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Abstract Antiferroelectrics are a promising class of materials for applications in capacitive energy storage and multi‐state memory, but comprehensive control of their functional properties requires further research. In thin films, epitaxial strain and size effects are important tuning knobs but difficult to probe simultaneously due to low critical thicknesses of common lead‐based antiferroelectrics. Antiferroelectric NaNbO₃enables opportunities for studying size effects under strain, but electrical properties of ultra‐thin films have not been thoroughly investigated due to materials challenges. Here, high‐quality, epitaxial, coherently‐strained NaNbO₃films are synthesized from 35‐ to 250‐ nm thickness, revealing a transition from a fully ferroelectric state to coexisting ferroelectric and antiferroelectric phases with increasing thickness. The electrical performance of this phase coexistence is analyzed through positive‐up negative‐down and first‐order reversal curve measurements. Further increasing thickness leads to a fully ferroelectric state due to a strain relief mechanism that suppresses the antiferroelectricity. The potential of engineering competing ferroic orders in NaNbO₃for multiple applications is evaluated, reporting significantly enhanced recoverable energy density (20.6 J cm⁻³at 35 nm) and energy efficiency (90% at 150 nm) relative to pure bulk NaNbO₃as well as strong retention and fatigue performance with multiple accessible polarization states in the intermediate thickness films. 

Abstract Phonon polaritons (PhPs), excitations arising from the coupling of light with lattice vibrations, enable light confinement and local field enhancement, which is essential for various photonic and thermal applications. To date, PhPs with high confinement and low loss are mainly observed in the mid‐infrared regime and mostly in manually exfoliated flakes of van der Waals (vdW) materials. In this work, the existence of low‐loss, thickness‐tunable phonon polaritons in the far‐infrared regime within transferable freestanding SrTiO₃membranes synthesized through a scalable approach, achieving high figures of merit is demonstrated, which are comparable to the previous record values from the vdW materials. Leveraging atomic precision in thickness control, large dimensions, and compatibility with mature oxide electronics, functional oxide membranes present a promising large‐scale 2D platform alternative to vdW materials for on‐chip polaritonic technologies in the infrared regime. 

Mechanical strain presents an effective control over symmetry-breaking phase transitions. In quantum paralelectric SrTiO3, strain can induce ferroelectric order via modification of the local Ti potential energy landscape. However, brittle bulk materials can only withstand limited strain range (~0.1%). Taking advantage of nanoscopically-thin freestanding membranes, we demonstrate an in-situ strain-induced reversible ferroelectric transition in freestanding SrTiO3 membranes. We measure the ferroelectric order by detecting the local anisotropy of the Ti 3d orbital signature using x-ray linear dichroism at the Ti-K pre-edge, while the strain is determined by x-ray diffraction. With reduced thickness, the SrTiO3 membranes remain elastic with >1% tensile strain cycles. A robust displacive ferroelectricity appears beyond a temperature-dependent critical strain. Interestingly, we discover a crossover from a classical ferroelectric transition to a quantum regime at low temperatures, which enhances strain-induced ferroelectricity. Our results offer new opportunities to strain engineer functional properties in low dimensional quantum materials and provide new insights into the role of ferroelectric fluctuations in quantum paraelectric SrTiO3.