Spatial mapping of collective modes and time-resolved tracking of excitonic order melting in Ta₂NiSe₅ at room temperature

The ternary compound Ta₂NiSe₅ has recently been proposed and investigated as a possible candidate for the long sought-after excitonic insulator (EI) phase in bulk materials. This fascinating phase of matter has attracted widespread attention as it could provide a toolset for the study of many-body physics phenomena and for the exploration of macroscopic coherent behaviors at room temperature. However, many questions remain open about the microscopic nature of the symmetry breaking process occurring in Ta₂NiSe₅ and other proposed EI candidates. In this talk, I will present our investigation of this material by imaging the spatial propagation of the condensate’s collective modes and detecting the effect of the destruction of the excitonic order on a femtosecond timescale. Through these attempts, we attempt to fill in some of the missing knowledge using ultrafast spectroscopic techniques.

We first use ultrafast microscopy measurements at room temperature to detect the propagation of coherent modes oscillating at the frequency of optical phonons but moving at electron-like velocities. We propose that this behavior can be explained by the existence of a hybridization process between the lattice degrees of freedom with the phase mode of an excitonic condensate hosted by Ta₂NiSe₅ at room temperature. Next, using broadband pump-probe measurements, we are able to identify a spectral range in the near infrared that maps the excitonic order of the condensate, which can be used to track the time evolution of the condensate out-of-equilibrium. Leveraging the 10 fs time resolution of our technique, we measure a sub-50 fs melting timescale for the condensate order, which suggests the existence of a strong electronic origin to the material’s broken symmetry. These results provide novel insights into the physics of EIs and showcase ultrafast pump-probe microscopy as an intriguing technique for the study of strongly-correlated materials.