Title: Stable Charged Antiparallel Domain Walls in Hyperferroelectrics,
Abstract: Charge-neutral 180$^\circ$ domain walls that separate domains of antiparallel
polarization directions are common structural topological defects in
ferroelectrics. In normal ferroelectrics, charged 180$^\circ$ domain walls
running perpendicular to the polarization directions are highly energetically
unfavorable because of the depolarization field and are difficult to stabilize.
We explore both neutral and charged 180$^\circ$ domain walls in
hyperferroelectrics, a class of proper ferroelectrics with persistent
polarization in the presence of a depolarization field, using density
functional theory. We obtain zero temperature equilibrium structures of
head-to-head and tail-to-tail walls in recently discovered $ABC$-type hexagonal
hyperferroelectrics. Charged domain walls can also be stabilized in canonical
ferroelectrics represented by LiNbO$_3$ without any dopants, defects or
mechanical clamping. First-principles electronic structure calculations show
that charged domain walls can reduce and even close the band gap of host
materials and support quasi-two-dimensional electron(hole) gas with enhanced
electrical conductivity.