probed by Raman scattering*

Excitonic insulator (EI) is a phase driven by Coulomb attraction between electrons and holes leading to a proliferation of particle-hole pairs. EIs break the lattice symmetries, raising the question of whether a particular transition is excitonic or structural. Recently, the transition origin in a candidate material Ta₂NiSe₅ has become a subject of interest, with both excitonic [1] and lattice [2] mechanisms proposed.

I will report the results of the study of collective excitations in Ta₂NiSe₅ [3] with polarization-resolved Raman scattering that allows to selectively probe the quadrupolar ones with the symmetry of the order parameter. We observe an overdamped electronic mode, consistent with excitonic fluctuations in a semimetal, softening at the transition temperature, indicating a strong electronic contribution to ordering. At the same time, the optical phonons do not soften, leaving the interplay of the excitonic mode and acoustic strain as the transition origin. On cooling, we demonstrate the gradual emergence of coherent superpositions of band states at the gap edge, with strong departures from mean-field theory predictions. Extending our measurements to the Ta₂Ni(Se_1-xS_x)₅ family, we find a strong suppression of the electronic contribution to ordering with x. In Ta₂NiS₅, we observe a sharp in-gap exciton that does not soften, instead of an overdamped mode, consistent with a suppression of the EI instability by the band gap . At the same time, we detect broken symmetry to be present for all x, indicating that the transition becomes fully elastically driven for large S content.

[1] Y. Wakisaka et al.,Phys. Rev. Lett. 103,026402 (2009); Y.F. Lu et al.,Nat. Commun.8,14408 (2017).
[2] E. Baldini et al.,arXiv:2007.02909; A. Subedi, Phys. Rev. Materials 4,083601 (2020).
[3] P.A. Volkov et al.,arXiv:2007.07344