Orthorhombic-to-monoclinic transition in Ta${}_2$NiSe${}_5$ due to a zone-center optical phonon instability

Abstract

I study dynamical instabilities in Ta${}_2$NiSe${}_5$ using density functional theory based calculations. The calculated phonon dispersions show two unstable optical branches. All the acoustic branches are stable, which shows that an elastic instability is not the primary cause of the experimentally observed orthorhombic-to-monoclinic structural transition in this material. The largest instability of the optical branches occurs at the zone center, consistent with the experimental observation that the size of the unit cell does not multiply across the phase transition. The unstable modes have the irreps $B_{1g}$ and $B_{2g}$. Full structural relaxations minimizing both the forces and stresses find that the monoclinic $C2/c$ structure corresponding to the $B_{2g}$ instability has the lowest energy. Electronic structure calculations show that this low-symmetry structure has a sizable band gap. This suggest that a $B_{2g}$ zone-center optical phonon instability is the primary cause of the phase transition. An observation of a softening of a $B_{2g}$ zone-center phonon mode as the transition is approached from above would confirm the mechanism proposed here. If none of the $B_{2g}$ modes present in the material soften, this would imply that the transition is caused by electronic or elastic instability.

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Cited by in corpus (25)

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• Photo-induced phase transition and associated time scales in the excitonic insulator Ta${}_2$NiSe${}_5$
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• Hybridization-gap Formation and Superconductivity in the Pressure-induced Semimetallic Phase of the Excitonic Insulator Ta${}_2$NiSe${}_5$
• Nonlinear spectroscopy of collective modes in excitonic insulator
• Unveiling the underlying interactions in Ta2NiSe5 from photo-induced lifetime change
• Disentangling lattice and electronic instabilities in the excitonic insulator candidate Ta${}_2$NiSe${}_5$ by nonequilibrium spectroscopy
• 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${}_5$
• Failed excitonic quantum phase transition in Ta${}_2$Ni(Se${}_{1-x}$S${}_x$)${}_5$
• Thermal transport signatures of the excitonic transition and associated phonon softening in the layered chalcogenide Ta${}_2$NiSe${}_5$
• Pressure suppression of the excitonic insulator state in Ta2NiSe5 observed by optical conductivity
• Theory of time-crystalline behaviour mediated by phonon squeezing in Ta2NiSe5
• Collective modes and Raman response in Ta${}_2$NiSe${}_5$
• Electronic and Optical Properties of Ta${}_2$NiSe${}_5$ Monolayer: A First-principles Study
• Photo-induced Dynamics of Quasicrystalline Excitonic Insulator
• Detecting photoelectrons from spontaneously formed excitons
• Gate tuning of coupled electronic and structural phase transition in atomically thin Ta${}_2$NiSe${}_5$
• 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
• 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