DOE OSTI · 3374413
Strong long-wavelength electron-phonon coupling in Ta 2 Ni(Se,S) 5
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, in this study, 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 $\frac{g}{ω_0}$ ∼ 10, revealing Ta 2 Ni(Se,S) 5 as a rare “ultrastrong coupling” material.
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Kang, Zhibo [Yale University, New Haven, CT (United States)] (ORCID:0000000239278128), Gurlek, Burak [Max Planck Institute for the Structure and Dynamics of Matter, Hamburg (Germany); Deutsches Elektronen-Synchrotron (DESY), Hamburg (Germany). Center for Free-Electron Laser Science] (ORCID:0000000290388488), Tang, Weichen [University of California, Berkeley, CA (United States); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)] (ORCID:0000000219455369), Chen, Xiang [University of California, Berkeley, CA (United States); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)] (ORCID:0000000339978148), Ruff, Jacob P. C. [Cornell University, Ithaca, NY (United States)] (ORCID:0000000227944795), Alatas, Ahmet [Argonne National Laboratory (ANL), Argonne, IL (United States)] (ORCID:000000016521856X), Said, Ayman H. [Argonne National Laboratory (ANL), Argonne, IL (United States)] (ORCID:0009000058228946), Birgeneau, Robert J. [University of California, Berkeley, CA (United States); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)] (ORCID:0000000311928333), Louie, Steven G. [University of California, Berkeley, CA (United States); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)] (ORCID:0000000306220170), Rubio, Angel [Max Planck Institute for the Structure and Dynamics of Matter, Hamburg (Germany); Deutsches Elektronen-Synchrotron (DESY), Hamburg (Germany). Center for Free-Electron Laser Science; Flatiron Institute, New York, NY (United States)] (ORCID:0000000320603151), Latini, Simone [Max Planck Institute for the Structure and Dynamics of Matter, Hamburg (Germany); Deutsches Elektronen-Synchrotron (DESY), Hamburg (Germany). Center for Free-Electron Laser Science; Technical University of Denmark, Lyngby (Denmark)] (ORCID:0000000195535259), He, Yu [Yale University, New Haven, CT (United States)] (ORCID:0000000304254529). 2026-05-04. Strong long-wavelength electron-phonon coupling in Ta 2 Ni(Se,S) 5. https://doi.org/10.1103/3wqb-7qq3
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