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Abstract

Since the discovery of ferroelectricity, a wide range of mechanisms has been identified that drive long-range ordering of electric dipoles. Despite this variety, research largely focused on “proper” ferroelectrics, where the polarization is the symmetry breaking order parameter and large saturation polarization can be reached. In contrast, materials with small or compensated polarization received much less attention and were deemed technologically irrelevant. However, this perception has changed completely in recent years and unconventional ferroelectrics are now intensively studied as functional materials, holding great promise for future applications in nanoelectronics and energy storage.

I my talk, I will explore various forms of electric long-range order beyond conventional proper ferroelectricity. To highlight the fascinating new physics emerging in these systems, I will present three cases: I will discuss (i) how improper ferroelectricity in hexagonal manganites and ferrites stabilizes charged interfaces, promoting the formation of electronic inversion layers, 3D oxygen vacancy order, and room-temperature multiferroicity. (ii) In hyperferroelectric lead germanate, usual anti-polar domain walls arise where the polarization direction alternates at the unit-cell level, facilitated by an intrinsic resilience against depolarizing fields. (iii) Furthermore, I will show how the noncollinear nature of antiferroelectric order in potassium niobate borate gives rise to unique hybrid antiferroelectric–ferroelectric responses.

Although hyper-, anti-, and improper ferroelectricity have been recognized for many years, experimental demonstrations of their unconventional properties beyond classical proper ferroelectricity have only recently emerged. As a result, novel physical phenomena in these material classes are becoming more that theoretical concept. Indeed, it is safe to say that we have only begun to uncover the extensive nanoscale phenomena and remarkable application possibilities that lie ahead.