Imaging the temperature dependence of excitonic insulator candidate, Ta₂NiSe₅, out of equilibrium.

The excitonic insulating phase is a predicted condensate formed of excitons, given that exciton binding energy is greater than the bandgap, expected to exist in Ta₂NiSe₅. However, the transition from insulator to excitonic insulator is masked by a structural phase transition, resulting in a similar bandgap opening at 328K. Here, we build a novel ultrafast, temperature dependent, optical pump-probe microscope, utilising a pulse shaper - resulting in a final pump of 20fs, focussed through a commercial 100x objective and cryostat. This ultrafast impulse is imaged using a widefield 12fs probe, from 10K to 370K, where Raman modes are identified in the time domain oscillating signal. Here, the 2.9THz mode experiences a redshift of 0.3meV as the temperature is increased, consistent with published CW Raman spectroscopy. Post subtraction of the oscillations, the electronic lifetimes are modelled by an exponentially modified Gaussian. Surprisingly, while the electronic lifetimes are found to increase upon cooling, there is a sharp decrease at 120K - typically indicative of a phase transition. This result is curious due to the lack of known phase transition at this temperature. Additionally, the spatial extent of the phonon modes change with temperature, however, at differing rates, indicating that the phonon propagation length is both temperature and mode dependent. These results demonstrate the possibility of interesting non-equilibrium phases and phonon transport in Ta₂NiSe₅.