Ta₂NiSe₅ Exhibits Microscopic Evidence of Excitonic Instability, Supporting Its Role as an Insulator

The search for materials exhibiting excitonic insulation, where electron-hole pairs condense into a charge-neutral state, promises revolutionary advances in optoelectronics and condensed matter physics. Seokjin Bae, Arjun Raghavan, and colleagues at the University of Illinois Urbana-Champaign, alongside Irena Feldman and Amit Kanigel from the Technion, Israel Institute of Technology, and Vidya Madhavan, now present compelling microscopic evidence for this phenomenon in the material Ta2NiSe5. Their research overcomes longstanding debate surrounding the material’s insulating ground state by demonstrating that the insulating gap persists even where structural distortions are absent, indicating the gap’s origin lies not in the material’s structure, but in the correlated behaviour of electrons and holes. By employing scanning tunneling microscopy and spectroscopy, the team reveals that localized charge puddles suppress the insulating gap and that the decay length of in-gap states closely matches the expected size of an exciton, strongly suggesting Ta2NiSe5 functions as an excitonic insulator and offers a promising avenue for exploring bosonic condensates in solid-state systems under ambient conditions.

Charge-neutral bosonic condensate of spontaneously formed electron-hole pairs represents an exotic quantum phenomenon, offering the potential for dissipationless charge neutral transport and significant advances in optoelectronic applications. This has driven an extensive search for intrinsic bulk materials that exhibit exciton insulation at ambient pressure, without the need for sophisticated device fabrication. Tantalum nickel diselenide has been proposed as a rare example of such a material, but its fundamental state has remained a subject of debate, as the insulating phase transition can be accompanied by structural changes. Researchers now employ scanning tunneling microscopy and spectroscopy to present microscopic evidence supporting an excitonic origin for the insulating phase observed in this material.

Ta2NiSe5 Insulating State, Defect Origins Investigated

This research investigates the origin of the insulating state in tantalum nickel diselenide. The team argues that local variations in carrier density, caused by defects or impurities, can suppress the insulating state locally, creating metallic regions within the material. They used scanning tunneling microscopy and spectroscopy to map the electronic structure at the nanoscale and demonstrate this suppression. They also estimated the size of the exciton wavefunction based on the spatial extent of these metallic regions. Tantalum nickel diselenide possesses a chain-like crystal structure, which influences its electronic properties.

It undergoes a structural phase transition at around 326K. The researchers used scanning tunneling microscopy to image the surface topography at the atomic scale and scanning tunneling spectroscopy to measure the local density of states, essentially the electronic structure, at different points on the surface. Experiments were performed at low temperatures to enhance the visibility of the electronic structure. The researchers observed significant variations in the local density of states across the surface of tantalum nickel diselenide. Some regions exhibited a clear insulating gap, while others showed metallic behaviour.

These metallic regions correlated with specific features in the topography, including charge puddles, regions of accumulated or depleted charge caused by sub-surface defects, and surface vacancies. The insulating gap significantly reduced or completely suppressed in the regions with charge puddles, indicating a transition to a metallic state. By fitting the spatial variation of the local density of states around the charge puddles, the researchers estimated the decay length of the metallic states, finding values of approximately 0. 49nm along the chain direction and 1. 57nm along the perpendicular direction. They interpret these decay lengths as being related to the size of the exciton wavefunction, suggesting that it is squeezed in the chain direction and more extended perpendicular to it.

Excitonic Insulation Survives Structural Disorder

Researchers have presented compelling microscopic evidence supporting the excitonic nature of the insulating state in tantalum nickel diselenide. The study demonstrates that an insulating gap of approximately 160 meV persists even at structural domain boundaries where the material’s characteristic shear distortion is absent, indicating that this distortion is not the primary cause of the insulating ground state. This finding strongly suggests that electron-hole interactions, rather than structural changes, drive the insulating behaviour. The team investigated the influence of localized charge, observing that regions with increased carrier density, termed “charge puddles”, significantly suppress the insulating gap.

Detailed scanning tunneling spectroscopy revealed that the decay length of in-gap metallic states at these charge puddles closely matches the estimated size of the exciton wavefunction, previously determined through photoemission studies. This consistency reinforces the idea that the insulating state arises from the condensation of electron-hole pairs, forming excitons. Experiments focused on subsurface defects which create localized charge puddles, revealing semi-metallic density of states within these regions, contrasting with the gapped insulating spectra of the surrounding material. Analysis of spatial decay lengths of in-gap metallic states at these defects provides further support for the excitonic origin of the insulating state, demonstrating a correlation between the localized charge and the suppression of the excitonic gap. These findings establish tantalum nickel diselenide as a promising candidate for an intrinsic excitonic insulator operating at ambient conditions, offering a versatile platform for exploring bosonic condensates in solid-state systems.

Excitonic Insulation Confirmed, Wavefunction Size Measured

This research presents compelling microscopic evidence supporting the identification of tantalum nickel diselenide as an excitonic insulator. Scientists utilized scanning tunneling microscopy and spectroscopy to investigate the material’s insulating state, revealing that the insulating gap persists even at boundaries where structural distortions are absent. This finding strongly suggests that the insulating behaviour does not originate primarily from the material’s structural changes, resolving a long-standing debate in the field. Further analysis demonstrated that increasing local carrier density suppresses the insulating state, and the extent of this suppression correlates with the estimated size of the exciton wavefunction, a bound state of an electron and a hole.

These observations collectively indicate that the insulating state arises from excitonic instability, where electron-hole pairs condense into a coherent state. The team successfully decoupled the structural transition from the electronic ground state, providing crucial insight into the material’s fundamental properties. This study establishes tantalum nickel diselenide as a promising platform for exploring bosonic condensates in solid-state systems at ambient conditions and contributes significantly to the broader understanding of excitonic insulators.