Phononic soft mode and strong electronic background behavior across the structural phase transition in the excitonic insulator Ta$_2$NiSe$_5$ (with Erratum)
Abstract
TaNiSe became one of the most investigated candidate materials for hosting an excitonic insulator ground state. Many studies describe the corresponding phase transition as a condensation of excitons breaking a continuous symmetry. This view got challenged recently pointing out the importance of the loss of two mirror symmetries at a structural phase transition that occurs together with the semiconductor-excitonic insulator transition. For such a scenario an unstable optical zone-center phonon at low energy is proposed to drive the transition. Here we report on the experimental observation of such a soft mode behavior using Raman spectroscopy. In addition we find a novel spectral feature, likely of electronic or joint electronic and phononic origin, that is clearly distinct from the lattice dynamics and that becomes dominant at Tc. This suggests a picture of joint structural and electronic order driving the phase transition.
Keywords
Cite
@article{arxiv.2007.01723,
title = {Phononic soft mode and strong electronic background behavior across the structural phase transition in the excitonic insulator Ta$_2$NiSe$_5$ (with Erratum)},
author = {Min-Jae Kim and Armin Schulz and Tomohiro Takayama and Masahiko Isobe and Hidenori Takagi and Stefan Kaiser},
journal= {arXiv preprint arXiv:2007.01723},
year = {2021}
}
Comments
Merged version with Erratum (1 page) in the original manuscript (21 pages, 8 figures). After publication of [Phys. Rev. Research 2, 042039 (2020), arXiv:2007.01723v2] [1] we got notified on an instrumental artifact in the low frequency part of the Raman spectrum of Ta$_2$NiSe$_5$. This erroneously led to a description of a soft phonon mode and a strong electronic background. Properly taking into account a low frequency electronic signal correctly explains the observed soft mode as Fano coupled phonon where a critical softening of an excitonic collective mode takes place as described by P.A. Volkov et al. in arXiv:2007.07344 [2] and M. Ye et al. in arXiv:2102.07912 [3]