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Strong long-wavelength electron-phonon coupling in ${Ta}_{2} {Ni (Se, S)}_{5}$

Zhibo Kang ¹, Burak Gurlek ², Weichen Tang ^3,4, Xiang Chen ^3,4, Jacob P. C. Ruff ⁵, Ahmet Alatas ⁶, Ayman H. Said ⁶, Robert J. Birgeneau ^3,4,7, Steven G. Louie ^3,4 et al.

Angel Rubio ^2,8, Simone Latini ^2,9, and Yu He ^1,*

¹Department of Applied Physics, Yale University, New Haven, Connecticut 06511, USA
²Max Planck Institute for the Structure and Dynamics of Matter and Center for Free-Electron Laser Science, Luruper Chaussee 149, 22761 Hamburg, Germany
³Physics Department, University of California, Berkeley, California 94720, USA
⁴Materials Sciences Division, Lawrence Berkeley National Lab, Berkeley, California 94720, USA
⁵Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York 14853, USA
⁶Argonne National Laboratory, Argonne, Illinois 60439, USA
⁷Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
⁸Initiative for Computational Catalysis (ICC), Flatiron Institute, New York, New York 10010, USA
⁹Department of Physics, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark

^*Contact author: yu.he@yale.edu

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Phys. Rev. Materials 10, L053201 – Published 4 May, 2026

DOI: https://doi.org/10.1103/3wqb-7qq3

Abstract

The search for intrinsic excitonic insulators (EI) has long been confounded by coexisting electron–phonon coupling in bulk materials. Although the ground state of an EI may be difficult to differentiate from density-wave orders or other structural instabilities, excited states offer distinctive signatures. One way to provide clarity is to directly inspect the phonon spectral function for long wavelength broadening caused by phonon interaction with the high velocity EI phason. Here, we report that the quasi-one-dimensional (quasi-1D) EI candidate ${Ta}_{2} {NiSe}_{5}$ shows extremely anisotropic phonon broadening and softening in the semimetallic normal state. In contrast, such behavior is completely absent in the broken symmetry state of ${Ta}_{2} {NiSe}_{5}$ and in the isostructural ${Ta}_{2} {NiS}_{5}$ , where the latter has a fully gapped normal state. By contrasting the expected phonon lifetimes in the BCS and BEC limits of a putative EI, our results suggest that the phase transition in ${Ta}_{2} Ni {(Se, S)}_{5}$ family is closely related to strong interband electron–phonon coupling. We experimentally determine the dimensionless coupling $\frac{g}{ω_{0}} \sim 10$ , revealing ${Ta}_{2} Ni {(Se, S)}_{5}$ as a rare “ultrastrong coupling” material.

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Strong long-wavelength electron-phonon coupling in ${Ta}_{2} {Ni (Se, S)}_{5}$

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Strong long-wavelength electron-phonon coupling in Ta2Ni(Se,S)5

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Strong long-wavelength electron-phonon coupling in ${Ta}_{2} {Ni (Se, S)}_{5}$