Thermal transport signatures of the excitonic transition and associated phonon softening in the layered chalcogenide Ta$_2$NiSe$_5$

TL;DR

This study investigates thermal conductivity anomalies at the excitonic transition in Ta$_2$NiSe$_5$, revealing phonon softening and anisotropic thermal transport changes linked to excitonic condensation and structural transition.

Contribution

It provides the first detailed thermal transport measurements across the excitonic transition in Ta$_2$NiSe$_5$, highlighting phonon softening and anisotropic effects associated with the transition.

Findings

Anomaly in in-plane thermal conductivities at $T_c$

Suppression of $ppa_a$ due to excitonic condensation

V-shaped suppression of $ppa_c$ from phonon softening

Abstract

The layered compound $\mathrm{Ta_2NiSe_5}$ is a quasi-one-dimensional and narrow-gap semiconductor, which is proposed to undergo a transition to an excitonic insulator at $T_\mathrm{c}=326$ K. We found a clear anomaly at $T_\mathrm{c}$ in the in-plane thermal conductivities both parallel ($\parallel$ $a$) and perpendicular ($\parallel$ $c$) to the one-dimensional chains, $\kappa_\mathrm{a}$ and $\kappa_\mathrm{c}$. While $\kappa_\mathrm{a}$ shows a rapid decrease below $T_\mathrm{c}$, $\kappa_\mathrm{c}$ shows a pronounced V-shaped suppression centered at $T_\mathrm{c}$. We argue that the decrease of $\kappa_\mathrm{a}$ represents the suppression of the quasiparticle contribution below $T_\mathrm{c}$ due to excitonic condensation. On the other hand, the V-shaped suppression of $\kappa_\mathrm{c}$ comes from the enhanced phonon scattering by soft phonons associated with the monoclinic transition with momentum $\mathbf{q}\parallel c$. The continued suppression of $\kappa_\mathrm{c}$ up to an extremely high temperature above $T_\mathrm{c}$ suggests the persistence of phonon softening likely coupled to electronic, presumably excitonic, fluctuations.