Failed excitonic quantum phase transition in Ta${}_2$Ni(Se${}_{1-x}$S${}_x$)${}_5$

Abstract

We study the electronic phase diagram of the excitonic insulator candidates Ta${}_2$Ni(Se${}_{1-x}$S${}_x$)${}_5$ [x=0, ... ,1] using 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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• Strong long-wavelength electron-phonon coupling in Ta${}_2$Ni(Se,S)${}_5$
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