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1. Antiferroelectric negative capacitance from a structural phase transition in zirconia

   https://doi.org/10.1038/s41467-022-28860-1  

   Hoffmann, Michael; Wang, Zheng; Tasneem, Nujhat; Zubair, Ahmad; Ravindran, Prasanna Venkatesan; Tian, Mengkun; Gaskell, Anthony Arthur; Triyoso, Dina; Consiglio, Steven; Tapily, Kandabara; et al (March 2022, Nature Communications)

   Abstract Crystalline materials with broken inversion symmetry can exhibit a spontaneous electric polarization, which originates from a microscopic electric dipole moment. Long-range polar or anti-polar order of such permanent dipoles gives rise to ferroelectricity or antiferroelectricity, respectively. However, the recently discovered antiferroelectrics of fluorite structure (HfO₂and ZrO₂) are different: A non-polar phase transforms into a polar phase by spontaneous inversion symmetry breaking upon the application of an electric field. Here, we show that this structural transition in antiferroelectric ZrO₂gives rise to a negative capacitance, which is promising for overcoming the fundamental limits of energy efficiency in electronics. Our findings provide insight into the thermodynamically forbidden region of the antiferroelectric transition in ZrO₂and extend the concept of negative capacitance beyond ferroelectricity. This shows that negative capacitance is a more general phenomenon than previously thought and can be expected in a much broader range of materials exhibiting structural phase transitions. 

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2. Two-dimensional ferroelectricity by design

   https://doi.org/10.1126/science.abi7296  

   Tsymbal, Evgeny Y. (June 2021, Science)

   The discovery of ferroelectricity marks its 100th anniversary this year ( 1 ), and this phenomenon continues to enrich our understanding of many fields of physics and material science, as well as creating subfields on its own. All of the ferroelectrics discovered have been limited to those exhibiting a polar space group of the bulk crystal that supports two or more topologically equivalent variants with different orientations of electric polarization. On pages 1458 and 1462 of this issue, Yasuda et al. ( 2 ) and Vizner Stern et al. ( 3 ), respectively, show that ferroelectricity can be engineered by artificially stacking a nonpolar in bulk, two-dimensional (2D) material, boron nitride (BN). A relatively weak van der Waals (vdW) coupling between the adjacent BN monolayers allows their parallel alignment in a metastable non-centrosymmetric coordination supporting 2D ferroelectricity with an out-of-plane electric polarization. These findings open opportunities to design 2D ferroelectrics out of parent nonpolar compounds. 

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3. Out-of-plane ferroelectricity and robust magnetoelectricity in quasi-two-dimensional materials

   https://doi.org/10.1126/sciadv.adi0138  

   Lu, Xue-Zeng; Zhang, Hui-Min; Zhou, Ying; Zhu, Tong; Xiang, Hongjun; Dong, Shuai; Kageyama, Hiroshi; Rondinelli, James M (November 2023, Science Advances)

   Thin-film ferroelectrics have been pursued for capacitive and nonvolatile memory devices. They rely on polarizations that are oriented in an out-of-plane direction to facilitate integration and addressability with complementary metal-oxide semiconductor architectures. The internal depolarization field, however, formed by surface charges can suppress the out-of-plane polarization in ultrathin ferroelectric films that could otherwise exhibit lower coercive fields and operate with lower power. Here, we unveil stabilization of a polar longitudinal optical (LO) mode in the n=2 Ruddlesden–Popper family that produces out-of-plane ferroelectricity, persists under open-circuit boundary conditions, and is distinct from hyperferroelectricity. Our first-principles calculations show the stabilization of the LO mode is ubiquitous in chalcogenides and halides and relies on anharmonic trilinear mode coupling. We further show that the out-of-plane ferroelectricity can be predicted with a crystallographic tolerance factor, and we use these insights to design a room-temperature multiferroic with strong magnetoelectric coupling suitable for magneto-electric spin-orbit transistors. 

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4. Ferroelectric nematic liquids with conics

   https://doi.org/10.1038/s41467-023-36326-1  

   Kumari, Priyanka; Basnet, Bijaya; Wang, Hao; Lavrentovich, Oleg D. (February 2023, Nature Communications)

   Abstract Spontaneous electric polarization of solid ferroelectrics follows aligning directions of crystallographic axes. Domains of differently oriented polarization are separated by domain walls (DWs), which are predominantly flat and run along directions dictated by the bulk translational order and the sample surfaces. Here we explore DWs in a ferroelectric nematic (N_F) liquid crystal, which is a fluid with polar long-range orientational order but no crystallographic axes nor facets. We demonstrate that DWs in the absence of bulk and surface aligning axes are shaped as conic sections. The conics bisect the angle between two neighboring polarization fields to avoid electric charges. The remarkable bisecting properties of conic sections, known for millennia, play a central role as intrinsic features of liquid ferroelectrics. The findings could be helpful in designing patterns of electric polarization and space charge. 

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5. Nanoscale Phase and Orientation Mapping in Multiphase Polycrystalline Hafnium Zirconium Oxide Thin Films Using 4D‐STEM and Automated Diffraction Indexing

   https://doi.org/10.1002/smtd.202400395  

   Baucom, Garrett; Hershkovitz, Eitan; Chojecki, Paul; Nishida, Toshikazu; Tabrizian, Roozbeh; Kim, Honggyu (May 2024, Small Methods)

   Abstract Ferroelectric hafnium zirconium oxide (HZO) holds promise for nextgeneration memory and transistors due to its superior scalability and seamless integration with complementary metal‐oxide‐semiconductor processing. A major challenge in developing this emerging ferroelectric material is the metastable nature of the non‐centrosymmetric polar phase responsible for ferroelectricity, resulting in a coexistence of both polar and non‐polar phases with uneven grain sizes and random orientations. Due to the structural similarity between the multiple phases and the nanoscale dimensions of the thin film devices, accurate measurement of phase‐specific information remains challenging. Here, the application of 4D scanning transmission electron microscopy is demonstrated with automated electron diffraction pattern indexing to analyze multiphase polycrystalline HZO thin films, enabling the characterization of crystallographic phase and orientation across large working areas on the order of hundreds of nanometers. This approach offers a powerful characterization framework to produce a quantitative and statistically robust analysis of the intricate structure of HZO films by uncovering phase composition, polarization axis alignment, and unique phase distribution within the HZO film. This study introduces a novel approach for analyzing ferroelectric HZO, facilitating reliable characterization of process‐structure‐property relationships imperative to accelerating the growth optimization, performance, and successful implementation of ferroelectric HZO in devices. 

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