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Excitonic insulators represent an exotic quantum state of matter in which excitons collectively condense into a
macroscopic ordered phase. Unlike superconductors, excitonic condensates, in principle, exist at
temperatures as high as room temperature, making them an attractive platform for studying macroscopic
quantum phenomena at elevated temperatures. Although this concept was proposed in solids more than 60
years ago, whether an excitonic insulator can be experimentally realized in a real material remains an open
question.
Promising candidate materials include 1T-TiSe₂ and Ta₂NiSe₅, both of which have been extensively
investigated experimentally and theoretically. A central difficulty is that, in these systems, the transition into
the proposed excitonic-insulator state coincides with a charge-density-wave or structural phase transition.
Whether these transitions are primarily structural or electronic in nature and which signatures can uniquely
identify excitonic order remains debated. Time-resolved techniques have contributed to this discussion by
revealing coherent collective excitations and their coupling to the lattice [1].
In this talk, I will present pump–probe experiments on Ta₂Pd₃Te₅, where an excitonic-insulator phase develops
without a detectable structural transition. We characterize the condensate through its coherent phonon-
amplitude response [2]. Most strikingly, the lifetimes of these modes increase dramatically below the
transition temperature, Tc. We identify this enhancement as a robust dynamical fingerprint associated with
exciton condensation. I will discuss how coupling between excitonic and lattice degrees of freedom may
account for the enhanced lifetimes, and address the broader question of what stabilizes an excitonic
condensate in real bulk materials.
[1] D. Werdehausen et al., “Coherent order parameter oscillations in the ground state of the excitonic insulator
Ta₂NiSe₅,” Science Advances 4, eaap8652 (2018). https://doi.org/10.1126/sciadv.aap8652
[2] Anjan Kumar N. M. et al., “Extraordinary Lifetime Enhancement of Coherent Phonon-Amplitude Modes in the
Excitonic Insulator Phase of Ta₂Pd₃Te₅,” arXiv:2609.26485 (2026). https://arxiv.org/abs/2609.26485 |