Layer-Confined Excitonic Insulating Phase in Ultrathin Ta2NiSe5 Crystals

Atomically thin nanosheets, as recently realized using van der Waals layered materials, offer a versatile platform for studying the stability and tunability of the correlated electron phases in the reduced dimension. Here, we investigate a thickness-dependent excitonic insulating (EI) phase on a layered ternary chalcogenide Ta2NiSe5. Using Raman spectroscopy, scanning tunneling spectroscopy, and in-plane transport measurements, we found no significant changes in crystalline and electronic structures as well as disorder strength in ultrathin Ta2NiSe5 crystals with a thickness down to five layers. The transition temperature, Tc, of ultrathin Ta2NiSe5 is reduced from its bulk value by ΔTc/Tcbulk ≈ −9%, which strongly contrasts the case of 1T-TiSe2, another excitonic insulator candidate, showing an increase of Tc by ΔTc/Tcbulk ≈ +30%. This difference is attributed to the dominance of interband Coulomb interaction over electron–phonon interaction and its zero-ordering wave vector due to the direct band gap str...