature is suppressed while a new interband transition appears around 0.07\,eV. From our analysis, we estimate that part of the spectral weight of the Drude response is transferred to a collective mode of the CDW phase. The remaining normal state charge dynamics appears to become strongly damped by interactions with the lattice as evidenced by a mass enhancement factor m$^{*}$/m$\approx$3. In addition to the changes taking place in the electronic structure, we observe the emergence of infrared active phonons below the critical temperature associated with the 4a x 4a lattice reconstruction.