Circumscribed ferroelectricity by phonon-decoupled oxygen tetrahedra in brownmillerite oxides

Abstract Ultimate scaling limit in ferroelectric switching has been attracting broad attention in the fields of materials science and nanoelectronics 1,2 . Despite the immense efforts to scale down ferroelectric features, however, only few materials have been shown to exhibit ferroelectricity at the unit cell level 3,4 . 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.