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

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Abstract
1T-TiSe$_2$ exhibits a charge-density-wave (CDW) transition below 200 K, which is believed to be driven by a hybrid exciton-phonon mechanism, making it a versatile platform for investigating the interplay between electronic and lattice degrees of freedom. Although the corresponding band structure modifications below the CDW transition temperature are well established, only a few reports discuss the occurrence of the CDW fluctuating phase and its spectral features well above the transition temperature. Here, we report the direct observation of spectral features associated with CDW fluctuations at 295 K, using time-resolved extreme ultraviolet momentum microscopy. We investigated the transient melting and recovery of CDW fluctuations upon nonresonant ultrafast photoexcitation. Surprisingly, our results reveal that the coherent amplitude mode modulating the ultrafast CDW recovery persists at these elevated temperatures. The time-, energy- and momentum-resolved photoemission data supported by density functional perturbation theory further confirm that CDW fluctuations at these elevated temperatures are dominated by electron-phonon interaction. The analysis of these very localized microscopic fluctuations consequently provides new insights into the complex interplay between the electronic and lattice degrees of freedom at elevated temperatures and, therefore, on the nature of this quantum phase transition.

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