Microscopic evidence of dominant excitonic instability in Ta2NiSe5

TL;DR

This study provides microscopic evidence that Ta2NiSe5 is an excitonic insulator, with insulating behavior driven by electron-hole pair correlations rather than structural distortions, making it a promising platform for studying bosonic condensates.

Contribution

The paper presents direct microscopic evidence supporting the excitonic origin of the insulating phase in Ta2NiSe5, distinguishing it from structural effects and highlighting charge correlations.

Findings

Insulating gap persists at structural domain boundaries.

Gap suppressed at localized charge puddles.

In-gap state decay length matches exciton size estimates.

Abstract

An excitonic insulator (EI) is a charge-neutral bosonic condensate of spontaneously formed electron-hole pairs. Exotic quantum phenomena such as dissipationless charge neutral transport and huge potential for optoelectronic applications have led to an extensive search for intrinsic bulk materials that are EIs at ambient pressure without sophisticated device fabrication. The narrow gap semimetal Ta2NiSe5 has been proposed as a rare example of an intrinsic EI, but its ground state has remained controversial since the EI phase transition is accompanied by a structural distortion. Here, we use scanning tunneling microscopy and spectroscopy to present microscopic evidence that supports an excitonic origin of the insulating phase in Ta2NiSe5. First, we find that the insulating gap persists at structural domain boundaries where the bulk structural distortion is absent suggesting that the structural distortion is not the primary origin of insulating ground state. Second, the insulating gap is suppressed at localized charge puddles which indicates that charge correlations are important in producing the insulating gap. Finally, the decay length of the in-gap states at the puddles shows similar value to the estimated size of the exciton wavefunction from previous photoemission studies. Taken together, these findings suggest that Ta2NiSe5 is an EI and may be a versatile platform to study the physics of bosonic condensates in solid-state system at ambient conditions.