Critical charge fluctuations and emergent coherence in a strongly correlated excitonic insulator

Abstract

Excitonic insulator is a coherent electronic phase that results from the formation of a macroscopic population of bound particle-hole pairs - excitons. With only a few candidate materials known, the collective excitonic behavior is challenging to observe, being obscured by crystalline lattice effects. Here we use polarization-resolved Raman spectroscopy to reveal the quadrupolar excitonic mode in the candidate zero-gap semiconductor Ta${}_2$NiSe${}_5$ disentangling it from the lattice phonons. The excitonic mode pronouncedly softens close to the phase transition, showing its electronic character, while its coupling to non-critical lattice modes is shown to enhance the transition temperature. On cooling, we observe the gradual emergence of coherent superpositions of band states at the correlated insulator gap edge, with strong departures from mean-field theory predictions. Our results demonstrate the realization of a strongly correlated excitonic state in an equilibrium bulk material.

References in corpus (11)

• Exciton-polariton condensates
• Inelastic Light Scattering From Correlated Electrons
• Pseudogap and charge density waves in two dimensions
• Excitonic Bose-Einstein condensation in Ta2NiSe5 above room temperature
• Nature of symmetry breaking at the excitonic insulator transition: Ta${}_2$NiSe${}_5$
• Band hybridisation at the semimetal-semiconductor transition of Ta${}_2$NiSe${}_5$ enabled by mirror-symmetry breaking
• Evolution of the gaps through the cuprate phase-diagram
• Orthorhombic-to-monoclinic transition in Ta${}_2$NiSe${}_5$ due to a zone-center optical phonon instability
• Doping-controlled transition from excitonic insulator to semimetal in Ta${}_2$NiSe${}_5$
• Possible Excitonic Insulating Phase in Quantum-Confined Sb Nanoflakes
• The spontaneous symmetry breaking in Ta${}_2$NiSe${}_5$ is structural in nature

Cited by in corpus (25)

• Imaging the coherent propagation of collective modes in the excitonic insulator candidate Ta${}_2$NiSe${}_5$ at room temperature
• 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
• Robust high-temperature topological excitonic insulator of transition-metal carbides (MXenes)
• A New Era of Excitonic Insulators
• 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
• Anomalous excitonic phase diagram in band-gap-tuned Ta2Ni(Se,S)5
• Terahertz parametric amplification as a reporter of exciton condensate dynamics
• Hidden orders in spin-orbit entangled correlated insulators
• Failed excitonic quantum phase transition in Ta${}_2$Ni(Se${}_{1-x}$S${}_x$)${}_5$
• Tunneling-tip-induced collapse of the charge gap in the excitonic insulator Ta${}_2$NiSe${}_5$
• 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
• Third-harmonic generation in excitonic insulators
• Theory of time-crystalline behaviour mediated by phonon squeezing in Ta2NiSe5
• Excitonic insulators and Gross-Neveu models
• Collective modes and Raman response in Ta${}_2$NiSe${}_5$
• Photo-induced Dynamics of Quasicrystalline Excitonic Insulator
• Anomalous phonon Grüneisen parameters in semiconductor Ta${}_2$NiS${}_5$
• Suppression of the charge fluctuations by nonlocal correlations close to the Mott transition
• 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
• Quantum coherent states of mass-imbalanced electron-hole system within optical microcavities
• Dynamical signatures and control of time-reversal breaking in twisted nodal superconductors