Evidence for short-range excitonic correlations in 1T-TiSe₂ probed by broadband extreme-ultraviolet transient absorption spectroscopy

The charge-density-wave (CDW) transition in 1T-TiSe₂ has been subjected to intense investigations due to a variety of novel states associated with the phase transition. In equilibrium, the transition is suggested to be accompanied by excitonic, gyrotropic, and orbital orders. After photoexcitation, a multitude of fascinating phenomena has also been reported, such as CDW dimension cross-over, creation of anisotropic topological defects, and emergence of a metastable state. In particular, despite decades of close scrutiny, whether and how excitonic correlations drive the CDW transition remains an unsettled question. Here, by employing broadband extreme-ultraviolet transient absorption spectroscopy, we identified clear core-level signatures of long-range CDW formation in 1T-TiSe₂, even though equilibrium photoemission and absorption measurements of the same core levels showed no spectroscopic singularity at the phase transition. Leveraging the high time resolution and intrinsic sensitivity to short-range charge excitations in transient core-level absorption, we observed compelling time-domain evidence for excitonic correlations in the normal-state of the material, whose presence has been subjected to a long-standing debate in equilibrium experiments because of interfering phonon fluctuations in a similar part of the phase space. Our findings support the scenario that short-range excitonic fluctuations prelude long-range order formation in the ground state, providing important insights in the mechanism of exciton condensation in a quasi-low-dimensional system. These results further demonstrate the importance of a simultaneous access to long- and short-range order with underlying dynamical processes spanning a multitude of time- and energy-scales, making transient core-level spectroscopy an indispensable tool for both understanding the equilibrium phase diagram and for discovering novel, nonequilibrium states in strongly correlated materials.