Revealing the order parameter dynamics of 1T-TiSe$_2$ following optical excitation

TL;DR

This study investigates the dynamics of the charge density wave order parameter in 1T-TiSe₂ after optical excitation, revealing the coexistence of excitonic and pseudogap orders and their distinct responses.

Contribution

It provides new insights into the dual nature of the gap in TiSe₂, distinguishing between excitonic and Jahn-Teller contributions through time-resolved photoelectron spectroscopy.

Findings

Identified two distinct orders contributing to the gap: CDW and pseudogap.

Revealed the excitonic long-range nature of the CDW gap.

Showed the pseudogap order's short-range Jahn-Teller character.

Abstract

The formation of a charge density wave state is characterized by an order parameter. The way it is established provides unique information on both the role that correlation plays in driving the charge density wave formation and the mechanism behind its formation. Here we use time and angle resolved photoelectron spectroscopy to optically perturb the charge-density phase in 1T-TiSe$_2$ and follow the recovery of its order parameter as a function of energy, momentum and excitation density. Our results reveal that two distinct orders contribute to the gap formation, a CDW order and pseudogap-like order, manifested by an overall robustness to optical excitation. A detailed analysis of the magnitude of the the gap as a function of excitation density and delay time reveals the excitonic long-range nature of the CDW gap and the short-range Jahn-Teller character of the pseudogap order. In contrast to the gap, the intensity of the folded Se$_{4p}$* band can only give access to the excitonic order. These results provide new information into the the long standing debate on the origin of the gap in TiSe$_2$ and place it in the same context of other quantum materials where a pseudogap phase appears to be a precursor of long-range order.