Lattice dynamics of the excitonic insulator <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Ta</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi>Ni</mml:mi><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:msub><mml:mi>Se</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">S</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:mo>)</mml:mo></mml:mrow><mml:mn>5</mml:mn></mml:msub></mml:mrow></mml:math>

Abstract

What role does the crystalline lattice play in the emergence of electronic order? Here the authors explore the lattice dynamics of the excitonic insulator candidate family Ta${}_{2}$Ni(Se${}_{1\ensuremath{-}x}$S${}_{x}$)${}_{5}$ by means of Raman scattering. Strong interactions between optical phonons and a continuum of soft quadrupolar excitons lead to broad asymmetric line shapes and anomalous temperature dependence of the phonon signatures. They further develop a model allowing disentanglement of the intrinsic responses of excitons and phonons, and assess their respective contributions to the excitonic insulator phase transition.

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