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

Abstract

The layered compound $\mathrm{T}{\mathrm{a}}_2\mathrm{NiS}{\mathrm{e}}_5$ 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.

References in corpus (4)

• Direct observation of excitonic instability in Ta2NiSe5
• Orthorhombic-to-monoclinic transition in Ta${}_2$NiSe${}_5$ due to a zone-center optical phonon instability
• Critical charge fluctuations and emergent coherence in a strongly correlated excitonic insulator
• Doping-controlled transition from excitonic insulator to semimetal in Ta${}_2$NiSe${}_5$

Cited by in corpus (2)

• Quantum coherent states of mass-imbalanced electron-hole system within optical microcavities
• Long-lived relaxation channel and exciton-phonon coupling in Ta2NiSe5 via non-degenerate pump-probe spectroscopy