Thermal transport signatures of the excitonic transition and associated phonon softening in the layered chalcogenide Ta2NiSe5

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 ($\ensuremath{\parallel}\phantom{\rule{4pt}{0ex}}a$) and perpendicular ($\ensuremath{\parallel}\phantom{\rule{4pt}{0ex}}c$) to the one-dimensional chains, ${\ensuremath{\kappa}}_{\mathrm{a}}$ and ${\ensuremath{\kappa}}_{\mathrm{c}}$. While ${\ensuremath{\kappa}}_{\mathrm{a}}$ shows a rapid decrease below ${T}_{\mathrm{c}}$, ${\ensuremath{\kappa}}_{\mathrm{c}}$ shows a pronounced V-shaped suppression centered at ${T}_{\mathrm{c}}$. We argue that the decrease of ${\ensuremath{\kappa}}_{\mathrm{a}}$ 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 ${\ensuremath{\kappa}}_{\mathrm{c}}$ comes from the enhanced phonon scattering by soft phonons associated with the monoclinic transition with momentum $\mathbf{q}\ensuremath{\parallel}c$. The continued suppression of ${\ensuremath{\kappa}}_{\mathrm{c}}$ up to an extremely high temperature above ${T}_{\mathrm{c}}$ suggests the persistence of phonon softening likely coupled to electronic, presumably excitonic, fluctuations.