Twisted bilayer hexagonal boron nitride: From stacking and lattice reconstruction to emergent quantum phenomena.

Twisted bilayer hexagonal boron nitride (h-BN) has become an important moiré system because its polar lattice and stacking-dependent interactions connect electronic, magnetic, ferroelectric, and optical phenomena. Unlike twisted bilayer graphene, its narrow band-edge states are not tied to a single magic angle. Instead, they emerge over a broader angular range as the interlayer potential, hybridization, and lattice reconstruction confine electrons and holes in distinct regions of the moiré pattern. These states have motivated proposals for spin-density-wave order, unconventional superconductivity, and itinerant magnetism, although the intrinsic minibands have not yet been observed directly, and controlled filling remains challenging. At small angles, reconstruction produces domains with opposite out-of-plane polarization and in-plane components near their boundaries. These polarization patterns make twisted h-BN a clear example of stacking-induced ferroelectricity and a platform for meron-like polar textures. The same reconstructed landscape also reshapes the excitonic response. Recent experiments on three-dimensional h-BN moiré quantum wells have demonstrated efficient deep-ultraviolet confinement and emission, while in atomically thin twisted bilayers the relative roles of moiré trapping, self-trapping, defects, and strain remain to be established. Beyond its intrinsic properties, the polar interface can impose a tunable periodic potential on nearby materials and act as an active moiré substrate. This topical review examines fabrication, structural classification, lattice reconstruction, electronic structure, correlated and magnetic phases, ferroelectricity, polar textures, and excitonic response, with emphasis on the current balance between theory and experiment.