Excitonic and lattice contributions to the charge density wave in 1T-TiSe₂ revealed by a relaxation bottleneck

Using time- and angle-resolved photoemission spectroscopy (TR-ARPES), we have studied the femtosecond dynamics of the charge density wave (CDW) in 1T-TiSe₂. This material exhibits a commensurate CDW with a (2 x 2 x 2) lattice distortion below 200 K for which the mechanism is still debated today. One of the recurring problems is disentangling the contributions from the electronic and lattice-driven order. Here, we have observed the photo-induced suppression of the CDW in real-time by tracking the valence band maximum at the Γ-point following an intense laser pulse. The timescales of gap closing and band-replica unfolding (< 200 fs), and the relatively low fluence required for these processes are suggestive of an excitonic mechanism. During recovery, coherent oscillations of the CDW gap are observed relating to the A_1g (3.4 THz) mode. A detailed pump fluence dependence reveals several regimes, including impeded recovery at high fluence. This bottleneck coincides with a change in the dominant frequency seen in the oscillations. Using complementary time-resolved reflectivity, we have established the threshold fluence of this phenomenon to be F > 60 μJ cm^-2. Our work is supported by a rate equation model which highlights the crucial role of excitons and phonons in this complex system.