Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Lattice dynamics of the excitonic insulator
  Ta$_2$Ni(Se$_{1-x}$S$_x$)$_5$   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.

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The categories of this paper are: plasm-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: solv-int
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The categories of this paper are: math
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The categories of this paper are: q-fin
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The categories of this paper are: nucl-th
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The categories of this paper are: gr-qc
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The categories of this paper are: stat
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The categories of this paper are: eess
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The categories of this paper are: nlin
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The categories of this paper are: nucl-ex
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The categories of this paper are: comp-gas
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The categories of this paper are: dg-ga
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The categories of this paper are: acc-phys
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The categories of this paper are: atom-ph
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The categories of this paper are: cs