Phononic soft mode and strong electronic background behavior across the structural phase transition in the excitonic insulator Ta${}_2$NiSe${}_5$ (with Erratum)

Abstract

Ta${}_2$NiSe${}_5$ 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.

arXiv commentMerged 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]

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• Imaging the coherent propagation of collective modes in the excitonic insulator candidate Ta${}_2$NiSe${}_5$ at room temperature
• Photoinduced multistage phase transitions in Ta2NiSe5
• Photo-induced phase transition and associated time scales in the excitonic insulator Ta${}_2$NiSe${}_5$
• Lattice dynamics of the excitonic insulator Ta${}_2$Ni(Se${}_{1-x}$S${}_x$)${}_5$
• Role of electron-phonon coupling in excitonic insulator candidate Ta2NiSe5
• A New Era of Excitonic Insulators
• Disentangling lattice and electronic instabilities in the excitonic insulator candidate Ta${}_2$NiSe${}_5$ by nonequilibrium spectroscopy
• Unveiling the underlying interactions in Ta2NiSe5 from photo-induced lifetime change
• Terahertz parametric amplification as a reporter of exciton condensate dynamics
• Evidence for an Excitonic Insulator State in Ta${}_2$Pd${}_3$Te${}_5$
• Tunneling-tip-induced collapse of the charge gap in the excitonic insulator Ta${}_2$NiSe${}_5$
• Evolution of the electronic structure in Ta${}_2$NiSe${}_5$ across the structural transition revealed by resonant inelastic x-ray scattering
• Pressure suppression of the excitonic insulator state in Ta2NiSe5 observed by optical conductivity
• Third-harmonic generation in excitonic insulators
• Theory of time-crystalline behaviour mediated by phonon squeezing in Ta2NiSe5
• Excitonic insulator phase and dynamics of condensate in a topological one-dimensional model
• Collective modes and Raman response in Ta${}_2$NiSe${}_5$
• Electronic and Optical Properties of Ta${}_2$NiSe${}_5$ Monolayer: A First-principles Study
• Detecting photoelectrons from spontaneously formed excitons
• Photo-induced Dynamics of Quasicrystalline Excitonic Insulator
• Elastic Soft Mode and Electric Quadrupole Response in Excitonic Insulator Candidate (Ta${}_{0.952}$V${}_{0.048}$)${}_2$NiSe${}_5$: Contribution of Electron-Phonon Interaction
• Gate tuning of coupled electronic and structural phase transition in atomically thin Ta${}_2$NiSe${}_5$
• Comment on "Direct observation of excitonic instability in Ta${}_2$NiSe${}_5$" [arXiv:2007.08212 (2020)]
• Long-lived relaxation channel and exciton-phonon coupling in Ta2NiSe5 via non-degenerate pump-probe spectroscopy
• Probing the collective excitations of excitonic insulators in an optical cavity