Almost zero-gap gap semiconductor/semimetal to insulator transition in layered chalcogenide Ta₂NiSe₅

The excitonic insulator is a long conjectured correlated electron phase of narrow gap semiconductors and semimetals, driven by weakly screened electron-hole interactions. Having been proposed more than 50 years ago, conclusive experimental evidence for its existence remains elusive. A new generation of candidate excitonic insulator, the layered chalcogenide Ta₂NiSe₅, was very recently proposed [1,2]. Band structure calculations show that the valence band comprises a hybridized Ni 3d/Se 4p state, while the conduction band contains the Ta 5d state. Since both the top of valence band and the bottom of conduction band are about touching at the Γ point, Ta₂NiSe₅ is a direct and almost zero-gap semiconductor/semimetal, which gives rise to an ideal playground for excitonic physics. An almost zero-gap semiconductor-to-insulator transition occurs at T_C ∼ 328 K, accompanied with a second-order structural transition without any superlattice formation. The optical gap formed below T_C is as large as ~0.2 eV, close to the exciton binding energy of 0.2-0.3 eV observed in the sister compound Ta₂NiS₅, with one electron energy gap of ~ a half eV[3]_.. The 0.2 eV excitation gap is observed also in the local DOS dI/dV measured by STM-STS at 4.2 K. We find a drastic collapse of the 0.2 eV gap with approaching the STM-tip to the sample surface, which we argue to indicate the many body nature of the gap. Those results provide strong support for the formation of excitonic insulating phase in Ta₂NiSe₅ below T_c.
[1] Wakisaka, Y. et al. , Phys. Rev. Lett. 103, 026402 (2009).
[2] Lu, Y. F. et al., Nat. Commun. 8 14408 (2017).
[3] Larkin, T. I. et al., Phys. Rev. B 95, 195144 (2017).