Circumscribed ferroelectricity by phonon-decoupled oxygen tetrahedra in brownmillerite oxides

Abstract

Abstract Ultimate scaling limit in ferroelectric switching has been attracting broad attention in the fields of materials science and nanoelectronics1,2. Despite the immense efforts to scale down ferroelectric features, however, only few materials have been shown to exhibit ferroelectricity at the unit cell level3,4. Here, we report a controllable unit cell-scale domain in the brownmillerite oxides consisting of alternating octahedral/tetrahedral unit cells. By combining atomic-scale imaging and in-situ transmission electron microscopy, we directly probed unit cell-wide ferroelectricity patterned by neutral unit cell-wide walls and its switchable characteristics. The first-principles phonon calculations confirm that the phonon modes related with oxygen-octahedra are fully decoupled from those with oxygen-tetrahedra in the brownmillerite oxides, and such localized oxygen-tetrahedral phonons stabilize the unit cell-wide domain. Unit cell-wide ferroelectricity in our study provides unprecedented scaling limit of ferroelectric thin films for designing high-density memory devices at the quantum limit.