Failed excitonic quantum phase transition in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>Ta</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:mi>Ni</mml:mi><mml:mo>(</mml:mo><mml:msub><mml:mrow><mml:mi>Se</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">S</mml:mi></mml:mrow><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mn>5</mml:mn></mml:msub></mml:mrow></mml:math>

Abstract

We study the electronic phase diagram of the excitonic insulator candidates ${\mathrm{Ta}}_{2}\mathrm{Ni}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{5}$ $(x=0,\ensuremath{\cdots},1)$ using polarization resolved Raman spectroscopy. Critical excitonic fluctuations are observed that diminish with $x$ and ultimately shift to high energies, characteristic of a quantum phase transition. Nonetheless, a symmetry-breaking transition at finite temperatures is detected for all $x$, exposing a cooperating lattice instability that takes over for large $x$. Our study reveals a failed excitonic quantum phase transition, masked by a preemptive structural order.

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