Electron-phonon-dominated charge-density-wave fluctuations in TiSe$_2$ accessed by ultrafast nonequilibrium dynamics

TL;DR

This study reveals that in TiSe2, charge density wave fluctuations above the transition temperature are primarily driven by electron-phonon interactions, not excitonic effects, using ultrafast spectroscopy and theoretical analysis.

Contribution

It demonstrates that CDW fluctuations in TiSe2 are dominated by electron-phonon coupling, challenging the previous excitonic-based understanding.

Findings

CDW fluctuations persist above T_CDW.

Electron-phonon interaction dominates over excitonic effects.

Ultrafast dynamics reveal spectral features of fluctuations.

Abstract

The complex phase diagram of 1T-TiSe2 consists of a charge density wave (CDW) below 200 K, and CDW fluctuations of still unknown origin at higher temperatures. Here, we use time-resolved extreme ultraviolet momentum microscopy and density functional perturbation theory to uncover the formation mechanism of CDW fluctuations and their spectral features at 295 K. We investigated the transient dynamics of fluctuations upon nonresonant ultrafast photoexcitation, and directly correlate it with the CDW soft-phonon hardening. Surprisingly, our results show that the coherent amplitude mode modulating ultrafast CDW recovery persists above TCDW, and reveal that CDW fluctuations are dominated by the electron-phonon interaction rather than excitonic correlations as commonly believed. Our findings on these microscopic CDW fluctuations clarify the complex interplay between electronic and lattice degrees of freedom at elevated temperatures and, therefore, could be useful in understanding the nature of the CDW phase transition in 1T-TiSe2 and similar quantum materials.