Ultrafast x-ray scattering reveals composite amplitude collective mode in the Weyl charge density wave material (TaSe$_4$)$_2$I

TL;DR

This study uses ultrafast x-ray scattering to observe a composite amplitude mode in the charge density wave material (TaSe4)2I, revealing how photoexcitation influences its collective excitations.

Contribution

It provides the first direct observation of a composite amplitude mode in a quasi-1D CDW material using ultrafast x-ray scattering.

Findings

Identification of a transverse acoustic amplitude mode

Mode dynamics well described by a displacive excitation model

Coupling between acoustic and optical phonons elucidated

Abstract

We report ultrafast x-ray scattering experiments of the quasi-1D charge density wave (CDW) material (TaSe$_4$)$_2$I following photoexcitation with femtosecond infrared laser pulses. From the time-dependent diffraction signal at the CDW sidebands we identify an amplitude mode derived primarily from the transverse acoustic component of the CDW static distortion. The dynamics of this acoustic amplitude mode are described well by a model of a displacive excitation, which we interpret as mediated through a coupling to the optical phonon component associated with the tetramerization of the Ta chains.