Giant exciton Fano resonance in quasi-one-dimensional Ta₂NiSe₅

We report the complex dielectric function of the quasi-one-dimensional chalcogenide Ta₂NiSe₅, which undergoes a structural phase transition presumably associated with exciton condensation below T_c = 326 K [Y. Wakisaka et al., Phys. Rev. Lett. 103, 026402 (2009); Y. F. Lu et al., Nat. Commun. 8, 14408 (2017)], and of the isostructural Ta2NiSe5, which does not exhibit such a transition. Using spectroscopic ellipsometry, we have detected exciton doublets with pronounced Fano line shapes in both the compounds. The exciton Fano resonances in Ta₂NiSe₅ display an order-of-magnitude higher intensity than those in Ta₂NiSe₅. In conjunction with prior theoretical work [E. Rashba, Sov. Phys. Semicond. 8, 807 (1975)], we attribute this observation to the giant oscillator strength of spatially extended exciton-phonon bound states in Ta₂NiSe₅. The formation of exciton-phonon complexes in Ta₂NiSe₅ and Ta₂NiSe₅ is confirmed by the pronounced temperature dependence of sharp interband transitions in the optical spectra, the peak energies and widths of which scale with the thermal population of optical phonon modes. The description of the optically excited states in terms of strongly overlapping exciton complexes is in good agreement with the hypothesis of an exciton insulator ground state.