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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. 

Abstract BaTiO₃is a technologically relevant material in the perovskite oxide class with above‐room‐temperature ferroelectricity and a very large electro‐optical coefficient, making it highly suitable for emerging electronic and photonic devices. An easy, robust, straightforward, and scalable growth method is required to synthesize epitaxial BaTiO₃thin films with sufficient control over the film's stoichiometry to achieve reproducible thin film properties. Here the growth of BaTiO₃thin films by hybrid molecular beam epitaxy is reported. A self‐regulated growth window is identified using complementary information obtained from reflection high energy electron diffraction, the intrinsic film lattice parameter, film surface morphology, and scanning transmission electron microscopy. Subsequent optical characterization of the BaTiO₃films by spectroscopic ellipsometry revealed refractive index and extinction coefficient values closely resembling those of stoichiometric bulk BaTiO₃crystals for films grown inside the growth window. Even in the absence of a lattice parameter change of BaTiO₃thin films, degradation of optical properties is observed, accompanied by the appearance of a wide optical absorption peak in the IR spectrum, attributed to optical transitions involving defect states present. Therefore, the optical properties of BaTiO₃can be utilized as a much finer and more straightforward probe to determine the stoichiometry level present in BaTiO₃films. 

Abstract Nitrogen vacancy (NV) centers, optically active atomic defects in diamond, have attracted tremendous interest for quantum sensing, network, and computing applications due to their excellent quantum coherence and remarkable versatility in a real, ambient environment. Taking advantage of these strengths, this paper reports on NV‐based local sensing of the electrically driven insulator‐to‐metal transition (IMT) in a proximal Mott insulator. The resistive switching properties of both pristine and ion‐irradiated VO₂thin film devices are studied by performing optically detected NV electron spin resonance measurements. These measurements probe thelocaltemperature and magnetic field in electrically biased VO₂devices, which are in agreement with theglobaltransport measurement results. In pristine devices, the electrically driven IMT proceeds through Joule heating up to the transition temperature while in ion‐irradiated devices, the transition occurs nonthermally, well below the transition temperature. The results provide direct evidence for nonthermal electrically induced IMT in a Mott insulator, highlighting the significant opportunities offered by NV quantum sensors in exploring nanoscale thermal and electrical behaviors in Mott materials.