  Two-dimensional ferroelectrics can maintain vertical polarization up to room
temperature, and are, therefore, promising for next-generation nonvolatile
memories. Although natural two-dimensional ferroelectrics are few, moir\'{e}
superlattices provide us with a generalized method to construct ferroelectrics
from non-ferroelectric parent materials. We report a realization of
ferroelectric hysteresis in a AB-BA stacked twisted double bilayer graphene
(TDBG) system. The ferroelectric polarization is prominent at zero external
displacement field and reduces upon increasing displacement fields. TDBG in the
AB-BA configuration possesses a superlattice of non-centrosymmetric domains,
exhibiting alternatively switchable polarities even without the assistance of
any boron nitride layers; however, in the AB-AB stacking case, the development
of polarized domains necessitates the presence of a second superlattice induced
by the adjacent boron nitride layer. Therefore, twisted multilayer graphene
systems offer us a fascinating field to explore two-dimensional
ferroelectricity.
