Direct observation of excitonic instability in Ta2NiSe5
Authors: B J Kim
Changil Kwon
Hoon Kim
Kwangrae Kim
Jonghwan Kim
Jun Sung Kim
Journal: Nature Communications (2021)
Discipline: Multidisciplinary
Keywords: Coulomb attraction; electrons; holes; narrow-gap semiconductor; semimetal
Affiliations: Department of Physics, Pohang University of Science and Technology, Pohang, South Korea., Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), Pohang, South Korea., Department of Physics, Pohang University of Science and Technology, Pohang, South Korea. .

Abstract:  Coulomb attraction between electrons and holes in a narrow-gap semiconductor or a semimetal is predicted to lead to an elusive phase of matter dubbed excitonic insulator. However, direct observation of such electronic instability remains extremely rare. Here, we report the observation of incipient divergence in the static excitonic susceptibility of the candidate material Ta2NiSe5 using Raman spectroscopy. Critical fluctuations of the excitonic order parameter give rise to quasi-elastic scattering of B2g symmetry, whose intensity grows inversely with temperature toward the Weiss temperature of TW ≈ 237 K, which is arrested by a structural phase transition driven by an acoustic phonon of the same symmetry at TC = 325 K. Concurrently, a B2g optical phonon becomes heavily damped to the extent that its trace is almost invisible around TC, which manifests a strong electron-phonon coupling that has obscured the identification of the low-temperature phase as an excitonic insulator for more than a decade. Our results unambiguously reveal the electronic origin of the phase transition. 

Retracted: No

PubPeer comments:
 Although the authors claimed to change the unphysical dataset, some analysis in the supplementary information still remains wrong. ![file] supplementary figure 2. ![file] supplementary figure 4. Above analyses were done on incorrect dataset, as authors also mentioned in their erratum. However neither the dataset nor the analysis is corrected in supplementary information. Although the analysis is not correct, authors are using the supplementary information multiple times in the main text to support their claims. For example,  The result is also in excellent agreement with that obtained using the standard coupled electron-phonon model (Supplementary Fig. 4) , which is not valid anymore due to the incorrect analysis. Moreover, the Fano-lineshape equation they presented in the paper is wrong. The denominator should be corrected. equation (4) supplementary information. ![file] , 
 One of the key observations in this paper is the transition accompanied by systematic evolution of the fano-shaped phonon at T between 326 K and 426 K. In order to signify this point authors carried out thorough multi-component fits. ![file] Fig. 3b of the main text The fit results evolves smoothly, and the parameters have small error bars, which usually means that the fits are very good, and naturally fosters their claim. However, looking at the actual fits, it is unclear how authors obtained the excellent fit results with extremely small error bars. Especially, between 336 and 426 K, the position and width of the peak barely match the results of the fit. ![file] Supplementary figure 2. ,