Voltage-tunable capacitors (varactors) are key to microwave circuits. Tunable dielectric varactors outperform competing technologies in almost every relevant metric but usually suffer from high dielectric loss. In contrast, Ruddlesden-Popper (RPs) dielectric thin films have remarkably low microwave loss. Unfortunately, their crystallographic symmetry has until recently dictated an in-plane device structure, precluding the favorable out-of-plane parallel-plate varactor design for minimized size and maximized electric field in the tunable dielectric. Guided by theory, we report RPs akin to the widely studied tunable microwave dielectric BaxSr1-xTiO3. Assembling these same atoms into the first RP phase with broken out-of-plane symmetry, we achieve a low-loss, out-of-plane tunable dielectric thin film. The highest performing film, (ATiO3)nAO film with A = Ba0.45Sr0.55 and n = 8, unlocks a tenfold improvement in the figure of merit for out-of-plane tunable dielectrics at 10 GHz, paving the way for a new generation of tunable monolithic microwave integrated circuits.

In correlated electronic states, observation of size-dependent nucleation and melting is rarely reported, likely due to the extremely small length scales necessary to observe such effects for electronic states. Here, using 1T-TiSe2 nanoflakes as a prototypical two-dimensional charge density wave (CDW) system, we perform in situ cryogenic electron microscopy with temperature down to 20 K and observe size-dependent melting of CDWs. Specifically, we observe a melting point depression of CDW for 1T-TiSe2 flakes with lateral sizes less than 100 nm. By fitting experimental data to a Ginzburg-Landau model, we estimate a zero-temperature correlation length of 10-50 nm, which matches the reported CDW domain size for 1T-TiSe2. As the flake size approaches the correlation length, the divergence of the CDW correlation length near the transition is cut off by the finite flake size, limiting long-range order and thereby lowering the transition temperature. For very small flakes whose size is close to the correlation length, we also observe absence of long-range CDW, as predicted by the model.

Scaling ferroelectrics to nanometer thicknesses remains a central challenge for low-power, nonvolatile electronics, as leakage currents increasingly dominate with reduced dimensions. Alkali-based, lead-free ferroelectrics offer an environmentally sustainable alternative to lead-based systems, yet their scaling is severely limited by leakage arising from volatile alkali constituents. Here, we show that this intrinsic limitation can be transformed into an advantageous degree of freedom through defect engineering. By precisely modulating alkali deficiency during thin-film synthesis, we engineer clustered defect complexes that function as deep trap states, strongly suppressing leakage and enabling robust ferroelectric operation in ultrathin films down to the sub-10 nm regime at voltages below 100 mV. Our results establish defect-enabled scaling as a viable pathway for advancing environmentally benign ferroelectrics toward ultra-low-power, non-volatile electronic technologies.

We report the molecular beam epitaxial growth conditions to realize coherently strained nitrogen-polar gallium nitride quantum wells on single-crystal bulk aluminum nitride substrates. The structural, optical, and electronic properties of these binary N-polar GaN/AlN heterostructures are discussed. The sharpness of the GaN/AlN interface and the preservation of the polarity across the heterojunction is studied by electron microscopy. Photoluminescence measurements reveal two peaks: one at similar to 3.6 eV corresponding to a GaN layer under compressive strain that produces a blue shift and the other at similar to 6.0 eV from the epitaxial AlN buffer layer. A high-density polarization-induced 2D electron gas is formed in the ultrathin N-polar GaN quantum well, whose transport properties are measured to cryogenic temperatures.

Conventional Li-S cells include both flammable DOL:DME liquid electrolyte and excess lithium metal, which poses a safety concern. This work presents a semi-solid anode-free Cu-Li2S cell design concept with Li[G4]TFSI solvate ionic liquid (SIL) electrolyte and a Li6PS5Cl solid-state electrolyte (SSE). Accelerated rate calorimetry (ARC) safety testing showed that the semi-solid cell had a thermal runaway onset temperature of 188 degrees C compared to only 116 degrees C for a conventional cell design. Improved safety of the semi-solid cell was attributed to less flammable SIL and reduced lithium content. Previous work also showed that Li-S cells with SIL electrolyte suffer from poor rate and capacity utilization because low polysulfide solubility in SIL results in sluggish reaction kinetics. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge/discharge experiments showed that 20 wt% SSE powder added to the cathode improved performance. Finally, low polysulfide concentration was harnessed to promote efficient Li stripping and plating without LiNO3. A semi-solid anode-free single-layer pouch cell achieved 50 cycles with 80% capacity retention. A fundamental study of solid electrolyte interphase (SEI) of Li metal in SIL was conducted using X-ray photoelectron spectroscopy (XPS) and cryogenic scanning transmission electron microscopy electron energy-loss spectroscopy (cryo-STEM/EELS). The data showed that limited SEI sulfurization promoted smooth and dense deposition of Li metal for good cycle life. The results presented here hold translational value to future work on anode-free cell design. (c) 2026 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

AlN has the largest bandgap in the wurtzite III-nitride semiconductor family, making it an ideal barrier for a thin GaN channel to achieve strong carrier confinement in field-effect transistors, analogous to silicon-on-insulator technology. Unlike /Si/, AlN/GaN/AlN can be grown fully epitaxially, enabling high carrier mobilities suitable for high-frequency applications. However, developing these heterostructures and related devices has been hindered by challenges in strain management, polarization effects, defect control, and charge trapping. Here, the AlN single-crystal high electron mobility transistor (XHEMT) is introduced, a new nitride transistor technology designed to address these issues. The XHEMT structure features a pseudomorphic GaN channel sandwiched between AlN layers, grown on single-crystal AlN substrates. XHEMTs demonstrate RF performance on par with the state-of-the-art GaN HEMTs, achieving 5.92 W/mm output power and 65% peak power-added efficiency at 10 GHz under 17 V drain bias. These devices overcome several limitations present in conventional GaN HEMTs, which are grown on lattice-mismatched foreign substrates that introduce undesirable dislocations and exacerbated thermal resistance. With the recent availability of 100-mm AlN substrates and AlN's high thermal conductivity (340 W/), XHEMTs show strong potential for next-generation RF electronics.

Scaling ferroelectrics to nanometer thicknesses remains a central challenge for low-power, nonvolatile electronics, as leakage currents increasingly dominate at reduced dimensions. Alkali-based, lead-free ferroelectrics offer an environmentally sustainable alternative to lead-based systems, yet their scaling is severely limited by leakage arising from volatile alkali constituents. Here, we show that this intrinsic limitation can be transformed into an advantageous degree of freedom through defect engineering. By precisely modulating alkali deficiency during thin-film synthesis, we engineer clustered defect complexes that function as deep trap states, strongly suppressing leakage and enabling robust ferroelectric operation in ultrathin films down to the sub-10 nm regime at voltages below 100 mV. Our results establish defect-enabled scaling as a viable pathway for advancing environmentally benign ferroelectrics toward ultra-low-power, nonvolatile electronic technologies.

Gallium nitride (GaN) is used in solid-state lighting and in high-performance radio frequency and power electronics. However, due to inefficient hole doping and low hole mobility, quantum oscillations in p-type GaN have not been observed, which limits studies of valence bands and hole transport engineering. Here we report high hole mobilities in a polarization-induced two-dimensional hole gas at a gallium nitride/aluminium nitride interface. The holes degenerately occupy two valence bands of GaN-the light-hole and heavy-hole bands-and have mobilities of 2,000 cm2 V-1 s-1 and 400 cm2 V-1 s-1 at 2 K, respectively. We use Shubnikov-de Haas oscillations of holes from both valence bands to extract their respective sheet densities and quantum scattering times and the effective masses of light holes and heavy holes. The hole mobilities of our heterostructure highlight the possibility of developing cryogenic GaN complementary metal-oxide-semiconductor technology with potential applications in quantum computing control electronics.

We report the structural and electronic properties of niobium nitride (NbN) thin films grown by molecular beam epitaxy on c-plane sapphire with miscut angles of 0.5 degrees, 2 degrees, 4 degrees, and 10 degrees toward m-axis. X-ray diffraction scans reveal that the full width at half maximum of the rocking curves around the 1 1 1 reflection of these NbN films decreases with increasing miscut. Starting from 76 arcsecs on 0.5 degrees miscut, the FWHM reduces to almost 20 arcsecs on 10 degrees miscut sapphire, indicating improved structural quality. Scanning transmission electron microscopy images indicate that NbN on c-sapphire has around 10 nm critical thickness, irrespective of the substrate miscut, above which it turns columnar. The improved structural property is correlated with a marginal increment in superconducting transition temperature T-c from 12.1 K for NbN on 0.5 degrees miscut sapphire to 12.5 K for NbN on 10 degrees miscut sapphire.

A new family of high-temperature superconductors was recently discovered in the n=2,3 Ruddlesden-Popper nickelates, where superconductivity emerges concomitant with suppression of parent density waves and structural octahedral rotations under hydrostatic pressure. Intriguingly, compressive strain mimics the structural effects of pressure in the n=2 phase, yielding ambient-pressure superconductivity. However, analogous strain-stabilized superconductivity has not been realized in the n=3. Here, we use atomically-precise synthesis, transport, picoscale electron microscopy, and synchrotron X-ray diffraction to probe n=3 La_4Ni_3O_10 thin films. Although compressive strain suppresses density wave order, we do not observe superconductivity even under the largest strain state. Importantly, we identify a structural distortion unique to strained n=3 thin films that may inhibit superconductivity: persistent, layer-inequivalent octahedral rotations around the c-axis. Our results highlight key differences between the n=3 and n=2 systems, suggesting that ambient-pressure superconductivity in the n=3 may require new methods beyond epitaxial strain engineering.

Enhancing intrinsic catalytic activity through material engineering remains a key objective in electrocatalysis research. To achieve this, constructing/assembling nanoscale core-shell structures has proven a particularly effective strategy, as it can simultaneously enhance catalytic activity and optimize atom utilization. The altered/modulated electrocatalytic properties of the shell can arise from lattice strain, induced by core-shell lattice mismatch, together with ligand effects, reflecting electronic interactions between heteroatoms. However, synthetically, it is difficult to isolate/separate the strain effect from electronic interactions, making the specific contribution/attribution to altered catalytic activity ambiguous. Here, we report a Pd@Pt core-shell nanocube (NC) system in which the Pd core, with expanded lattice parameters, minimizes its strain effect on the Pt shell. The Pt shell exhibits a more than 10-fold increase in catalytic activity compared to pure Pt NCs for both the hydrogen oxidation (HOR) and oxygen reduction (ORR) reactions under alkaline conditions. X-ray photoelectron spectroscopy analysis revealed a downshift of the Pt d-band center, and density functional theory calculations indicate that this change arises predominantly from electronic effects rather than strain effects, weakening the binding strength of reaction intermediates and thereby enhancing electrocatalytic activity. This work highlights the significance of electronic effects in tuning electrocatalytic activity.

Epitaxial all-nitride Josephson junctions are promising components for high coherence superconducting qubits, yet nanoscale defects often limit their performance. The key to mitigating these defects lies in understanding the atomic-scale relationship between polymorph selection, defect chemistry, and device performance. Here, we investigate structural and chemical defects in epitaxially grown 𝛿-NbN/AlN/𝛿-NbN, 𝛾-Nb4N3/AlN/𝛾-Nb4N3, β-Nb2N/AlN/β-Nb2N heterostructures on c-plane sapphire using molecular beam epitaxy. Advanced microscopy integrated with density functional theory shows varying impurity distribution across different polymorphs. The chemical distribution reveals that δ-NbN electrodes contain significant oxygen, whereas in β-Nb2N/AlN/β-Nb2N heterostructures oxygen preferentially segregates to the AlN barrier. DFT calculations indicate that these differences arise from phase-dependent oxygen energetics and diffusion kinetics, with oxygen remaining kinetically trapped in δ-NbN while exhibiting greater mobility in β-Nb2N. These structural and chemical differences are consistent with the distinct transport behavior observed in the two junction architectures and provide mechanistic insight into the role of defect chemistry in epitaxial nitride Josephson junctions. The observed impurity distribution may influence superconducting properties and contribute to the formation of two-level systems, a major source of loss and decoherence in superconducting quantum circuits.

High-resolution transmission electron microscopy (HRTEM) is an important method for imaging beam sensitive materials often under cryo conditions. Electron ptychography in the scanning transmission electron microscope (STEM) has been shown to reconstruct low-noise phase data at a reduced fluence for such materials. This raises the question of whether ptychography or HRTEM provides a more fluence-efficient imaging technique. Even though the transfer function is a common metric for evaluating the performance of an imaging method, it only describes the signal transfer with respect to spatial frequency, irrespective of the noise transfer. It can also not be well defined for methods, such as ptychography, that use an algorithm to form the final image. Here we apply the concept of detective quantum efficiency (DQE) to electron microscopy as a fluence independent and sample independent measure of technique performance. We find that, for a weak-phase object, ptychography can never reach the efficiency of a perfect Zernike phase imaging microscope but that ptychography is more robust to partial coherence.

The discovery of superconductivity in square-planar nickelates has offered a rich materials platform to explore the origins of high-temperature superconductivity. However, experimental investigations have largely been limited to the infinite-layer RNiO2 (R, rare earth) nickelates. We constructed a phase diagram of multilayer square-planar Ndn+1NinO2n+2 compounds and found signatures of superconductivity for dimensionality n = 4 to 8. Upon decreasing n, the superconducting anisotropy evolves owing to 4f electron effects, and electronic structure characteristics approach cuprate-like behavior. Magnetic fluctuations persist from within the superconducting regime and into the overdoped, nonsuperconducting regime. The superconducting regime overlaps with that of chemically doped infinite-layer nickelates, demonstrating underlying commonalities as well as differences across varying structural realizations of square-planar nickelates. Our work establishes this layered template for creating new nickel-based superconductors.

Abstract Van der Waals layered magnetic materials have recently received significant attention for their ability to exhibit antiferromagnetic or ferromagnetic (FM) properties, even at the few-layer or monolayer scale. Among them, Fe 3 GeTe 2 is one of the most extensively studied systems, crystallizing in a hexagonal structure as an itinerant FM with a Curie temperature ( T C ) of ∼220 K in bulk form and strong magnetic anisotropy. In this study, temperature and magnetic field dependence of the four-probe resistance ( R x x ) , thermopower (TEP) ( S ), and Hall resistance ( R x y ) were investigated in thick Fe 3 GeTe 2 flakes with different thicknesses to understand electron and spin transport, as well as spin and magnetic states. R x x decreased with decreasing temperature, confirming metallic behavior, consistent with the observed reduction in the magnitude of the negative TEP. Negative magnetoresistance (MR) with the magnetic field normal to the sample plane exhibited a quadratic field dependence below T C . An anomalous Hall effect was observed below T C , where R x y ( B ) showed a linear field dependence at low fields and saturation at higher fields. The anomalous Hall resistance ( R x y A ) followed a dependence of α R x x + β R x x 2 . A positive in-plane MR was observed when the current was perpendicular to the magnetic field, attributed to increased scattering from the enhanced Lorentz force and related orbital effects. Additionally, a hysteresis behavior was observed when cycling the in-plane magnetic field, likely due to the delay in domain alignment in response to the changing field.

A large challenge in determining the physics of helimagnetic SrFeO3 is in stabilizing the stoichiometric chemical phase over long enough time scales to conduct extensive measurements. Degradation in SrFeO3 manifests mainly as a crossover from metallic to insulating behavior. Using a combination of electronic transport and density functional theory, we show that this degradation is dominated by oxygen loss, possibly on the order of one percent. We further demonstrate that high quality SrFeO3 thin films can be stabilized long-term by combining a nanoscale band insulator capping layer with an ex situ ozone anneal. We show that this produces a nearly-pristine cation sublattice and preserves metallicity for at least several weeks. These results establish a reliable pathway for producing chemically stable SrFeO3 thin films, enabling reproducible studies of its unusual helimagnetism.

Two-dimensional (2D) magnets provide a versatile platform for exploring emergent quantum phases and developing next-generation spintronic devices. Despite this potential, high-throughput chemical vapor deposition (CVD) of ternary phase 2D magnets remains a significant challenge and is rarely explored. Here, we report a seeded-CVD method to synthesize nanoplates of a 2D magnetic material, Fe3GeTe2 (FGT), with lateral sizes of 10 μm and thicknesses of 20-80 nm. Synthesized nanoplates exhibit high Curie temperature (Tc ∼ 206 K) and large coercive field (∼1 T) based on reflective magnetic circular dichroism (RMCD) measurements. Cross-sectional scanning transmission electron microscopy and multislice electron ptychography directly reveal widespread and 3D-inhomogeneous Fe intercalation within the vdW gaps that is quantified to be Fe3+xGeTe2 with x ≈ 0.4, which resolves the atomic structural origins of the magnetic enhancement. These results enable a scalable route to synthesize 2D ternary magnet Fe3GeTe2 directly for property studies and device integration.

Next-generation semiconductor devices are adopting three-dimensional (3D) architectures with feature sizes in the few-nanometer regime, creating a need for atomic-scale metrology to identify and resolve performance-limiting fabrication challenges. X-ray methods provide 3D information but lack atomic resolution, while conventional electron microscopy offers limited depth sensitivity. Here we show how multislice electron ptychography, a computational microscopy technique with sub-Ångström lateral and nanometer-scale depth resolution, enables 3D imaging of buried device structures. We image prototype gate-all-around transistors and directly quantify roughness, strain, and defects at the interface of the 3D gate oxide wrapped around the channel. We find that silicon in the 5-nm-thick channel relaxes away from the interfaces, leaving only ~60% of atoms in a bulk-like structure. From a single dataset, ptychography provides quantitative metrology of atomic-scale interface roughness in 3D, previously accessible only through indirect inference, along with strain and other structural parameters needed for device modeling and process development.