Two-Dimensional Flat Bands in Moiré Diamonds.

The discovery of flat bands in magic-angle twisted bilayer graphene established moiré engineering as a powerful route to correlated electronic states, but such flat bands typically require extremely small twist angles and remain fragile due to weak van der Waals coupling. Here, we demonstrate an alternative mechanism for flat-band formation based on interlayer sp3 hybridization in twisted graphite, which stabilizes large-angle moiré superstructures through covalent bonding. Using twisted graphite as a prototype, we identify a family of three-dimensional diamond-like carbon phases with moiré features (moiré diamonds) hosting two-dimensional flat bands: the electronic bands are nearly dispersionless in the in-plane directions while remaining dispersive along kz. These flat bands emerge at relatively large twist angles with short moiré periods and show strong robustness against thermal fluctuations and structural perturbations. Our results establish covalently bonded moiré diamonds as a new platform for flat-band physics beyond the van der Waals regime.