Enabling ultra-low-voltage switching in BaTiO3.

Single crystals of BaTiO3 exhibit small switching fields and energies, but thin-film performance is considerably worse, thus precluding their use in next-generation devices. Here, we demonstrate high-quality BaTiO3 thin films with nearly bulk-like properties. Thickness scaling provides access to the coercive voltages (<100 mV) and fields (<10 kV cm-1) required for future applications and results in a switching energy of <2 J cm-3 (corresponding to <2 aJ per bit in a 10 × 10 × 10 nm3 device). While reduction in film thickness reduces coercive voltage, it does so at the expense of remanent polarization. Depolarization fields impact polar state stability in thicker films but fortunately suppress the coercive field, thus driving a deviation from Janovec-Kay-Dunn scaling and enabling a constant coercive field for films <150 nm in thickness. Switching studies reveal fast speeds (switching times of ~2 ns for 25-nm-thick films with 5-µm-diameter capacitors) and a pathway to subnanosecond switching. Finally, integration of BaTiO3 thin films onto silicon substrates is shown. We also discuss what remains to be demonstrated to enable the use of these materials for next-generation devices.

Local negative permittivity and topological phase transition in polar skyrmions.

Topological solitons such as magnetic skyrmions have drawn attention as stable quasi-particle-like objects. The recent discovery of polar vortices and skyrmions in ferroelectric oxide superlattices has opened up new vistas to explore topology, emergent phenomena and approaches for manipulating such features with electric fields. Using macroscopic dielectric measurements, coupled with direct scanning convergent beam electron diffraction imaging on the atomic scale, theoretical phase-field simulations and second-principles calculations, we demonstrate that polar skyrmions in (PbTiO3)n/(SrTiO3)n superlattices are distinguished by a sheath of negative permittivity at the periphery of each skyrmion. This enhances the effective dielectric permittivity compared with the individual SrTiO3 and PbTiO3 layers. Moreover, the response of these topologically protected structures to electric field and temperature shows a reversible phase transition from the skyrmion state to a trivial uniform ferroelectric state, accompanied by large tunability of the dielectric permittivity. Pulsed switching measurements show a time-dependent evolution and recovery of the skyrmion state (and macroscopic dielectric response). The interrelationship between topological and dielectric properties presents an opportunity to simultaneously manipulate both by a single, and easily controlled, stimulus, the applied electric field.