We report the complex dielectric function of the quasi-one-dimensional chalcogenide ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, 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 ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$, 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 ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ display an order-of-magnitude higher intensity than those in ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$. 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 ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$. The formation of exciton-phonon complexes in ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ and ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ 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.

• Received 7 September 2016
• Revised 16 February 2017

DOI:https://doi.org/10.1103/PhysRevB.95.195144