Layer-Confined Excitonic Insulating Phase in Ultrathin Ta2NiSe5 Crystals

Abstract

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 of ultrathin Ta2NiSe5 is reduced from its bulk value by Delta T-c/T-c(bulk) approximate to -9%, which strongly contrasts the case of 1T-TiSe2, another excitonic insulator candidate, showing an increase of T-c by Delta T-c/T-c(bulk) approximate to +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.