The synthesis of high-quality Ruddlesden-Popper (RP) nickelates remains challenging due to variations in oxygen content and the prevalence of intergrown RP phases. Precisely controlling the stoichiometry and characterizing the resulting physical properties are essential for understanding the mechanism of high-T_c superconductivity in these materials. In this work, we synthesize a series of La_3Ni_2O_7+δ samples with systematically controlled oxygen content and perform comprehensive structural and compositional analyses. Precise oxygen tuning enables us to tailor the microstructure, yielding a pure bilayer phase, a mixture of bilayer and hybrid single-layer-bilayer phases, and a predominantly bilayer phase containing trilayer intergrowths. High-pressure transport measurements reveal distinct superconducting transitions with contrasting T_c values, corresponding to the bilayer phase, the hybrid phase, and trilayer inclusions. Notably, we find that oxygen content not only governs the phase purity-i.e., the presence of intergrowth phases-but also directly modulates the upper critical field (H_c2) of the bilayer superconductivity. By establishing a phase diagram of T_c and H_c2 as functions of oxygen content in La_3Ni_2O_7+δ, this work advances synthetic control and provides new insights into the superconducting mechanism of RP nickelates.

In lithium-excess layered oxide cathodes, the extra lithium within transition metal (TM) layers (i.e., Li[TM]) can trigger anionic redox to provide additional capacity. However, substantial extraction of Li[TM] may induce irreversible/detrimental local structural rearrangements. The conventional edge-shared connecting configuration between LiO6 and TMO6 octahedra facilitates interlayer migration of Li[TM]. In contrast, the face-shared configuration has the potential to limit the mobility of Li[TM], but this metastable configuration cannot be harvested via traditional calcination. Herein, during Na-to-Li ion exchange in P2 phase Na0.66[Li0.22TM0.78]O2 precursor, we observe that the random gliding of TMO2 layers yielding Li0.66[Li0.22TM0.78]O2 with O2/O6 intergrowth structure. Despite the random gliding, the functional face-shared configuration is successfully obtained and proven to effectively restrict interlayer migration of Li[TM] during charging. Consequently, we observe the suppressed formation of aggregated vacancies and O-O dimers within the TM layers. Ultimately, our synthesized O2/O6 Li-excess demonstrates enhanced structural reversibility and improved capacity/voltage retention. This oxygen-stacking engineering provides a compelling strategy for developing cathodes with enhanced local structural reversibility.

Cryogenic energy storage is vital for frontier technologies including deep-space exploration and quantum computing, yet conventional electrochemical energy systems fail below 230 K due to frozen ion migration. While relaxor-based dielectric capacitors provide high efficiency at room temperature, the intrinsic freezing/growth of polar nanodomains at extended cryogenic regime limits their applications with deteriorated hysteresis losses. Here, we realize superior cryogenic energy-storage performance by designing unit-cell-level disordered dipole-glass state in Pb0.6Sr0.4ZrO3 thin films with composition near antiferroelectric-paraelectric phase boundary. The antiferroelectric-derived dipole-glass introduces enhanced unit-cell-level complexity of dipole interaction that suppresses long-range ferroelectric order. This enables ultralow-hysteresis operation (efficiency > 88

Topological phases, as characterized by their topological invariants, have been considered as distinct states from the raw phases and hold great promise as tiny yet robust information carriers for the era of artificial intelligence1,2. However, these nontrivial states are typically found under non-equilibrium conditions, or stabilized by extrinsic electrical or mechanical boundary constraints3-6, which limit their applications. Particularly in ferroelectrics, it usually entails a maximized depolarization field produced by interfacial bound charges to balance the large elastic and gradient energies as dipole whirling at the atomic scale7-10. Despite substantial attempts, achieving highly ordered topological polar crystals in bulk ferroelectrics still remains a challenge11-14. Here we show that a two-dimensional polar hedgehog lattice with a period down to 4 nm can crystallize spontaneously free from any external boundary constraints in a family of A-site layer-ordered perovskites. Using advanced scanning transmission electron microscopy, we observe the polar hedgehog vortices in real space and disclose the physical nature as the cooperative assembly of modulated in-phase and out-of-phase octahedral rotations, further underpinned by hybrid improper ferroelectricity. Theoretical calculations show that the exchange interaction of phonons describing the octahedral rotations is the primary driving force of this intriguing dipole topology. Our findings not only clarify the ambiguity in the structure and origin of the widespread superstructure in layer-ordered perovskites but also demonstrate a viable framework for designing nontrivial structures and functionalities beyond perovskites.

Hydrogen plays critical roles in materials science, particularly for advancing technologies in hydrogen storage and phase manipulation, while also posing challenges like hydrogen embrittlement. Understanding its behavior, vital for improving material properties, requires precise determination of atomic-scale distribution-a persistent challenge due to hydrogen's weak electron scattering and high mobility, as well as the limitations of conventional transmission electron microscopy. We demonstrate that multislice electron ptychography (MEP) overcomes these constraints through three key advances: exceptional sensitivity for hydrogen occupancy, three-dimensional quantification, and picometer-level precision in atomic positioning. Experimentally, MEP resolves heterogeneous hydrogen distributions and quantifies hydrogen-induced lattice displacements with picometer precision in multi-principal-element alloy hydrides. This work demonstrates MEP as a transformative method for directly probing hydrogen atoms in solids, unlocking fundamental understanding of hydrogen's impact on material properties.

To overcome the spatial resolution limit set by aperture-limited diffraction in traditional scanning transmission electron microscopy, microscopists have developed ptychography enabled by iterative phase retrieval algorithms and high-dynamic-range pixel array detectors. Current detector designs are limited by the data rate off chip, so a high-pixel-count detector has a proportionally lower frame rate than the few-segment detectors used for differential phase contrast (DPC) imaging. This slower acquisition speed leads to heightened vulnerability to scan noise, drift, and potential sample damage. This creates opportunities for repurposing fast segmented detectors for ptychography by trading a reduction in reciprocal space pixels for an increase in real space pixels. Here, we explore a strategy of oversampling in real space and instead apply detector pixel upsampling during the reconstruction process. We demonstrate the viability of achieving super-resolution ptychography on thin objects using only 2 x 2 detector pixels, surpassing the resolution of integrated DPC (iDPC) imaging. With optimization using simulated datasets and experiments on MoTe2/WSe2 bilayer moir & eacute; superlattices, we achieved super-resolution ptychography reconstructions under rapid acquisition conditions (37.5 pA, 1 mu s dwell time), yielding over 50% improvements in contrast and information limit compared to annular dark field and iDPC imaging on the same detectors.

Real-space, three-dimensional imaging of atomic structures in materials science is a critical yet challenging task. Although scanning transmission electron microscopy has achieved sub-angstrom lateral resolution through techniques like electron ptychography, depth resolution remains limited to only 2 to 3 nanometers using single-projection setups. Attaining better depth resolution often requires large sample tilt angles and numerous projections, as demonstrated in atomic electron tomography. Here, we introduce an extension of multislice electron ptychography, which couples only a few small-angle projections to improve depth resolution by more than threefold, reaching the sub-nanometer scale and potentially approaching the atomic level. This technique maintains high resolving power for both light and heavy atoms, significantly enhancing the detection of individual dopants. We experimentally demonstrate three-dimensional visualization of dilute praseodymium dopants in a brownmillerite oxide, Ca2Co2O5, along with the accompanying lattice distortions. This approach can be implemented on widely available transmission electron microscopes equipped with hybrid pixel detectors, with data processing achievable using high-performance computing systems. The authors implement a computational strategy that couples electron diffraction datasets from a few small-angle projections to more than triple the depth resolution to sub-nanometers and improve dopant detection in bulk crystals.

The discovery of 80 K superconductivity in pressurized bilayer Ruddlesden-Popper (RP) nickelate La_3Ni_2O_7 has established a new high-temperature superconductor family. The quest to understand the governing principles of RP nickelate superconductivity has become a central focus in condensed matter physics. Here, we report a critical advance by synthesizing and investigating a distinct structural polymorph of the same compound: the monolayer-trilayer (1313) hybrid phase of La_3Ni_2O_7. Under high pressure, synchrotron X-ray diffraction and Raman spectroscopy reveal a structural transition from the orthorhombic Cmmm to the tetragonal P4/mmm space group at 13 GPa. Above 19 GPa, the phase exhibits a clear superconducting transition, confirmed by a zero-resistance state, albeit at a significantly reduced temperature of 3.6 K. The stark contrast with the 80 K transition in the bilayer phase provides a uniquely clean experimental comparison. Our results demonstrate that the superconducting transition temperature is directly governed by the nature of the interlayer coupling, and the bilayer NiO_6 block as the essential structural motif for achieving high-T_c superconductivity in the RP nickelates.

Freestanding oxide films offer significant potential for integrating exotic quantum functionalities with semiconductor technologies. However, their performance is critically limited by surface roughness and interfacial imperfection caused by dangling bonds, which disrupt coherent interactions and suppress quantum phenomena at heterointerfaces. To address the challenge of structural characterization of surfaces and interfaces, we develop a metrological approach achieving atomic-scale precision in mapping the topography of both free surfaces and buried interfaces within ultrathin oxide heterostructures leveraging three-dimensional structures reconstructed from multislice electron ptychography. This method also allows for counting the number of atoms, even including light elements such as oxygen, along the electron trajectory in electron microscopy, leading to the identification of surface termination in oxide films. The planar-view of measurement geometry, allowing for large field-of-view imaging, provides remarkably rich information and high statistics about the atomic-scale structural inhomogeneities in freestanding membranes. This quantitative analysis provides unprecedented capabilities for correlating structural imperfection with quantum device performance, offering critical insights for engineering robust heterointerfaces in next-generation oxide electronics.

The recent discovery of superconductivity in La3Ni2O7-δ under high pressure with a transition temperature around 80 K has sparked extensive experimental and theoretical efforts. Several key questions regarding the pairing mechanism remain to be answered, such as the most relevant atomic orbitals and the role of atomic deficiencies. Here, we develop a new energy-filtered multislice electron ptychography technique, assisted with electron energy loss spectroscopy, to address these critical issues. Oxygen vacancies are directly visualized and are found to primarily occupy the inner apical sites, which have been proposed to be crucial to superconductivity. We precisely determine the nanoscale stoichiometry and its correlation to the oxygen K edge spectra, which reveals a significant inhomogeneity in the oxygen content and electronic structure within the sample. The spectroscopic results also unveil that stoichiometric La3Ni2O7 is in the charge-transfer regime, with holes that are self-doped from Ni sites into O sites. The outer apical oxygen is found to be less relevant to the low-energy physics and can be safely disregarded in theoretical models. These observations will assist in further development and understanding of superconducting nickelate materials. Our imaging technique for quantifying atomic deficiencies can also be widely applied in materials science and condensed matter physics.

Doping impurity atoms is a strategy commonly used to tune the functionality of materials including catalysts, semiconductors, and quantum emitters. The location of dopants and their interaction with surrounding atoms could significantly modulate the transport, optical, or magnetic properties of materials. However, directly imaging individual impurity atoms inside materials remains a generally unaddressed need. Here, we demonstrate how single atoms can be detected and located in three dimensions via multislice electron ptychography.Interstitial atoms in a complex garnet oxide heterostructure are resolved with a depth resolution better than 2.7 nm, together with a deep-sub-Ångstrom lateral resolution. Single-scan atomic-layer depth resolution should be possible using strongly divergent electron probe illumination. Our results provide a new approach to detecting individual atomic defects and open doors to characterize the local environments and spatial distributions that underlie a broad range of systems such as single-atom catalysts, nitrogen-vacancy centers, and other atomic-scale quantum sensors.

Noncolinear spin textures, including chiral stripes and skyrmions, have shown great potential in spintronics. Basic configurations of spin textures are either Bloch or Néel types, and the intermediate hybrid type has rarely been reported. A major challenge in identifying hybrid spin textures is to quantitatively determine the hybrid angle, especially in ferrimagnets with weak net magnetization. Here, we develop an approach to quantify magnetic parameters, including chirality, saturation magnetization, domain wall width, and hybrid angle with sub-5 nm spatial resolution, based on Lorentz four-dimensional scanning transmission electron microscopy (Lorentz 4D-STEM). We find strong nanometer-scale variations in the hybrid angle and domain wall width within structurally and chemically homogeneous FeGd ferrimagnetic films. These variations fluctuate during different magnetization circles, revealing intrinsic local magnetization inhomogeneities. Furthermore, hybrid skyrmions can also be nucleated in FeGd films. These analyses demonstrate that the Lorentz 4D-STEM is a quantitative tool for exploring complex spin textures.

Topological spin textures have been proposed as candidate information carriers for next generation electronic devices [1].There are still many challenging problems to be solved before topological spin textures reach device level applications, such as small size, controlled generation/annihilation, and good current-driven properties.Revealing their internal structures can provide insightful information to address these issues [2].Lorentz electron microscopy techniques are among the highest resolution tools for magnetic imaging.However, quantitative measurements of key intrinsic properties of spin textures with high accuracy are still challenging [3,4].In this talk, I will present our recent efforts in quantifying spin textures using four-dimensional Lorentz scanning transmission electron microscopy (4D-LSTEM).LSTEM, operated in weak or zero external magnetic field conditions, requires specific electron optics that are not routinely available in commercial TEMs, although differential phase-contrast (DPC)-LSTEM has been used to image magnetic domain walls since the 1970s [5].Compared to other techniques, such as Lorentz TEM and electron holography, LSTEM was not commonly used until recently, partly due to the limited sensitivity and low throughput.4D-LSTEM techniques with a high dynamic range pixel array detector, such as EMPAD [6], can largely overcome these problems.Several works have demonstrated the advantages of 4D-LSTEM in efficiency and precision [7,8].We recently demonstrated that Lorentz electron ptychography (LEP) using 4D-LSTEM datasets can further improve the precision and outperform results from electron holography [9].I will show that high throughput can also be achieved using LEP with a large defocused probe.We have achieved field-of-view up to 10 × 10 μm 2 with resolution better than 3 nm.We also optimized imaging conditions to reach sub-nm resolution using the field-free mode of a conventional ThermoFisher electron microscope, which has an aberration-limited resolution worse than 5 nm.I will also show a few applications of 4D-LSTEM and LEP in imaging of nanoscale spin textures.Finally, I will discuss the main procedures and key parameters in the alignment and calibration of 4D-LSTEM [10].

Journal Article Robust Imaging of Three-dimensional Polar Textures Using 4D-STEM Diffraction Imaging and Multislice Electron Ptychography Get access Yu-Tsun Shao, Yu-Tsun Shao Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA, USASchool of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Search for other works by this author on: Oxford Academic Google Scholar Zhen Chen, Zhen Chen School of Materials Science and Engineering, Tsinghua University, Beijing, China Search for other works by this author on: Oxford Academic Google Scholar Chenyu Zhang, Chenyu Zhang School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Search for other works by this author on: Oxford Academic Google Scholar Harikrishnan K.P., Harikrishnan K.P. School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Search for other works by this author on: Oxford Academic Google Scholar David A Muller David A Muller School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USAKavli Institute at Cornell for Nanoscale Science, Ithaca, NY, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 29, Issue Supplement_1, 1 August 2023, Page 276, https://doi.org/10.1093/micmic/ozad067.126 Published: 22 July 2023