Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session M50: Is Ta2NiSe5 an Excitonic Insulator?
11:30 AM–2:30 PM,
Wednesday, March 17, 2021
Sponsoring Unit: DCMP
Chair: Girsh Blumberg, Rutgers University, New Brunswick
Abstract: M50.00004 : Electronic phase diagram of the excitonic insulator candidates Ta2Ni(Se1-xSx)5 probed by Raman scattering*
1:18 PM–1:54 PM
I will report the results of the study of collective excitations in Ta2NiSe5  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(Se1-xSx)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.
 Y. Wakisaka et al.,Phys. Rev. Lett. 103,026402 (2009); Y.F. Lu et al.,Nat. Commun.8,14408 (2017).
 E. Baldini et al.,arXiv:2007.02909; A. Subedi, Phys. Rev. Materials 4,083601 (2020).
 P.A. Volkov et al.,arXiv:2007.07344
*Work was performed in collaboration with M. Ye, H. Lohani, I. Feldman, A. Kanigel, M. Kim, K. Haule and G. Blumberg and was supported by NSF Grant No. DMR-1709161
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