Observation of an isolated flat band in the van der Waals crystal NbOCl$_2$

TL;DR

This paper reports the direct observation of a fully isolated, dispersionless flat electronic band in the van der Waals crystal NbOCl$_2$, offering a new platform for studying flat-band physics and correlated electronic phenomena.

Contribution

The study provides the first direct mapping of an entire flat band in NbOCl$_2$ using photoelectron momentum microscopy, highlighting its isolation and tunability.

Findings

Identified a flat band with ~100 meV width across the Brillouin zone.

Demonstrated high tunability of the quasiparticle band gap with surface caesium deposition.

Correlated the flat band with optical transmission measurements to determine the optical gap.

Abstract

Dispersionless electronic bands lead to an extremely high density of states and suppressed kinetic energy, thereby increasing electronic correlations and instabilities that can shape emergent ordered states, such as excitonic, ferromagnetic, and superconducting phases. A flat band that extends over the entire momentum space and is well isolated from other dispersive bands is, therefore, particularly interesting. Here, the band structure of the van der Waals crystal NbOCl$_2$ is revealed by utilizing photoelectron momentum microscopy. We directly map out an electronic band that is flat throughout the entire Brillouin zone and features a width of only $\sim$100 meV. This band is well isolated from both the conduction and remote valence bands. Moreover, the quasiparticle band gap shows a high tunability upon the deposition of caesium atoms on the surface. By combining the single-particle band structure with the optical transmission spectrum, the optical gap is identified. The fully isolated flat band in a van der Waals crystal provides a qualitatively new testbed for exploring flat-band physics.