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1. Removing the Critical Thickness of Improper Ferroelectrics via Interfacial Reconstruction

   https://doi.org/10.1021/acs.nanolett.5c02517  

   Li, Xin; Yun, Yu; Ren, Guodong; Thind, Arashdeep Singh; Shah, Amit Kumar; Mishra, Rohan; Xu, Xiaoshan (August 2025, Nano Letters)

   The atomic structures at epitaxial film–substrate interfaces determine the scalability of thin films and can result in new phenomena. However, it is challenging to control the structure of the interface. In this work, we report the strong tunability of the epitaxial interface of improper ferroelectric hexagonal ferrites deposited on spinel ferrites, achieving the artificial selection of two types of interfaces that are related by a 90° rotation of in-plane epitaxial relations and feature either disordered or hybrid reconstruction. The hybrid-type interface exhibits characteristic structures of both hexagonal ferrites and spinel ferrites, which remove the critical thickness for improper ferroelectricity. This tunable interfacial structure provides critical insight into controlling interfacial clamping to maintain robust improper ferroelectricity at the two-dimensional limit. 

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2. Defect‐Enhanced Polarization Switching in the Improper Ferroelectric LuFeO ₃

   https://doi.org/10.1002/adma.202000508  

   Barrozo, Petrucio; Småbråten, Didrik_René; Tang, Yun‐Long; Prasad, Bhagwati; Saremi, Sahar; Ozgur, Rustem; Thakare, Vishal; Steinhardt, Rachel_A; Holtz, Megan_E; Stoica, Vladimir_Alexandru; et al (April 2020, Advanced Materials)

   Abstract Results of switching behavior of the improper ferroelectric LuFeO₃are presented. Using a model set of films prepared under controlled chemical and growth‐rate conditions, it is shown that defects can reduce the quasi‐static switching voltage by up to 40% in qualitative agreement with first‐principles calculations. Switching studies show that the coercive field has a stronger frequency dispersion for the improper ferroelectrics compared to a proper ferroelectric such as PbTiO₃. It is concluded that the primary structural order parameter controls the switching dynamics of such improper ferroelectrics. 

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3. Subregular Tree Transductions, Movement, Copies, Traces, and the Ban on Improper Movement

   https://doi.org/10.7275/tk1n-q855  

   Graf, Thomas (January 2023, Proceedings of the Society for Computation in Linguistics)

   Extending prior work in Graf (2018, 2020, 2022c), I show that movement is tier-based strictly local (TSL) even if one analyzes it as a transformation, i.e. a tree transduction from derivation trees to output trees. I define input strictly local (ISL) tree-to-tree transductions with (lexical) TSL tests as a tier-based extension of ISL tree-to-tree transductions. TSL tests allow us to attach each mover to all its landing sites. In general, this class of transductions fails to attach each mover to its final landing site to the exclusion of all its intermediate landing sites, which is crucial for producing output trees with the correct string yield. The problem is avoided, though, if syntax enforces a variant of the Ban on Improper Movement. Subregular complexity thus provides a novel motivation for core restrictions on movement while also shedding new light on the choice between copies and traces in syntax. 

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4. Misspecification in Prediction Problems and Robustness via Improper Learning

   Marsden, Annie; Duchi, John; Valiant, Gregory (January 2021, AISTATS)

   null (Ed.)

   We study probabilistic prediction games when the underlying model is misspecified, investigating the consequences of predicting using an incorrect parametric model. We show that for a broad class of loss functions and parametric families of distributions, the regret of playing a ``proper'' predictor--one from the putative model class--relative to the best predictor in the same model class has lower bound scaling at least as sqrt(c n), where c is a measure of the model misspecification to the true distribution in terms of total variation distance. In contrast, using an aggregation-based (improper) learner, one can obtain regret d log n for any underlying generating distribution, where d is the dimension of the parameter; we exhibit instances in which this is unimprovable even over the family of all learners that may play distributions in the convex hull of the parametric family. These results suggest that simple strategies for aggregating multiple learners together should be more robust, and several experiments conform to this hypothesis. 

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5. Suppressing the ferroelectric switching barrier in hybrid improper ferroelectrics

   https://doi.org/10.1038/s41524-020-00436-x  

   Li, Shutong; Birol, Turan (November 2020, npj Computational Materials)

   Abstract Integration of ferroelectric materials into novel technological applications requires low coercive field materials, and consequently, design strategies to reduce the ferroelectric switching barriers. In this first principles study, we show that biaxial strain, which has a strong effect on the ferroelectric ground states, can also be used to tune the switching barrier of hybrid improper ferroelectric Ruddlesden–Popper oxides. We identify the region of the strain-tolerance factor phase diagram where this intrinsic barrier is suppressed, and show that it can be explained in relation to strain-induced phase transitions to nonpolar phases. 

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