Polarisation topology at the nominally charged domain walls in uniaxial ferroelectrics

Ferroelectric domain walls provide a fertile environment for novel materials physics. If a polarisation discontinuity arises, it can drive a redistribution of electronic carriers and changes in band structure, which often result in emergent two-dimensional conductivity. If such a discontinuity is not tolerated, then its amelioration usually involves the formation of complex topological patterns, such as flux-closure domains, dipolar vortices, skyrmions, merons or Hopfions. The degrees of freedom required for the development of such patterns, in which dipolar rotation is a hall mark, are readily found in poly-axial systems. In uniaxial ferroelectrics, where only two opposite polar orientations are possible, it has been assumed that discontinuities are unavoidable when antiparallel components of polarisation meet. This perception has been borne out by the appearance of charged conducting domain walls in systems such as hexagonal manganites and lithium niobate. Here, experimental and theoretical investigations on lead germanate (Pb5Ge3O11) reveal that polar discontinuities can be obviated at head-to-head and tail-to-tail domain walls by mutual domain bifurcation along two different axes, creating a characteristic saddle-point domain wall morphology and associated novel dipolar topology, removing the need for screening charge accumulation and associated conductivity enhancement.