Electronic tuning of the soft-phonon transport anomaly in Ta${}_2$Ni(S${}_x$Se${}_{1-x}$)${}_5$

Abstract

Ta${}_2$NiSe${}_5$ continues to be investigated for its phase transition at $T_{\text{c}}$ = 326 K, where it develops both an electronic gap and a distortion of its Ta/Ni chains. One intriguing feature at $T_{\text{c}}$ seen in thermal transport is the giant anisotropic scattering of phonons moving perpendicular to the chains, which is apparently associated with the softening of a transverse acoustic phonon, but whose microscopic origin and significance demand clarification. By tuning the normal-state band overlap/gap with S substitution, we uncover a close connection between this soft-phonon transport anomaly and underlying electronic instabilities: When Ta${}_2$Ni(S${}_x$Se${}_{1-x}$)${}_5$ approaches a band insulator at high $x$, and signatures of the electronic transition are suppressed, the soft-phonon transport anomaly concomitantly vanishes. Our results establish the following picture for the Ta${}_2$Ni(S${}_x$Se${}_{1-x}$)${}_5$ family: Near the S end, a sole lattice instability gives rise to a weak structural transition with $T_{\text{c}}$ approaching 130 K. Near the Se end, additional electronic instabilities boost $T_{\text{c}}$ up to 326 K and amplify experimental signatures of the transition. The strong interaction between electrons, holes, and the lattice is manifested as a soft-phonon transport anomaly accompanied by electronic fluctuations, which include excitonic and hybridization-gap fluctuations.

7 pages, 4 figures