Imaging the coherent propagation of collective modes in the excitonic insulator Ta2NiSe5 at room temperature

Bretcher, Hope 

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Excitonic insulators (EIs) are materials predicted to host a condensate of electron-hole pairs in their ground state, giving rise to collective many- body effects. Although several bulk materials have been proposed as EIs, an observation of the characteristic long-range coherent behavior is miss- ing and thus the origin of the ordered phase remains controversial. Here, we use ultrafast, spatially-resolved, pump-probe microscopy to investigate the low-temperature phase of the EI candidate Ta2NiSe5. Below the crit- ical temperature (328 K), we observe anomalous micrometer-scale propa- gation of coherent oscillatory modes at velocities of the order of 105 m/s. We attribute these observations to a hybridization between phonon modes and a gapless or low-lying phase mode of the excitonic insulator. We de- velop a theoretical framework to support this explanation and propose that the ordered phase in Ta2NiSe5 is driven predominantly by interac- tions of electronic nature and that the behavior of this system falls close to the BCS-BEC crossover regime. These results allow us to understand how the condensate’s collective modes transport energy through the material and how they interact with other degrees of freedom. Our study provides a new paradigm for the investigation of these properties in strongly cor- related materials and paves the way toward the use of these phenomena in new quantum architectures operating up to room temperature.

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Science Advances
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American Association for the Advancement of Science
Engineering and Physical Sciences Research Council (EP/M006360/1)
European Research Council (758826)