Excitonic condensation in bulk crystals is hard to separate from charge-density-wave formation, and probes that depend on long-range order lose sensitivity above Tc. In this work we used few-femtosecond broadband extreme-ultraviolet absorption spectroscopy (UBXAS) to follow exciton dissociation in 1T-TiSe2 as a function of pump fluence and temperature. The Se M4,5 edge reports on the transient population of Se 4p holes through local dipole transitions, so it stays sensitive when long-range phase coherence is gone.
We found that the initial response time scales as one over square root of fluence both below and above Tc ≈ 200 K, the signature of exciton breaking by plasmon screening. Short-range excitonic correlations therefore emerge before long-range order forms in the ground state. Close to Tc the response becomes fast and independent of fluence, consistent with a diverging excitonic susceptibility. A dominant phonon contribution is ruled out, since the response time drops threefold while the CDW phonon frequency renormalizes by only 20%.
This gives a direct observable for excitonic fluctuations outside the thermodynamically ordered phase, and it transfers to other excitonic insulator candidates in quasi-one- and quasi-two-dimensional crystals.
Congratulations to Alfred, Sheng-Chih, Emma, Bailey, Marcus and the whole team, and thanks to our collaborators at Shanghai Jiao Tong University, ShanghaiTech University, Tohoku University and SSRL.
The paper is now out open access in Nature Physics at https://doi.org/10.1038/s41567-026-03423-z
