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

The quasi-one-dimensional layered compound ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ has been 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 the excitonic transition. 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.

Figure 1

(a) Crystal structure of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ showing the stack of layers along the $b$ axis (left) and a single layer in the $a\text{−}c$ plane consisting of an array of two chains of Ta octahedrally coordinated with Se and one chain of Ni in between, tetrahedrally coordinated with Se (right) [33]. (b) Thermal conductivities ${\kappa }_{\mathrm{a}}$ with heat current parallel to the chains ($\parallel a$; red) and ${\kappa }_{\mathrm{c}}$ with heat current perpendicular to the chains ($\parallel c$; blue) as a function of temperature. The dashed black line indicates $T_{\mathrm{c}}=326$ K. Inset: Enlarged view of ${\kappa }_{\mathrm{a}}$ and ${\kappa }_{\mathrm{c}}$ near $T_{\mathrm{c}}$.

Figure 2

Separation of electron and phonon contributions to the thermal conductivities ${\kappa }_{\mathrm{a}}$ with heat current parallel to the chains ($\parallel a$; red) and ${\kappa }_{\mathrm{c}}$ with heat current perpendicular to the chains ($\parallel c$; blue) in Fig. 1, using the W-F law. (a) The temperature-dependent electrical conductivities, ${\sigma }_{\mathrm{a}}\left(T\right)$ (red) parallel to the chains and ${\sigma }_{\mathrm{c}}\left(T\right)$ (blue) perpendicular to the chains. (b) The total, electron, and phonon thermal conductivities along the $a$ axis, parallel to the chains, are denoted by ${\kappa }_{\mathrm{a}}$ (circles), ${\kappa }_{\mathrm{a}}^{\left(\mathrm{e}\right)}$ (broken), and ${\kappa }_{\mathrm{a}}^{\left(\mathrm{ph}\right)}$ (solid), respectively. (c) The total, electron, and phonon thermal conductivities along the $c$ axis, perpendicular to the chains, are denoted ${\kappa }_{\mathrm{c}}$ (circles), ${\kappa }_{\mathrm{c}}^{\left(\mathrm{e}\right)}$ (broken), and ${\kappa }_{\mathrm{c}}^{\left(\mathrm{ph}\right)}$ (solid), respectively.

Figure 3

Comparison of temperature-dependent thermal conductivities $\kappa \left(T\right)$ for ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ ($\parallel a$ red; $\parallel c$ blue) with typical order- disorder-type ferroelectrics, ${\mathrm{NH}}_4\mathrm{Cl}$ (reproduced from [40]), and ${\mathrm{KH}}_2{\mathrm{PO}}_4$ (reproduced from [41]). The solid black line is a guide line for the $1/T$ power law.