Structural Dynamics of incommensurate Charge-Density Waves tracked by Ultrafast Low-Energy Electron Diffraction

TL;DR

This study uses ultrafast low-energy electron diffraction to investigate the non-equilibrium structural dynamics of charge-density waves in 1T-TaS2, revealing long-lived suppression and phase fluctuations of the order parameter after excitation.

Contribution

It provides new insights into the ultrafast quench and recovery processes of charge-density waves, highlighting the role of lattice thermalization and phonon populations.

Findings

Long-lived partial suppression of the CDW order parameter up to 60 ps.

Delayed return to thermal equilibrium linked to acoustic phonon population.

Observation of nonlinear phase fluctuation generation at high fluences.

Abstract

We study the non-equilibrium structural dynamics of the incommensurate and nearly-commensurate charge-density wave phases in 1T-TaS$_2$. Employing ultrafast low-energy electron diffraction (ULEED) with 1 ps temporal resolution, we investigate the ultrafast quench and recovery of the CDW-coupled periodic lattice distortion. Sequential structural relaxation processes are observed by tracking the intensities of main lattice as well as satellite diffraction peaks as well as the diffuse scattering background. Comparing distinct groups of diffraction peaks, we disentangle the ultrafast quench of the PLD amplitude from phonon-related reductions of the diffraction intensity. Fluence-dependent relaxation cycles reveal a long-lived partial suppression of the order parameter for up to 60 picoseconds, far outlasting the initial amplitude recovery and electron-phonon scattering times. This delayed return to a quasi-thermal level is controlled by lattice thermalization and coincides with the population of zone-center acoustic modes, as evidenced by a structured diffuse background. The long-lived non-equilibrium order parameter suppression suggests hot populations of CDW-coupled lattice modes. Finally, a broadening of the superlattice peaks is observed at high fluences, pointing to a nonlinear generation of phase fluctuations.