Gate tuning of coupled electronic and structural phase transition in atomically thin Ta$_2$NiSe$_5$

TL;DR

This study investigates the phase transition in atomically thin Ta$_2$NiSe$_5$, revealing it is driven by coupled electronic and structural changes rather than excitonic effects, with phase stability tunable by doping.

Contribution

It provides systematic experimental evidence that challenges the excitonic insulator hypothesis in Ta$_2$NiSe$_5$, emphasizing the role of coupled electronic-structural transitions.

Findings

Phase transition persists down to monolayer.

Doping suppresses or enhances the transition depending on carrier type.

Quasi-elastic scattering does not diminish with doping, contradicting excitonic fluctuation expectations.

Abstract

Realizing an excitonic insulator phase from narrow-gap semiconductors remains challenging, as unambiguous experimental signatures are difficult to establish. Ta$_2$NiSe$_5$ has been widely regarded as a leading candidate, yet the nature of its phase transition and insulating state remains controversial. Here, we report a systematic Raman spectroscopy study of Ta$_2$NiSe$_5$ as a function of thickness and field-effect doping, complemented by electrical transport measurements. The phase transition persists down to the monolayer limit, with the critical temperature increasing as thickness decreases. In bilayer samples, both electron and hole doping suppress the insulating state, with electron doping lowering and hole doping raising the transition temperature. Importantly, the quasi-elastic scattering, previously attributed to excitonic fluctuations, evolves monotonically across the entire doping range, inconsistent with the expected suppression of excitonic correlations by Coulomb screening. These findings rule out a dominant excitonic mechanism and instead point to a coupled electronic and structural phase transition, whose stability is tunable by carrier doping. Our doping-based approach offers a general strategy for evaluating the role of excitonic effects in candidate excitonic insulators.