title: 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, Tc, of ultrathin Ta2NiSe5 is reduced from its bulk value by ΔTc/Tc(bulk) ≈ -9%, which strongly contrasts the case of 1T-TiSe2, another excitonic insulator candidate, showing an increase of Tc by ΔTc/Tc(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.
authors.forename: So Young
authors.initials: SY
authors.lastname: Kim
authors.forename: Youngwook
authors.initials: Y
authors.lastname: Kim
authors.forename: Chang-Jong
authors.initials: CJ
authors.lastname: Kang
authors.forename: Eun-Su
authors.initials: ES
authors.lastname: An
authors.affiliation: Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS) , Pohang 37673, Korea.
authors.forename: Hyoung Kug
authors.initials: HK
authors.lastname: Kim
authors.forename: Man Jin
authors.initials: MJ
authors.lastname: Eom
authors.affiliation: Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS) , Pohang 37673, Korea.
authors.forename: Minkyung
authors.initials: M
authors.lastname: Lee
authors.affiliation: Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS) , Pohang 37673, Korea.
authors.forename: Chibeom
authors.initials: C
authors.lastname: Park
authors.affiliation: Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS) , Pohang 37673, Korea.
authors.forename: Tae-Hwan
authors.initials: TH
authors.lastname: Kim
authors.affiliation: Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS) , Pohang 37673, Korea.
authors.forename: Hee Cheul
authors.initials: HC
authors.lastname: Choi
authors.forename: Byung Il
authors.initials: BI
authors.lastname: Min
authors.forename: Jun Sung
authors.initials: JS
authors.lastname: Kim
keywords: direct band gap semiconductor; excitonic insulator; interband Coulomb interaction; ternary chalcogenides; ultrathin crystals; van der Waals materials