January 28, 2026 -- A team of researchers at Monash University has uncovered a powerful new way to engineer exotic quantum states, revealing a robust and tunable three‑dimensional flat electronic band in an ultrathin kagome metal, an achievement long thought to be nearly impossible.

The discovery centres on Mn₃Sn films just three nanometres thick. Despite their extreme thinness, these films host a 3D flat band that spans the entire momentum space, offering an unprecedented platform for exploring strongly correlated quantum phases and designing future low‑energy electronic technologies.

“Until now, 3D flat bands had only been observed in a few bulk materials with special lattice geometries,” said PhD candidate and co‑lead author James Blyth, from the Monash University School of Physics and Astronomy. “It wasn’t clear whether such a state could survive in an ultrathin film, so seeing it emerge in a structure only a few atoms thick was remarkable.”

Flat bands are electronic states where electrons lose almost all mobility, allowing interactions to dominate and giving rise to unconventional superconductivity, unusual magnetism and other exotic phases.

Achieving a flat band in all three dimensions, however, has been one of the field’s most persistent challenges. Even when electrons are confined to a two‑dimensional plane, they typically retain some ability to move between layers, destroying true 3D flatness.

The Monash team overcame this by combining precision thin‑film growth with the intrinsic properties of Mn₃Sn. Using molecular beam epitaxy, they produced high‑quality, single‑crystal films with the atomic‑level control needed to preserve delicate electronic states.

“The 3D flat band states in the film were revealed by photon‑energy‑dependent angle‑resolved photoemission spectroscopy, which allowed us to map the band dispersion along all three directions of momentum space,” said Dr Mengting Zhao, postdoctoral researcher and co‑lead author. She explains that scanning tunnelling microscopy and spectroscopy further confirmed electron localisation at the corner‑sharing triangles characteristic of the kagome lattice.

The team’s experiments, supported by advanced theoretical modelling, show that the 3D flat band arises from two effects working in tandem: the strong electron correlations inherent to Mn₃Sn’s magnetic kagome lattice, and the powerful quantum confinement introduced by the ultrathin geometry. This confinement not only restricts out‑of‑plane motion but also enhances flat‑band features within the plane, producing more pronounced flat bands than those found in bulk Mn₃Sn.

Together, these effects lock electrons in place in all three dimensions, stabilising a genuine 3D flat band in a film just three nanometres thick.

This breakthrough demonstrates a practical and scalable route to creating 3D flat bands without relying on twisting, stacking or complex artificial structures. By uniting intrinsic magnetic correlations with precision thin‑film engineering, the researchers have opened a new pathway for designing correlated and topological quantum states in low‑dimensional materials.

The study, 3D Flat Band in Ultra‑Thin Kagome Metal Mn₃Sn Film, by M. Zhao, J. Blyth, T. Yu and collaborators, appears in Advanced Materials.