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.