Light-induced dimension crossover in 1T-TiSe$_2$ dictated by excitonic correlations

TL;DR

This study reveals that ultrafast laser pulses can induce a transition from three-dimensional to two-dimensional charge-density-wave states in 1T-TiSe$_2$, driven by excitonic correlations that preserve out-of-plane coherence.

Contribution

It demonstrates that excitonic correlations are essential for maintaining 3D CDW coherence and that optical excitation can control the system's dimensionality in strongly correlated materials.

Findings

Photoexcitation suppresses 3D CDW and creates 2D CDW.

Dimension crossover depends on breaking electron-hole pairs.

Excitonic correlations are crucial for out-of-plane coherence.

Abstract

In low-dimensional systems with strong electronic correlations, the application of an ultrashort laser pulse often yields novel phases that are otherwise inaccessible. The central challenge in understanding such phenomena is to determine how dimensionality and many-body correlations together govern the pathway of a non-adiabatic transition. To this end, we examine a layered compound, 1T-TiSe$_2$, whose three-dimensional charge-density-wave (3D CDW) state also features exciton condensation due to strong electron-hole interactions. We find that photoexcitation suppresses the equilibrium 3D CDW while creating a nonequilibrium 2D CDW. Remarkably, the dimension reduction does not occur unless bound electron-hole pairs are broken. This relation suggests that excitonic correlations maintain the out-of-plane CDW coherence, settling a long-standing debate over their role in the CDW transition. Our findings demonstrate how optical manipulation of electronic interaction enables one to control the dimensionality of a broken-symmetry order, paving the way for realizing other emergent states in strongly correlated systems.