3D Flat Band in Ultra-Thin Kagome Metal Mn3Sn Film.

Flat bands with narrow energy dispersion can give rise to strongly correlated electronic and topological phases, especially when located at the Fermi level. Whilst flat bands are experimentally realized in 2D twisted van der Waals heterostructures, they are highly sensitive to twist angle, necessitating complex fabrication techniques. Geometrically frustrated kagome lattices have emerged as an attractive alternative platform as they can natively host flat bands that are observed experimentally in quasi-2D bulk-crystal kagome metals. An outstanding experimental question is whether flat bands can be realized in ultra-thin metals, with opportunities for stronger electron-electron interactions through tuning of the surrounding dielectric environment. Here, angle-resolved photoelectron spectroscopy, scanning tunnelling microscopy, and band structure calculations are used to show that ultra-thin films of the kagome metal Mn3Sn host a robust dispersionless flat band with a bandwidth of 50 meV. Furthermore, chemical tuning of the flat band to near the Fermi level via manganese defect engineering is demonstrated. The realization of tunable kagome-derived flat bands in an ultra-thin kagome metal represents a promising platform to study strongly correlated and topological phenomena, with applications in quantum computing, spintronics and low-energy electronics.