Ta 2 NiSe 5 : Best of three worlds?

Max Planck Institute for Solid State Research

Zhang et al. demonstrate that the debated 326-K phase transition in Ta₂NiSe₅ involves a collaboration of three microscopic factors

The excitonic insulator is a macroscopic condensate of electron-hole pairs, i.e., excitons, predicted to emerge in a narrow-gap semiconductor or semimetal. Despite apparent analogies to a superconductor, which is a macroscopic condensate of electron-electron pairs, the excitonic insulator has yet to be firmly established as the ground state of a bulk compound. The quasi-one-dimensional chalcogenide Ta₂NiSe₅ undergoes an insulator transition below T_c = 326 K and is currently a leading candidate for a bulk excitonic insulator. However, a simultaneous lattice distortion at T_c has led to a polarizing debate as to whether the transition is electronically or structurally driven.

Zhang et al. approach this question by uniquely applying high-temperature thermal transport, which is not only sensitive to charge carriers, but also neutral heat carriers, such as excitons or phonons. They uncover a giant and anisotropic soft-phonon transport anomaly around T_c. By tracking this anomaly as a function of sulfur substitution, the authors explain how a bare lattice instability of 130 K is boosted to T_c = 326 K by its coupling to electronic instabilities, which include both the excitonic insulator and a single-particle hybridization gap. Ta₂NiSe₅ therefore represents the best of these three instabilities.

This project was carried out in collaboration with the Crystal Growth group and partly supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – TRR 360 – 492547816, “Constrained Quantum Matter.”