Comment on "Direct observation of excitonic instability in Ta$_2$NiSe$_5$" [arXiv:2007.08212 (2020)]

TL;DR

This paper critically examines recent Raman scattering studies on Ta$_2$NiSe$_5$, revealing inconsistencies in data and parameters used to interpret its excitonic phase transition, challenging prior conclusions about its excitonic insulator nature.

Contribution

It provides a critical analysis of recent experimental data, highlighting inconsistencies that question previous interpretations of the excitonic transition in Ta$_2$NiSe$_5$.

Findings

Identifies inconsistencies in recent Raman scattering data.

Challenges the validity of parameters used for excitonic transition.

Questions the interpretation of Ta$_2$NiSe$_5$ as an excitonic insulator.

Abstract

The origin of the second order phase transition at 328K in Ta$_2$NiSe$_5$, a prominent candidate for direct gap excitonic insulator, remains under fervent debate. The driving force for the transition can be revealed by identification of the soft mode's origin that may be deducted from polarization resolved Raman scattering experiments. Such studies were recently reported in [arXiv:2007.07344 (2020)], [arXiv:2102.07912 (2021)], [arXiv:2007.01723 (2020)] and [arXiv:2007.08212 (2020)]. In this Comment, it is shown that the parameters derived in a recent arXiv by Kwangrae Kim et. al. [arXiv:2007.08212 (2020)], including the Weiss temperature for excitonic transition, are based on inconsistent data.