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1. Realization of Non‐Equilibrium Wurtzite Structure in Heterovalent Ternary MgSiN ₂ Film Grown by Reactive Sputtering

   https://doi.org/10.1002/aelm.202400880  

   Kageyama, Sotaro; Okamoto, Kazuki; Yasuoka, Shinnosuke; Ide, Keisuke; Hanzawa, Kota; Hiranaga, Yoshiomi; Hsieh, Pochun; Hazra, Sankalpa; Suceava, Albert; Saha, Akash; et al (February 2025, Advanced Electronic Materials)

   Abstract The piezoelectric and ferroelectric applications of heterovalent ternary materials are not well explored. Epitaxial MgSiN₂films are grown at 600 °C on (111)Pt//(001)Al₂O₃substrates by the reactive sputtering method using metallic Mg and Si under the N₂atmosphere. Detailed X‐ray diffraction measurements and transmission electron microscopy observations revealed that the epitaxially grown films on the substrates have a hexagonal wurtzite structure withc‐axis out‐of‐plane orientation. The random occupation of this structure by Mg and Si differs from that of the previously reported structure in which these two cations periodically occupy the cationic sites. However, the lattice spacings closely approximate those that are previously reported, irrespective of the ordering, and they are almost comparable with those of (Al_0.8Sc_0.2)N. The wide bandgap of >5.0 eV in deposited MgSiN₂is compatible with that of AlN and suggests durability against the application of strong external electric fields, possibly to induce polarization switching. In addition, MgSiN₂is shown to have piezoelectric properties with an effectived₃₃value of 2.3 pm V⁻¹for the first time. This work demonstrates the compositional expansion of hexagonal wurtzite to heterovalent ternary nitrides for novel piezoelectric materials, whose ferroelectricity is expected. 

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2. Quadrupling the depairing current density in the iron-based superconductor SmFeAsO1–xHx

   https://doi.org/10.1038/s41563-024-01952-7  

   Miura, Masashi; Eley, Serena; Iida, Kazumasa; Hanzawa, Kota; Matsumoto, Jumpei; Hiramatsu, Hidenori; Ogimoto, Yuki; Suzuki, Takumi; Kobayashi, Tomoki; Ozaki, Toshinori; et al (October 2024, Nature Materials)

   Abstract Iron-based 1111-type superconductors display high critical temperatures and relatively high critical current densitiesJ_c. The typical approach to increasingJ_cis to introduce defects to control dissipative vortex motion. However, when optimized, this approach is theoretically predicted to be limited to achieving a maximumJ_cof only ∼30% of the depairing current densityJ_d, which depends on the coherence length and the penetration depth. Here we dramatically boostJ_cin SmFeAsO_1–xH_xfilms using a thermodynamic approach aimed at increasingJ_dand incorporating vortex pinning centres. Specifically, we reduce the penetration depth, coherence length and critical field anisotropy by increasing the carrier density through high electron doping using H substitution. Remarkably, the quadrupledJ_dreaches 415 MA cm^–2, a value comparable to cuprates. Finally, by introducing defects using proton irradiation, we obtain highJ_cvalues in fields up to 25 T. We apply this method to other iron-based superconductors and achieve a similar enhancement of current densities. 

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3. Large Enhancements in Optical and Piezoelectric Properties in Ferroelectric Zn _{1‐ x} Mg _x O Thin Films through Engineering Electronic and Ionic Anharmonicities

   https://doi.org/10.1002/apxr.202300003  

   Zu, Rui; Ryu, Gyunghyun; Kelley, Kyle P.; Baksa, Steven M.; Jacques, Leonard C.; Wang, Bo; Ferri, Kevin; He, Jingyang; Chen, Long‐Qing; Dabo, Ismaila; et al (May 2023, Advanced Physics Research)

   Abstract Multifunctionality as a paradigm requires materials exhibiting multiple superior properties. Integrating second‐order optical nonlinearity and large bandgap with piezoelectricity can, for example, enable broadband, strain‐tunable photonics. Though very different phenomena at distinct frequencies, both second‐order optical nonlinearity and piezoelectricity are third‐rank polar tensors present only in acentric crystal structures. However, simultaneously enhancing both phenomena is highly challenging since it involves competing effects with tradeoffs. Recently, a large switchable ferroelectric polarization of ≈80 μC cm⁻²was reported in Zn_1‐xMg_xO films. Here, ferroelectric Zn_1‐xMg_xO is demonstrated to be a platform that hosts simultaneously a 30% increase in the electronic bandgap, a 50% enhancement in the second harmonic generation (SHG) coefficients, and a near 200% improvement in the piezoelectric coefficients over pure ZnO. These enhancements are shown to be due to a 400% increase in the electronic anharmonicity and a ≈200% decrease in the ionic anharmonicity with Mg substitution. Precisely controllable periodic ferroelectric domain gratings are demonstrated down to 800 nm domain width, enabling ultraviolet quasi‐phase‐matched optical harmonic generation as well as domain‐engineered piezoelectric devices. 

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4. Mapping domain structures near a grain boundary in a lead zirconate titanate ferroelectric film using X-ray nanodiffraction

   https://doi.org/10.1107/S1600576724009026  

   Udovenko, Stanislav; Son, Yeongwoo; Tipsawat, Pannawit; Knox, Reilly J; Hruszkewycz, Stephan O; Yan, Hanfei; Huang, Xiaojing; Pattammattel, Ajith; Zajac, Marc; Cha, Wonsuk; et al (December 2024, Journal of Applied Crystallography)

   The effect of an electric field on local domain structure near a 24° tilt grain boundary in a 200 nm-thick Pb(Zr_0.2Ti_0.8)O₃bi-crystal ferroelectric film was probed using synchrotron nanodiffraction. The bi-crystal film was grown epitaxially on SrRuO₃-coated (001) SrTiO₃24° tilt bi-crystal substrates. From the nanodiffraction data, real-space maps of the ferroelectric domain structure around the grain boundary prior to and during application of a 200 kV cm⁻¹electric field were reconstructed. In the vicinity of the tilt grain boundary, the distributions of densities ofc-type tetragonal domains with thecaxis aligned with the film normal were calculated on the basis of diffracted intensity ratios ofc- anda-type domains and reference powder diffraction data. Diffracted intensity was averaged along the grain boundary, and it was shown that the density ofc-type tetragonal domains dropped to ∼50% of that of the bulk of the film over a range ±150 nm from the grain boundary. This work complements previous results acquired by band excitation piezoresponse force microscopy, suggesting that reduced nonlinear piezoelectric response around grain boundaries may be related to the change in domain structure, as well as to the possibility of increased pinning of domain wall motion. The implications of the results and analysis in terms of understanding the role of grain boundaries in affecting the nonlinear piezoelectric and dielectric responses of ferroelectric materials are discussed. 

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5. 2D Amorphous Homojunction Transistors Exfoliated from Liquid Metals

   https://doi.org/10.1002/adfm.202512590  

   Le, Minh Nhut; Rahman, Md Saifur; Agnew, Simon A; Ong, Samuel W; Scheideler, William J (July 2025, Advanced Functional Materials)

   Abstract Ultrathin 2D metal oxides are a high‐performance class of transparent conducting materials capable of overcoming the traditional limitations of inorganic flexible electronics. The low temperature, thermodynamically favorable synthesis of 2D oxides at liquid metal interfaces offers the potential for printing these materials over large areas at unprecedented speeds with sub‐nanometer scale precision. However, these native oxides are sub‐stoichiometric and highly conductive, so new strategies are needed that can precisely engineer the electrostatics and enhance stability. In this work, the crystalline vs. amorphous phase of 2D oxides is engineered via alloying of ternary In_1‐ySn_yO_xand ultralow deposition temperatures (120–160 °C) are afforded by In‐Sn eutectics. This approach is extended to rapid assembly of nanoscale (3–5 nm per layer) vertical 2D homojunctions with electrostatically favorable grading from high density of states front channels to lower density of states back‐channels. Detailed materials characterization reveals how this platform enhances electron mobility while improving resilience under bias‐stress in metal oxide transistors. Devices based on amorphous 2D oxide homojunctions with high‐k sol‐gel ZrO_xdielectrics achieve excellent electron mobility (30 cm²/V·s), steep switching (SS of 100 mV dec⁻¹), I_on/offof 10⁷and 10X reduced bias‐stress shifts, presenting an ideal strategy for high‐performance flexible oxide electronics. 

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