Imaging the coherent propagation of collective modes in the excitonic insulator candidate Ta${}_2$NiSe${}_5$ at room temperature

Abstract

Excitonic insulators host a condensate of electron-hole pairs at equilibrium, giving rise to collective many-body effects. Although several materials have emerged as excitonic insulator candidates, evidence of long-range coherence is lacking and the origin of the ordered phase in these systems remains controversial. Here, using ultrafast pump-probe microscopy, we investigate the possible excitonic insulator Ta${}_2$NiSe${}_5$. Below 328 K, we observe the anomalous micrometer-scale propagation of coherent modes at velocities of the order of $\sim 10^5$ m/s, which we attribute to the hybridization between phonon modes and the phase mode of the condensate. We develop a theoretical framework to support this explanation and propose that electronic interactions provide a significant contribution to the ordered phase in Ta${}_2$NiSe${}_5$. These results allow us to understand how the condensate's collective modes transport energy and interact with other degrees of freedom. Our study provides a unique paradigm for the investigation and manipulation of these properties in strongly correlated materials.

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• A New Era of Excitonic Insulators
• Unveiling the underlying interactions in Ta2NiSe5 from photo-induced lifetime change
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• Leaky exciton condensates in transition metal dichalcogenide moiré bilayers
• Anomalous excitonic phase diagram in band-gap-tuned Ta2Ni(Se,S)5
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• Direct Observation of Ultrafast Singlet Exciton Fission in Three Dimensions
• Third-harmonic generation in excitonic insulators
• Theory of time-crystalline behaviour mediated by phonon squeezing in Ta2NiSe5
• Entangled phase of simultaneous fermion and exciton condensations realized
• Collective modes and Raman response in Ta${}_2$NiSe${}_5$
• Detecting photoelectrons from spontaneously formed excitons
• Spin-Triplet Excitonic Insulator in the Ultra-Quantum Limit of HfTe5
• Photo-induced Dynamics of Quasicrystalline Excitonic Insulator
• Stabilization of sliding ferroelectricity through exciton condensation
• Ab initio Approach to Collective Excitations in Excitonic Insulators
• Photoinduced melting dynamics and collective mode in a correlated charge-ordered system
• Probing the collective excitations of excitonic insulators in an optical cavity