LAUSR

There is growing interest in the quasi-one-dimensional chalcogenide Ta${}_{2}$NiSe${}_{5}$ due to proposals that its ground state is an excitonic insulator. Here, the authors report a comparative spectroscopic study of Ta${}_{2}$NiSe${}_{5}$ and Ta${}_{2}$NiS${}_{5}$ single crystals and show that the excited states of Ta${}_{2}$NiSe${}_{5}$ can be described in terms of strongly overlapping exciton complexes along the Ta-Ni chains -- conditions highly conducive for the formation of an excitonic condensate. Their results offer a wealth of information on lattice and quasiparticle dynamics, as well as on excitons and their many-body interactions, that will support detailed assessments of the excitonic insulator hypothesis.