Thermal Transport Signatures Of The Excitonic Transition And Associated Phonon Softening In The Layered Chalcogenide Ta2nise5

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