Observation of Excitonic Instability in a Monolayer Ta₂NiSe₅ With Strain Disorder

Excitonic insulating phase, a long-sought exotic quantum state, is formed by spontaneous condensation of electron-hole pairs or excitons. Stabilizing such a fragile many-body state in reduced dimensions requires precise control of multiple electronic parameters and external perturbations, which makes its material realization challenging, particularly at high temperatures. Here, using van der Waals layers of ${\mathrm{Ta}}_2$ ${\mathrm{NiSe}}_5$ as a material platform, we show that the excitonic insulating phase is stable up to $T_c$ $\sim$ 190 K in the two-dimensional limit. The distinctive signatures of the excitonic insulating phase transition, such as the hybridization gap opening and critical fluctuations of the excitonic order observed in Raman spectroscopy, persist even in a monolayer, although systematically suppressed with thickness. The large gap ratio, nearly independent of thickness, suggests the importance of strong exciton-phonon coupling in maintaining a high-temperature excitonic instability, while substrate-induced strain disorder, as observed by scanning tunneling microscopy, lowers $T_c$ in monolayer ${\mathrm{Ta}}_2$ ${\mathrm{NiSe}}_5$ than in the bulk. Our findings establish the monolayer ${\mathrm{Ta}}_2$ ${\mathrm{NiSe}}_5$ as a promising model system for studying and manipulating correlated exciton-lattice coupling at the two dimensional limit.