Short-range excitonic correlations and enhanced excitonic susceptibility in 1T-TiSe2

Abstract Excitons—bound states of electrons and holes—can spontaneously condense into a quantum coherent ground state. 1 T -TiSe 2 , a layered transition metal dichalcogenide, is a candidate for realizing this phase in a bulk material, but its putative excitonic condensation is accompanied by charge-density-wave formation that obscures signatures of excitonic instability and fluctuations in this structural transition. Here we clarify the mechanism of exciton condensation in quasi-low-dimensional systems by tracking exciton dissociation as a function of photoexcitation fluence and temperature using few-femtosecond broadband extreme-ultraviolet absorption spectroscopy. We find that short-range excitonic fluctuations emerge before long-range-order formation in the ground state. We also observe enhanced excitonic susceptibility, identified by faster exciton dissociation near the phase transition temperature, consistent with excitonic instability in this material. These results provide a framework for understanding other excitonic insulator candidates proposed in quasi-one- or quasi-two-dimensional crystals.