Lattice dynamics of the excitonic insulator Ta${}_2$Ni(Se${}_{1-x}$S${}_x$)${}_5$

Abstract

Recently, we employed electronic polarization-resolved Raman spectroscopy to reveal the strongly correlated excitonic insulator (EI) nature of Ta2NiSe5, Volkov et al. [arXiv:2007.07344], and also showed that for Ta${}_2$Ni(Se${}_{1-x}$S${}_x$)${}_5$ alloys the critical excitonic fluctuations diminish with sulfur concentration x exposing a cooperating lattice instability that takes over for large x, Volkov et al. [arXiv:2104.07032]. Here we focus on the lattice dynamics of this EI family. We identify all Raman-active optical phonons of fully symmetric and ac-quadrupole-like symmetries and study their evolution with temperature and sulfur concentration. We demonstrate the change of selection rules at temperatures below the orthorhombic-to-monoclinic transition at Tc(x) that is related to the EI phase. We find that Tc(x) decrease monotonically from 328 K for Ta2NiSe5 to 120 K for Ta2NiS5 and that the magnitude of lattice distortion also decreases with the sulfur concentration x. For x < 0.7, the two lowest-frequency B2g phonon modes show strongly asymmetric lineshapes at high temperatures due to Fano interference with the broad excitonic continuum present in a semimetallic state. Within the framework of extended Fano model, we develop a quantitative description of the interacting exciton-phonon excitation lineshape, enabling us to derive the intrinsic phonon parameters and determine the exciton-phonon interaction strength, that affects the transition temperature Tc(x). We also observe signatures of the acoustic mode scattered assisted by the structural domain walls formed below Tc. Based on our results, we additionally present a consistent interpretation of the origin of oscillations observed in time-resolved pump-probe experiments.

References in corpus (11)

• Advanced capabilities for materials modelling with Quantum ESPRESSO
• Evidence for an excitonic insulator phase in 1T-TiSe${}_2$
• Excitonic Bose-Einstein condensation in Ta2NiSe5 above room temperature
• Direct observation of excitonic instability in Ta2NiSe5
• Photo-induced enhancement of excitonic order
• Photoinduced multistage phase transitions in Ta2NiSe5
• Ultrafast melting and recovery of collective order in the excitonic insulator Ta${}_2$NiSe${}_5$
• Orthorhombic-to-monoclinic transition in Ta${}_2$NiSe${}_5$ due to a zone-center optical phonon instability
• Phononic soft mode and strong electronic background behavior across the structural phase transition in the excitonic insulator Ta${}_2$NiSe${}_5$ (with Erratum)
• Critical charge fluctuations and emergent coherence in a strongly correlated excitonic insulator
• Signatures of ultrafast reversal of excitonic order in Ta${}_2$NiSe${}_5$

Cited by in corpus (16)

• 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
• Terahertz parametric amplification as a reporter of exciton condensate dynamics
• Failed excitonic quantum phase transition in Ta${}_2$Ni(Se${}_{1-x}$S${}_x$)${}_5$
• Crystal-field excitations and vibronic modes in triangular-lattice spin-liquid candidate TbInO${}_3$
• Third-harmonic generation in excitonic insulators
• Strong electron-phonon coupling and enhanced phonon Grüneisen parameters in valence-fluctuating metal EuPd${}_2$Si${}_2$
• Collective modes and Raman response in Ta${}_2$NiSe${}_5$
• Hidden excitonic quantum states with broken time-reversal symmetry
• Anomalous phonon Grüneisen parameters in semiconductor Ta${}_2$NiS${}_5$
• Strong long-wavelength electron-phonon coupling in Ta${}_2$Ni(Se,S)${}_5$
• 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
• Unveiling Excitonic Insulator Signatures in Ta${}_2$NiSe${}_5$
• Dynamical signatures and control of time-reversal breaking in twisted nodal superconductors
• Long-lived relaxation channel and exciton-phonon coupling in Ta2NiSe5 via non-degenerate pump-probe spectroscopy