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, T_c, of ultrathin Ta₂NiSe₅ is reduced from its bulk value by ΔT_c/T_c^bulk ≈ -9%, which strongly contrasts the case of 1T-TiSe₂, another excitonic insulator candidate, showing an increase of T_c by ΔT_c/T_c^bulk ≈ +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 structure of Ta₂NiSe₅. The out-of-plane correlating length of the EI phase is estimated to have monolayer thickness, suggesting that the EI phase in Ta₂NiSe₅ is highly layer-confined and in the strong coupling limit.