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

Abstract

The quasi-one-dimensional layered compound Ta2NiSe5 has been proposed to undergo a transition to an excitonic insulator at Tc=326 K. We found a clear anomaly at Tc in the in-plane thermal conductivities both parallel (∥a) and perpendicular (∥c) to the one-dimensional chains, κa and κc. While κa shows a rapid decrease below Tc, κc shows a pronounced V-shaped suppression centered at Tc. We argue that the decrease of κa represents the suppression of the quasiparticle contribution below Tc due to the excitonic transition. On the other hand, the V-shaped suppression of κc comes from the enhanced phonon scattering by soft phonons associated with the monoclinic transition with momentum q∥c. The continued suppression of κc up to an extremely high temperature above Tc suggests the persistence of phonon softening likely coupled to electronic, presumably excitonic, fluctuations.