Trans-switching in layered ferroelectrics

Ferroelectric materials exhibit a spontaneous electric polarization that can be reversed byan electric field ¹ , a property central to non-volatile memories ^2,3 , sensors ⁴ and actuators ^5,6 . In most conventional ferroelectric oxides, the polarization originates from a softening of a polar mode ⁷ . The amplitude of the mode along a given direction couples with the applied field along the same direction ⁸ . A long-standing challenge has been to achieve controlled switching of the in-plane polarization component using an out-of-plane electric field. We have discovered that a trilinear coupling between the in-plane and out-of-plane polarization, mediated via the octahedral tilts and rotations in the layered ferroelectric Bi ₄ Ti ₃ O ₁₂ naturally fulfills these requirements and enables trans-switching of the polarization state. In its monoclinic phase, this material hosts a large in-plane polarization (∼ 50 μC cm ^-2 ) driven by a proper ferroelectric instability, together with smaller out-of-plane polarization (∼ 5 μC cm ^-2 ) of improper origin, induced by oxygen-octahedral distortions. We demonstrate that in c-axis-oriented epitaxial films the in-plane polarization switches deterministically under an out-of-plane electric field. This cross-coupling between orthogonal polarization components provides a route to transverse manipulation of ferroic order parameters and establishes layered ferroelectrics as a platform for capacitive computing concepts, analogous to the transconductance of modern CMOS electronics.