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C-H bond activation enables the facile synthesis of new chemicals. While C-H activation in short-chain alkanes has been widely investigated, it remains largely unexplored for long-chain organic molecules. Here, we report light-driven C-H activation in complex organic materials mediated by 2D transition metal dichalcogenides (TMDCs) and the resultant solid-state synthesis of luminescent carbon dots in a spatially-resolved fashion. We unravel the efficient H adsorption and a lowered energy barrier of C-C coupling mediated by 2D TMDCs to promote C-H activation. Our results shed light on 2D materials for C-H activation in organic compounds for applications in organic chemistry, environmental remediation, and photonic materials.

We report the synthesis and characterization of thin films of the Weylsemimetal NbAs grown on GaAs (100) and GaAs (111)B substrates. By choosing theappropriate substrate, we can stabilize the growth of NbAs in the (001) and(100) directions. We combine x-ray characterization with high-angle annulardark field scanning transmission electron microscopy to understand both themacroscopic and microscopic structure of the NbAs thin films. We show thatthese films are textured with domains that are tens of nanometers in size andthat, on a macroscopic scale, are mostly aligned to a single crystallinedirection. Finally, we describe electrical transport measurements that revealsimilar behavior in films grown in both crystalline directions, namely carrierdensities of $\sim 10^{21} - 10^{22} $

Four-dimensional scanning transmission electron microscopy (4D-STEM) hasrecently gained widespread attention for its ability to image atomic electricfields with sub-{\AA}ngstrom spatial resolution. These electric field mapsrepresent the integrated effect of the nucleus, core electrons and valenceelectrons, and separating their contributions is non-trivial. In this paper, weutilized simultaneously acquired 4D-STEM center of mass (CoM) images andannular dark field (ADF) images to determine the electron charge density inmonolayer MoS2. We find that both the core electrons and the valence electronscontribute to the derived electron charge density. However, due to blurring bythe probe shape, the valence electron contribution forms a nearly featurelessbackground while most of the spatial modulation comes from the core electrons.Our findings highlight the importance of probe shape in interpreting chargedensities derived from 4D STEM.

A spontaneous ferromagnetic moment can be induced in Bi$_{2}$Te$_{3}$ thinfilms below a temperature T $\approx$ 16 K by the introduction of Mn dopants.We demonstrate that films grown via molecular beam epitaxy with thestoichiometry Mn$_{0.14}$Bi$_{1.86}$Te$_3$ maintain the crystal structure ofpure Bi$_{2}$Te$_{3}$. The van der Waals nature of inter-layer forces in theMn$_{0.14}$Bi$_{1.86}$Te$_3$ crystal causes lattice mismatch with theunderlayer to have a limited effect on the resulting crystal structure, as wedemonstrate by thin film growth on tetragonal MgF$_{2}$ (110) and NiF$_{2}$(110). Electronic transport and magnetic moment measurements show that theferromagnetic moment of the Mn$_{0.14}$Bi$_{1.86}$Te$_3$ thin films is enhancedas the Fermi level moves from the bulk conduction band and towards the bulkband gap, suggesting that electronic surface states play an important role inmediating the ferromagnetic order. FerromagneticMn$_{0.14}$Bi$_{1.86}$Te$_3$/antiferromagnetic NiF$_{2}$ bilayers show evidencethat the ferromagnetic moment of the Mn$_{0.14}$Bi$_{1.86}$Te$_3$ film issuppressed, suggesting the existence of an interface effect between the twomagnetic layers.

Transmission electron microscopy (TEM) is a potent technique for thedetermination of three-dimensional atomic scale structure of samples instructural biology and materials science. In structural biology,three-dimensional structures of proteins are routinely determined usingphase-contrast single-particle cryo-electron microscopy from thousands ofidentical proteins, and reconstructions have reached atomic resolution forspecific proteins. In materials science, three-dimensional atomic structures ofcomplex nanomaterials have been determined using a combination of annular darkfield (ADF) scanning transmission electron microscopic (STEM) tomography andsubpixel localization of atomic peaks, in a method termed atomic electrontomography (AET). However, neither of these methods can determine thethree-dimensional atomic structure of heterogeneous nanomaterials containinglight elements. Here, we perform mixed-state electron ptychography from 34.5million diffraction patterns to reconstruct a high-resolution tilt series of adouble wall-carbon nanotube (DW-CNT), encapsulating a complex $\mathrm{ZrTe}$sandwich structure. Class averaging of the resulting reconstructions andsubpixel localization of the atomic peaks in the reconstructed volume revealsthe complex three-dimensional atomic structure of the core-shellheterostructure with 17 picometer precision. From these measurements, we solvethe full $\mathrm{Zr_{11}Te_{50}}$ structure, which contains a previouslyunobserved $\mathrm{ZrTe_{2}}$ phase in the core. The experimental realizationof ptychographic atomic electron tomography (PAET) will allow for structuraldetermination of a wide range of nanomaterials which are beam-sensitive orcontain light elements.

We report the development of deep learning coherent electron diffractiveimaging at sub-angstrom resolution using convolutional neural networks (CNNs)trained with only simulated data. We experimentally demonstrate this method byapplying the trained CNNs to directly recover the phase images from electrondiffraction patterns of twisted hexagonal boron nitride, monolayer graphene anda Au nanoparticle with comparable quality to those reconstructed by aconventional ptychographic method. Fourier ring correlation between the CNN andptychographic images indicates the achievement of a spatial resolution in therange of 0.70 and 0.55 angstrom (depending on different resolution criteria).The ability to replace iterative algorithms with CNNs and perform real-timeimaging from coherent diffraction patterns is expected to find broadapplications in the physical and biological sciences.

Heterogeneous catalysts play a key role in the chemical and energy industries 1 . To date, most industrial-scale heterogeneous catalytic reactions have relied on nanocatalysts 2,3 . However, despite significant progress from theoretical, experimental and computational studies 4-18 , identifying the active sites of alloy nanocatalysts remains a major challenge. This limitation is mainly due to an incomplete understanding of the three-dimensional (3D) atomic and chemical arrangement of different constituents and structural reconstructions driven by catalytic reactions 19-22 . Here, we use atomic electron tomography 23 to determine the 3D local atomic structure, surface morphology and chemical composition of 11 Pt alloy nanocatalysts for the electrochemical oxygen reduction reaction (ORR). We reveal the facet, surface concaveness, structural and chemical order/disorder, coordination number, and bond length with unprecedented 3D atomic detail. The experimental 3D atomic coordinates are used by first-principles trained machine learning to identify the active sites of the nanocatalysts, which are corroborated by electrochemical measurements. A striking feature is the difference of the ORR activity of the surface Pt sites on the nanocatalysts by several orders of magnitude. Furthermore, by analyzing the structure-activity relationship, we formulate an equation named the local environment descriptor to balance the strain and ligand effects and gain quantitative insights into the ORR active sites of the Pt alloy nanocatalysts. The ability to determine the 3D atomic structure and chemical composition of realistic nanoparticles coupled with machine learning could transform our fundamental understanding of the catalytic active sites and provide a guidance for the rational design of optimal nanocatalysts.

We present micromagnetic simulations on resonant spin wave modes of magnetic Hopfions up to 15 GHz driven by external magnetic fields. A sharp transition is found around 66 mT coinciding with a transition from Hopfions to magnetic torons. The modes exhibit characteristic amplitudes in frequency space accompanied by unique localization patterns in real space and are found to be robust to damping around topological features, particularly vortex lines in Hopfions and Bloch points in torons. The marked differences in spin wave spectra between Hopfions, torons, and target skyrmions can serve as fingerprints in future experimental validation studies of these novel 3D topological spin textures.

Compressed sensing has become a widely accepted paradigm to construct high dimensional cluster expansion models used for statistical mechanical studies of atomic configuration in complex multicomponent crystalline materials. However, strict sampling requirements necessary to obtain minimal coherence measurements for compressed sensing to guarantee accurate estimation of model parameters are difficult and in some cases impossible to satisfy due to the inability of physical systems to access certain configurations. Nevertheless, the dependence of energy on atomic configuration can still be adequately learned without these strict requirements by using compressed sensing by way of coherent measurements using redundant function sets known as frames. We develop a particular frame constructed from the union of all occupancy-based cluster expansion basis sets. We illustrate how using this highly redundant frame yields sparse expansions of the configuration energy of complex oxide materials that are competitive and often surpass the prediction accuracy and sparsity of models obtained from standard cluster expansions.

The BaAl4 prototype crystal structure is the most populous of all structure types, and is the building block for a diverse set of sub-structures including the famous ThCr2Si2 family that hosts high-temperature superconductivity and numerous magnetic and strongly correlated electron systems. The MA4 family of materials (M = Sr, Ba, Eu; A = Al, Ga, In) themselves present an intriguing set of ground states including charge and spin orders, but have largely been considered as uninteresting metals. We predict the exemplary compound BaAl4 to harbor a three-dimensional Dirac spectrum with non-trivial topology and possible nodal lines crossing the Brillouin zone, wherein one pair of semi-Dirac points with linear dispersion along the kz direction and quadratic dispersion along the kx/ky direction resides on the rotational axis with C4v point group symmetry. An extremely large, unsaturating positive magnetoresistance in BaAl4 despite an uncompensated band structure is revealed, and quantum oscillations and angle-resolved photoemission spectroscopy measurements confirm the predicted multiband semimetal structure with pockets of Dirac holes and a Van Hove singularity (VHS) remarkably consistent with the theoretical prediction. We thus present BaAl4 as a topological semimetal, casting its prototype status into a role as a building block for a vast array of topological materials.

When a three-dimensional material is constructed by stacking different two-dimensional layers into an ordered structure, new and unique physical properties can emerge. An example is the delafossite PdCoO2, which consists of alternating layers of metallic Pd and Mott-insulating CoO2 sheets. To understand the nature of the electronic coupling between the layers that gives rise to the unique properties of PdCoO2, we revealed its layer-resolved electronic structure combining standing-wave X-ray photoemission spectroscopy and ab initio many-body calculations. Experimentally, we have decomposed the measured VB spectrum into contributions from Pd and CoO2 layers. Computationally, we find that many-body interactions in Pd and CoO2 layers are highly different. Holes in the CoO2 layer interact strongly with charge-transfer excitons in the same layer, whereas holes in the Pd layer couple to plasmons in the Pd layer. Interestingly, we find that holes in states hybridized across both layers couple to both types of excitations (charge-transfer excitons or plasmons), with the intensity of photoemission satellites being proportional to the projection of the state onto a given layer. This establishes satellites as a sensitive probe for inter-layer hybridization. These findings pave the way towards a better understanding of complex many-electron interactions in layered quantum materials.

Atomic-scale characteristics of surfaces dictate the principles governingnumerous scientific phenomena ranging from catalysis to friction. Despite thisfact, our ability to visualize and alter surfaces on the atomic scale isseverely hampered by the strict conditions under which the related methods areoperated to achieve high spatial resolution. In particular, the two prominentmethods utilized to achieve atomic-resolution imaging - scanning tunnelingmicroscopy (STM) and noncontact atomic force microscopy (NC-AFM) - aretypically performed under ultrahigh vacuum (UHV) and often at low temperatures.Perhaps more importantly, results obtained under such well-controlled, pristineconditions bear little relevance for the great majority of processes andapplications that often occur under ambient conditions. As such, a method thatcan robustly image surfaces on the atomic scale under ambient conditions haslong been thought of as a "holy grail" of surface science. Here, by way of aproof-of-principle measurement on molybdenum disulfide (MoS2), we report thatthe method of conductive atomic force microscopy (C-AFM) can be utilized toachieve true atomic-resolution imaging under ambient conditions as proven bythe imaging of a single atomic vacancy, without any control over theoperational environment or elaborate sample preparation. While the physicalmechanisms behind this remarkable observation are not elucidated yet, ourapproach overcomes many of the classical limitations associated with STM andNC-AFM, and the findings herald the potential emergence of C-AFM as a powerfultool for atomic-resolution imaging under ambient conditions.

Novel phenomena in magnetically-intercalated graphite has been a subject ofmuch research, pioneered and promoted by M.~S. and G.~Dresselhaus and manyothers in the 1980s. Among the most enigmatic findings of that era was adramatic, roller-coaster-like behavior of the magnetoresistivity in EuC$_6$compound, in which magnetic Eu$^{2+}$ ions form a triangular lattice that iscommensurate to graphite honeycomb planes. In this study, we provide along-awaited {\it microscopic} explanation of this behavior, demonstrating thatthe resistivity of EuC$_6$ is dominated by spin excitations in Eu-planes andtheir highly nontrivial evolution with the magnetic field. Together with ourtheoretical analysis, the present study showcases the power of the synthetic 2Dmaterials as a source of potentially significant insights into the nature ofexotic spin excitations.

Quantum materials hosting Weyl fermions have opened a new era of research in condensed matter physics. First proposed in 1929 in the context of particle physics, Weyl fermions have yet to be observed as elementary particles. In 2015, Weyl fermions were detected as collective electronic excitations in the strong spin–orbit coupled material tantalum arsenide, TaAs. This discovery was followed by a flurry of experimental and theoretical explorations of Weyl phenomena in materials. Weyl materials naturally lend themselves to the exploration of the topological index associated with Weyl fermions and their divergent Berry curvature field, as well as the topological bulk–boundary correspondence, giving rise to protected conducting surface states. Here, we review the broader class of Weyl topological phenomena in materials, starting with the observation of emergent Weyl fermions in the bulk and Fermi arc states on the surface of the TaAs family of crystals by photoemission spectroscopy. We then discuss several exotic optical and magnetic responses observed in these materials, as well as progress in developing related chiral materials. We discuss the conceptual development of high-fold chiral fermions, which generalize Weyl fermions, and we review the observation of high-fold chiral fermion phases by taking the rhodium silicide, RhSi, family of crystals as a prime example. Lastly, we discuss recent advances in Weyl line phases in magnetic topological materials. With this Review, we aim to provide an introduction to the basic concepts underlying Weyl physics in condensed matter, and to representative materials and their electronic structures and topology as revealed by spectroscopic studies. We hope this work serves as a guide for future theoretical and experimental explorations of chiral fermions and related topological quantum systems with potentially enhanced functionalities.

New interaction potentials were developed for molecular dynamics simulations to study the role of Mg and Ca in modifying the structure and properties of alkaline earth silicates and borates. Competition between the depolymerization of the silica network and the formation of new bonds between oxygen atoms and modifiers leads to the enhancement of the elastic moduli with increasing modifier content in alkaline earth silicate glasses. Compared with calcium silicate, the higher elastic moduli of magnesium silicate result from a higher connectivity of the overall glass network due to the incorporation of fourfold coordinated magnesium and a more rigid connection between oxygen atoms and modifiers. In contrast to the silicates, the effect of modifier on the elastic moduli of alkaline earth borates is dominated by the formation of fourfold coordinated boron (N4). Calcium borate with higher N4 shows a more rigid network structure and higher elastic moduli.

Phase-contrast transmission electron microscopy (TEM) is a powerful tool for imaging the local atomic structure of materials. TEM has been used heavily in studies of defect structures of two-dimensional materials such as monolayer graphene due to its high dose efficiency. However, phase-contrast imaging can produce complex nonlinear contrast, even for weakly scattering samples. It is, therefore, difficult to develop fully automated analysis routines for phase-contrast TEM studies using conventional image processing tools. For automated analysis of large sample regions of graphene, one of the key problems is segmentation between the structure of interest and unwanted structures such as surface contaminant layers. In this study, we compare the performance of a conventional Bragg filtering method with a deep learning routine based on the U-Net architecture. We show that the deep learning method is more general, simpler to apply in practice, and produces more accurate and robust results than the conventional algorithm. We provide easily adaptable source code for all results in this paper and discuss potential applications for deep learning in fully automated TEM image analysis.

New interaction potentials were developed for molecular dynamics simulations to study the role of Mg and Ca in modifying the structure and properties of alkaline earth silicates and borates. Competition between the depolymerization of the silica network and the formation of new bonds between oxygen atoms and modifiers leads to the enhancement of the elastic moduli with increasing modifier content in alkaline earth silicate glasses. Compared with calcium silicate, the higher elastic moduli of magnesium silicate result from a higher connectivity of the overall glass network due to the incorporation of fourfold coordinated magnesium and a more rigid connection between oxygen atoms and modifiers. In contrast to the silicates, the effect of modifier on the elastic moduli of alkaline earth borates is dominated by the formation of fourfold coordinated boron (N4). Calcium borate with higher N4 shows a more rigid network structure and higher elastic moduli.

High-throughput grain mapping with sub-nanometer spatial resolution is demonstrated using scanning nanobeam electron diffraction (also known as 4D scanning transmission electron microscopy, or 4D-STEM) combined with high-speed direct-electron detection. An electron probe size down to 0.5 nm in diameter is used and the sample investigated is a gold–palladium nanoparticle catalyst. Computational analysis of the 4D-STEM data sets is performed using a disk registration algorithm to identify the diffraction peaks followed by feature learning to map the individual grains. Two unsupervised feature learning techniques are compared: principal component analysis (PCA) and non-negative matrix factorization (NNMF). The characteristics of the PCA versus NNMF output are compared and the potential of the 4D-STEM approach for statistical analysis of grain orientations at high spatial resolution is discussed.

van der Waals materials exhibit naturally passivated surfaces and an ability to form versatile heterostructures to enable an examination of carrier transport mechanisms not seen in traditional materials. Here, we report a new type of homojunction termed a "band-bending junction" whose potential landscape depends solely on the difference in thickness between the two sides of the junction. Using MoS2 on Au as a prototypical example, we find that surface potential differences can arise from the degree of vertical band bending in thin and thick regions. Furthermore, by using scanning ultrafast electron microscopy, we examine the spatiotemporal dynamics of charge carriers generated at this junction and find that lateral carrier separation is enabled by differences in the band bending in the vertical direction, which we verify with simulations. Band-bending junctions may therefore enable new optoelectronic devices that rely solely on band bending arising from thickness variations to separate charge carriers.

Van der Waals heteroepitaxy allows deterministic control over lattice mismatch or azimuthal orientation between atomic layers to produce long-wavelength superlattices. The resulting electronic phases depend critically on the superlattice periodicity and localized structural deformations that introduce disorder and strain. In this study we used Bragg interferometry to capture atomic displacement fields in twisted bilayer graphene with twist angles

Mastery of order-disorder processes in highly nonequilibrium nanostructured oxides has significant implications for the development of emerging energy technologies. However, we are presently limited in our ability to quantify and harness these processes at high spatial, chemical, and temporal resolution, particularly in extreme environments. Here, we describe the percolation of disorder at the model oxide interface LaMnO3/SrTiO3, which we visualize during in situ ion irradiation in the transmission electron microscope. We observe the formation of a network of disorder during the initial stages of ion irradiation and track the global progression of the system to full disorder. We couple these measurements with detailed structural and chemical probes, examining possible underlying defect mechanisms responsible for this unique percolative behavior.

Autonomous synthesis and characterization of inorganic materials require the automatic and accurate analysis of X-ray diffraction spectra. For this task, we designed a probabilistic deep learning algorithm to identify complex multi-phase mixtures. At the core of this algorithm lies an ensemble convolutional neural network trained on simulated diffraction spectra, which are systematically augmented with physics-informed perturbations to account for artifacts that can arise during experimental sample preparation and synthesis. Larger perturbations associated with off-stoichiometry are also captured by supplementing the training set with hypothetical solid solutions. Spectra containing mixtures of materials are analyzed with a newly developed branching algorithm that utilizes the probabilistic nature of the neural network to explore suspected mixtures and identify the set of phases that maximize confidence in the prediction. Our model is benchmarked on simulated and experimentally measured diffraction spectra, showing exceptional performance with accuracies exceeding those given by previously reported methods based on profile matching and deep learning. We envision that the algorithm presented here may be integrated in experimental workflows to facilitate the high-throughput and autonomous discovery of inorganic materials.

We present INQ, a new implementation of density functional theory (DFT) andtime-dependent DFT (TDDFT) written from scratch to work on graphical processingunits (GPUs). Besides GPU support, INQ makes use of modern code design featuresand takes advantage of newly available hardware. By designing the code aroundalgorithms, rather than against specific implementations and numericallibraries, we aim to provide a concise and modular code. The result is a fairlycomplete DFT/TDDFT implementation in roughly 12,000 lines of open-source C++code representing a modular platform for community-driven applicationdevelopment on emerging high-performance computing architectures for thesimulation of materials.

Diamine-appended metal-organic frameworks (MOFs) of the form Mg2(dobpdc)(diamine)2 adsorb CO2 in a cooperative fashion, exhibiting an abrupt change in CO2 occupancy with pressure or temperature. This change is accompanied by hysteresis. While hysteresis is suggestive of a first-order phase transition, we show that hysteretic temperature-occupancy curves associated with this material are qualitatively unlike the curves seen in the presence of a phase transition; they are instead consistent with CO2 chain polymerization, within one-dimensional channels in the MOF, in the absence of a phase transition. Our simulations of a microscopic model reproduce this dynamics, providing a physical understanding of cooperative adsorption in this industrially important class of materials.

Perovskite SrTiO3 (STO) is an attractive photocatalyst for solar watersplitting, but suffers from a limited photoresponse in the ultraviolet spectralrange due to its wide band gap. By means of hybrid density functional theorycalculations, we systematically study engineering its band gap via doping 4dand 5d transition metals M (M=Zr, Nb, Mo, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Irand Pt) and chalcogen elements Y (Y=S and Se). We find that transition metaldopant M either has no effect on STO band gap or introduces detrimental mid-gapstates, except for Pd and Pt that are able to reduce the STO band gap. Incontrast, doping S and Se significantly reduces STO's direct band gap, thusleading to appreciable optical absorption transitions in the visible spectralrange. Our findings provide that Pd, S and Se doped STO are potential promisingphotocatalysts for water splitting under visible light irradiation, therebyproviding insightful theoretical guides for experiments to improve thephotocatalytic activity of STO.

Solid-state synthesis from powder precursors is the primary processing route to advanced multicomponent ceramic materials. Designing reaction conditions and precursors for ceramic synthesis can be a laborious, trial-and-error process, as heterogeneous mixtures of precursors often evolve through a complicated series of reaction intermediates. Here, ab initio thermodynamics is used to model which pair of precursors has the most reactive interface, enabling the understanding and anticipation of which non-equilibrium intermediates form in the early stages of a solid-state reaction. In situ X-ray diffraction and in situ electron microscopy are then used to observe how these initial intermediates influence phase evolution in the synthesis of the classic high-temperature superconductor YBa2 Cu3 O6+ x   (YBCO). The model developed herein rationalizes how the replacement of the traditional BaCO3 precursor with BaO2 redirects phase evolution through a low-temperature eutectic melt, facilitating the formation of YBCO in 30 min instead of 12+ h. Precursor selection plays an important role in tuning the thermodynamics of interfacial reactions and emerges as an important design parameter in planning kinetically favorable synthesis pathways to complex ceramic materials.

We report the structural and magnetic ground state properties of the monoclinic compound barium iridium oxide Ba4Ir3O10 using a combination of resonant x-ray scattering, magnetometry, and thermodynamic techniques. Magnetic susceptibility exhibits a pronounced antiferromagnetic transition at TN≈25 K, a weaker anomaly at TS≈142 K, and strong magnetic anisotropy at all temperatures. Resonant elastic x-ray scattering experiments reveal a second order structural phase transition at TS and a magnetic transition at TN. Both structural and magnetic superlattice peaks are observed at L = half integer values. The magnetization anomaly at TS implies the presence of magnetoelastic coupling, which conceivably facilitates the symmetry lowering. Mean field critical scattering is observed above TS. The magnetic structure of the antiferromagnetic ground state is discussed based on the measured magnetic superlattice peak intensity. Our study not only presents essential information for understanding the intertwined structural and magnetic properties in Ba4Ir3O10 but also highlights the necessary ingredients for exploring novel ground states with octahedra trimers.

The advent of long-range magnetic order in non-centrosymmetric compounds hasstimulated interest in the possibility of exotic spin transport phenomena andtopologically protected spin textures for applications in next-generationspintronics. This work reports a novel wurtzite-structure polar magnetic metal,identified as AA'-stacked (Fe0.5Co0.5)5-xGeTe2, which exhibits a Neel-typeskyrmion lattice as well as a Rashba-Edelstein effect at room temperature.Atomic resolution imaging of the structure reveals a structural transition as afunction of Co-substitution, leading to the polar phase at 50% Co. Thisdiscovery reveals an unprecedented layered polar magnetic system forinvestigating intriguing spin topologies and ushers in a promising newframework for spintronics.

Stochastic density functional theory (sDFT) is becoming a valuable tool for studying ground-state properties of extended materials. The computational complexity of describing the Kohn-Sham orbitals is replaced by introducing a set of random (stochastic) orbitals leading to linear and often sub-linear scaling of certain ground-state observables at the account of introducing a statistical error. Schemes to reduce the noise are essential, for example, for determining the structure using the forces obtained from sDFT. Recently, we have introduced two embedding schemes to mitigate the statistical fluctuations in the electron density and resultant forces on the nuclei. Both techniques were based on fragmenting the system either in real space or slicing the occupied space into energy windows, allowing for a significant reduction in the statistical fluctuations. For chemical accuracy, further reduction of the noise is required, which could be achieved by increasing the number of stochastic orbitals. However, the convergence is relatively slow as the statistical error scales as 1/Nχ according to the central limit theorem, where Nχ is the number of random orbitals. In this paper, we combined the embedding schemes mentioned above and introduced a new approach that builds on overlapped fragments and energy windows. The new approach significantly lowers the noise for ground-state properties, such as the electron density, total energy, and forces on the nuclei, as demonstrated for a G-center in bulk silicon.

Polar textures have attracted significant attention in recent years as apromising analog to spin-based textures in ferromagnets. Here, using opticalsecond harmonic generation based circular dichroism, we demonstratedeterministic and reversible control of chirality over mesoscale regions inferroelectric vortices using an applied electric field. The microscopic originsof the chirality, the pathway during the switching, and the mechanism forelectric-field control are described theoretically via phase-field modeling andsecond-principles simulations, and experimentally by examination of themicroscopic response of the vortices under an applied field. The emergence ofchirality from the combination of non-chiral materials and subsequent controlof the handedness with an electric field has far-reaching implications for newelectronics based on chirality as a field controllable order parameter.

Topological superconductors (TSCs) are unconventional superconductors with bulk superconducting gap and in-gap Majorana states on the boundary that may be used as topological qubits for quantum computation. Despite their importance in both fundamental research and applications, natural TSCs are very rare. Here, combining state of the art synchrotron and laser-based angle-resolved photoemission spectroscopy, we investigated a stoichiometric transition metal dichalcogenide (TMD), 2M-WS2 with a superconducting transition temperature of 8.8 K (the highest among all TMDs in the natural form up to date) and observed distinctive topological surface states (TSSs). Furthermore, in the superconducting state, we found that the TSSs acquired a nodeless superconducting gap with similar magnitude as that of the bulk states. These discoveries not only evidence 2M-WS2 as an intrinsic TSC without the need of sensitive composition tuning or sophisticated heterostructures fabrication, but also provide an ideal platform for device applications thanks to its van der Waals layered structure.

Halide perovskites are strongly influenced by large amplitude anharmoniclattice fluctuations at room temperature. We develop a tight binding model fordynamically disordered MAPbI$_3$ based on density functional theory (DFT)calculations to calculate electronic structure for finite temperature crystalstructures at the length scale of thermal disorder and carrier localization.The model predicts individual Hamiltonian matrix elements and band structureswith high accuracy, owing to the inclusion of additional matrix elements anddescriptors for non-Coulombic interactions. We apply this model to electronicstructure at length and time scales inaccessible to first principles methods,finding an increase in band gap, carrier mass, and the sub-picosecondfluctuations in these quantities with increasing temperature as well as theonset of carrier localization in large supercells induced by thermal disorderat 300 K. We identify the length scale $L^*= 5$ nm as the onset of localizationin the electronic structure, associated with associated with decreasing bandedge fluctuations, increasing carrier mass, and Rashba splitting approachingzero.

The prediction of material properties through electronic-structuresimulations based on density-functional theory has become routinely common,thanks, in part, to the steady increase in the number and robustness ofavailable simulation packages. This plurality of codes and methods aiming tosolve similar problems is both a boon and a burden. While providing greatopportunities for cross-verification, these packages adopt different methods,algorithms, and paradigms, making it challenging to choose, master, andefficiently use any one for a given task. Leveraging recent advances inmanaging reproducible scientific workflows, we demonstrate how developingcommon interfaces for workflows that automatically compute material propertiescan tackle the challenge mentioned above, greatly simplifying interoperabilityand cross-verification. We introduce design rules for reproducible and reusablecode-agnostic workflow interfaces to compute well-defined material properties,which we implement for eleven different quantum engines and use to computethree different material properties. Each implementation encodes carefullyselected simulation parameters and workflow logic, making the implementer'sexpertise of the quantum engine directly available to non-experts. Fullprovenance and reproducibility of the workflows is guaranteed through the useof the AiiDA infrastructure. All workflows are made available as open-sourceand come pre-installed with the Quantum Mobile virtual machine, making theiruse straightforward.

In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter, and for their applications in advanced technological fields. Among these compounds, LaB6 has a special place, being a traditional d-band metal without additional f bands. In order to understand the bulk electronic structure of the more complex rare-earth hexaborides, in this paper we investigate the bulk electronic structure of LaB6 using tender/hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. Furthermore, we compare the La 3d core level spectrum to cluster model calculations in order to understand the bulklike core-hole screening effects. The results show that the La 3d well-screened peak is at a lower binding energy compared to the main poorly screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulklike band structure of LaB6 determined by tender/hard x-ray angle-resolved photoemission spectroscopy (HARPES). We compare HARPES experimental results to the free-electron final-state calculations and to the more precise one-step photoemission theory including matrix element and phonon excitation effects. The agreement between the features present in the experimental ARPES data and the theoretical calculations is very good. In addition, we consider the nature and the magnitude of phonon excitations in order to interpret HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that the one-step theory of photoemission and HARPES experiments provides, at present, the only approach capable of probing, both experimentally and theoretically, true "bulklike"electronic band structure of rare-earth hexaborides and strongly correlated materials.

In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter, and for their applications in advanced technological fields. Among these compounds, LaB6 has a special place, being a traditional d-band metal without additional f bands. In order to understand the bulk electronic structure of the more complex rare-earth hexaborides, in this paper we investigate the bulk electronic structure of LaB6 using tender/hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. Furthermore, we compare the La 3d core level spectrum to cluster model calculations in order to understand the bulklike core-hole screening effects. The results show that the La 3d well-screened peak is at a lower binding energy compared to the main poorly screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulklike band structure of LaB6 determined by tender/hard x-ray angle-resolved photoemission spectroscopy (HARPES). We compare HARPES experimental results to the free-electron final-state calculations and to the more precise one-step photoemission theory including matrix element and phonon excitation effects. The agreement between the features present in the experimental ARPES data and the theoretical calculations is very good. In addition, we consider the nature and the magnitude of phonon excitations in order to interpret HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that the one-step theory of photoemission and HARPES experiments provides, at present, the only approach capable of probing, both experimentally and theoretically, true "bulklike"electronic band structure of rare-earth hexaborides and strongly correlated materials.

We present micromagnetic simulations on resonant spin wave modes of magneticHopfions up to 15 GHz driven by external magnetic fields. A sharp transition isfound around 32 mT coinciding with a transition from Hopfions to magnetictorons. The modes exhibit characteristic amplitudes in frequency spaceaccompanied by unique localization patterns in real space, and are found to berobust to damping around topological features, particularly vortex lines inHopfions and Bloch points in torons. The marked differences in spin wavespectra between Hopfions, torons and target skyrmions can serve as fingerprintsin future experimental validation studies of these novel 3d topological spintextures.

It has been shown previously that the presence of a Dzyaloshinskii-Moriyainteraction in perpendicularly magnetized thin films stabilizes N\'eel typedomain walls. We demonstrate, using micromagnetic simulations and analyticalmodeling, that the presence of a uniaxial in-plane magnetic anisotropy can alsolead to the formation of N\'eel walls in the absence of a Dzyaloshinskii-Moriyainteraction. It is possible to abruptly switch between Bloch and N\'eel wallsvia a small modulation of both the in-plane, but also the perpendicularmagnetic anisotropy. This opens up a route towards electric field control ofthe domain wall type with small applied voltages through electric fieldcontrolled anisotropies.

Machine learning has emerged as a powerful approach in materials discovery. Its major challenge is selecting features that create interpretable representations of materials, useful across multiple prediction tasks. We introduce an end-to-end machine learning model that automatically generates descriptors that capture a complex representation of a material's structure and chemistry. This approach builds on computational topology techniques (namely, persistent homology) and word embeddings from natural language processing. It automatically encapsulates geometric and chemical information directly from the material system. We demonstrate our approach on multiple nanoporous metal-organic framework datasets by predicting methane and carbon dioxide adsorption across different conditions. Our results show considerable improvement in both accuracy and transferability across targets compared to models constructed from the commonly-used, manually-curated features, consistently achieving an average 25-30% decrease in root-mean-squared-deviation and an average increase of 40-50% in R2 scores. A key advantage of our approach is interpretability: Our model identifies the pores that correlate best to adsorption at different pressures, which contributes to understanding atomic-level structure-property relationships for materials design.

We study how the intrinsic anomalous Hall conductivity is modified in two-dimensional crystals with broken time-reversal symmetry due to weak inhomogeneity of the applied electric field. Focusing on a clean noninteracting two-band system without band crossings, we derive the general expression for the Hall conductivity at small finite wave vector q to order q^{2}, which governs the Hall response to the second gradient of the electric field. Using the Kubo formula, we show that the answer can be expressed through the Berry curvature, Fubini-Study quantum metric, and the rank-3 symmetric tensor which is related to the quantum geometric connection and physically corresponds to the gauge-invariant part of the third cumulant of the position operator. We further compare our results with the predictions made within the semiclassical approach. By deriving the semiclassical equations of motion, we reproduce the result obtained from the Kubo formula in some limits. We also find, however, that the conventional semiclassical description in terms of the definite position and momentum of the electron is not fully consistent because of singular terms originating from the Heisenberg uncertainty principle. We thus present a clear example of a case when the semiclassical approach inherently suffers from the uncertainty principle, implying that it should be applied to systems in nonuniform fields with extra care.

Controlling electronic properties via band structure engineering is at the heart of modern semiconductor devices. Here, we extend this concept to semimetals where, using LuSb as a model system, we show that quantum confinement lifts carrier compensation and differentially affects the mobility of the electron and hole-like carriers resulting in a strong modification in its large, nonsaturating magnetoresistance behavior. Bonding mismatch at the heteroepitaxial interface of a semimetal (LuSb) and a semiconductor (GaSb) leads to the emergence of a two-dimensional, interfacial hole gas. This is accompanied by a charge transfer across the interface that provides another avenue to modify the electronic structure and magnetotransport properties in the ultrathin limit. Our work lays out a general strategy of using confined thin-film geometries and heteroepitaxial interfaces to engineer electronic structure in semimetallic systems, which allows control over their magnetoresistance behavior and simultaneously provides insights into its origin.

The Dzyaloshinskii-Moriya interaction (DMI) has drawn much attention, as it stabilizes magnetic chirality, with important implications in fundamental and applied research. This antisymmetric exchange interaction is induced by the broken inversion symmetry at interfaces or in noncentrosymmetric lattices. Significant interfacial DMIs are often found at magnetic/heavy-metal interfaces with large spin-orbit coupling. Recent studies have shown promise for induced DMI at interfaces involving light elements such as carbon (graphene) or oxygen. Here, we report direct observation of induced DMI by chemisorption of the lightest element, hydrogen, on a ferromagnetic layer at room temperature, which is supported by density functional theory calculations. We further demonstrate a reversible chirality transition of the magnetic domain walls due to the induced DMI via hydrogen chemisorption and desorption. These results shed new light on the understanding of DMI in low atomic number materials and the design of novel chiral spintronics and magneto-ionic devices.

The ongoing COVID-19 pandemic highlights the severe health risks posed by deep submicrometer-sized airborne viruses and particulates in the spread of infectious diseases. There is an urgent need for the development of efficient, durable, and reusable filters for this size range. Here we report the realization of efficient particulate filters using nanowire-based low-density metal foams which combine extremely large surface areas with excellent mechanical properties. The metal foams exhibit outstanding filtration efficiencies (>96.6%) in the PM0.3 regime, with the potential for further improvement. Their mechanical stability, light weight, chemical and radiation resistance, ease of cleaning and reuse, and recyclability further make such metal foams promising filters for combating COVID-19 and other types of airborne particulates.

Imposing additional confinement in two-dimensional (2D) materials yields further control over their electronic, optical, and topological properties. However, synthesis of ultranarrow nanoribbons (NRs) remains challenging, particularly for transition metal dichalcogenides (TMDs), and synthesizing TMD NRs narrower than 50 nm has remained elusive. Here, we report the vapor-phase synthesis of ultranarrow TaS2 NRs. The NRs are grown within carbon nanotubes, limiting their width and layer number, while stabilizing them against the environment. The NRs reach monolayer thickness and exhibit widths down to 2.5 nm. Atomic-resolution scanning transmission electron microscopy reveals the detailed atomic structure of the ultranarrow NRs and we observe a hitherto unseen atomic structure supermodulation of ordered defect arrays within the NRs. Density functional theory calculations show the presence of flat bands and boundary-localized states, and help identify the atomic configuration of the supermodulation. Nanotube-templated synthesis represents a unique, transferable, and broadly deployable route toward ultranarrow TMD NR growth.

The electronic transport behaviour of materials determines their suitability for technological applications. We develop a computationally efficient method for calculating carrier scattering rates of solid-state semiconductors and insulators from first principles inputs. The present method extends existing polar and non-polar electron-phonon coupling, ionized impurity, and piezoelectric scattering mechanisms formulated for isotropic band structures to support highly anisotropic materials. We test the formalism by calculating the electronic transport properties of 23 semiconductors, including the large 48 atom CH3NH3PbI3 hybrid perovskite, and comparing the results against experimental measurements and more detailed scattering simulations. The Spearman rank coefficient of mobility against experiment (rs = 0.93) improves significantly on results obtained using a constant relaxation time approximation (rs = 0.52). We find our approach offers similar accuracy to state-of-the art methods at approximately 1/500th the computational cost, thus enabling its use in high-throughput computational workflows for the accurate screening of carrier mobilities, lifetimes, and thermoelectric power.

The arrival of direct electron detectors (DED) with high frame-rates in thefield of scanning transmission electron microscopy has enabled manyexperimental techniques that require collection of a full diffraction patternat each scan position, a field which is subsumed under the name fourdimensional-scanning transmission electron microscopy (4D-STEM). DED framerates approaching 100 kHz require data transmission rates and data storagecapabilities that exceed commonly available computing infrastructure. Currentcommercial DEDs allow the user to make compromises in pixel bit depth, detectorbinning or windowing to reduce the per-frame file size and allow higher framerates. This change in detector specifications requires decisions to be madebefore data acquisition that may reduce or lose information that could havebeen advantageous during data analysis. The 4D Camera, a DED with 87 kHzframe-rate developed at Lawrence Berkeley National Laboratory, reduces the rawdata to a linear-index encoded electron event representation (EER). Here weshow with experimental data from the 4D Camera that linear-index encoded EERand its direct use in 4D-STEM phase contrast imaging methods enables real-time,interactive phase-contrast from large-area 4D-STEM datasets. We detail thecomputational complexity advantages of the EER and the necessary computationalsteps to achieve real-time interactive ptychography and center-of-massdifferential phase contrast using commonly available hardware accelerators.

We propose the use of silicon carbide (SiC) for direct detection of sub-GeV dark matter. SiC has properties similar to both silicon and diamond but has two key advantages: (i) it is a polar semiconductor which allows sensitivity to a broader range of dark matter candidates; and (ii) it exists in many stable polymorphs with varying physical properties and hence has tunable sensitivity to various dark matter models. We show that SiC is an excellent target to search for electron, nuclear and phonon excitations from scattering of dark matter down to 10 keV in mass, as well as for absorption processes of dark matter down to 10 meV in mass. Combined with its widespread use as an alternative to silicon in other detector technologies and its availability compared to diamond, our results demonstrate that SiC holds much promise as a novel dark matter detector.

ACo2Se2 (A=K, Rb, Cs) is a homologue of the iron-based superconductor AFe2Se2. From a comprehensive study of RbCo2Se2 via measurements of magnetization, transport, neutron diffraction, angle-resolved photoemission spectroscopy, and first-principles calculations, we identify a ferromagnetic order accompanied by an orbital-dependent spin splitting of the electronic dispersions. Furthermore, we identify the ordered moment to be dominated by a dx2-y2 flat band near the Fermi level, which exhibits the largest spin splitting across the ferromagnetic transition, suggesting an itinerant origin of the ferromagnetism. In the broader context of the iron-based superconductors, we find this dx2-y2 flat band to be a common feature in the band structures of both iron chalcogenides and iron pnictides, accessible via heavy electron doping.

Second harmonic generation (SHG) spectroscopy ubiquitously enables theinvestigation of surface chemistry, interfacial chemistry as well as symmetryproperties in solids. Polarization-resolved SHG spectroscopy in the visible toinfrared regime is regularly used to investigate electronic and magnetic ordersthrough their angular anisotropies within the crystal structure. However, theincreasing complexity of novel materials and emerging phenomena hamper theinterpretation of experiments solely based on the investigation of hybridizedvalence states. Here, polarization-resolved SHG in the extreme ultraviolet(XUV-SHG) is demonstrated for the first time, enabling element-resolved angularanisotropy investigations. In non-centrosymmetric LiNbO$_3$, elementalcontributions by lithium and niobium are clearly distinguished by energydependent XUV-SHG measurements. This element-resolved and symmetry-sensitiveexperiment suggests that the displacement of Li ions in LiNbO$_3$, which isknown to lead to ferroelectricity, is accompanied by distortions to the Nb ionenvironment that breaks the inversion symmetry of the NbO$_{6}$ octahedron aswell. Our simulations show that the measured second harmonic spectrum isconsistent with Li ion displacements from the centrosymmetric position by$\sim$0.5 Angstrom while the Nb-O bonds are elongated/contracted bydisplacements of the O atoms by $\sim$0.1 Angstrom. In addition, thepolarization-resolved measurement of XUV-SHG shows excellent agreement withnumerical predictions based on dipole-induced SHG commonly used in the opticalwavelengths. This constitutes the first verification of the dipole-based SHGmodel in the XUV regime. The findings of this work pave the way for futureangle and time-resolved XUV-SHG studies with elemental specificity in condensedmatter systems.

Scanning transmission electron microscopy (STEM) is an extremely versatilemethod for studying materials on the atomic scale. Many STEM experiments aresupported or validated with electron scattering simulations. However, using theconventional multislice algorithm to perform these simulations can requireextremely large calculation times, particularly for experiments with millionsof probe positions as each probe position must be simulated independently.Recently, the PRISM algorithm was developed to reduce calculation times forlarge STEM simulations. Here, we introduce a new method for STEM simulation:partitioning of the STEM probe into "beamlets," given by a natural neighborinterpolation of the parent beams. This idea is compatible with PRISMsimulations and can lead to even larger improvements in simulation time, aswell requiring significantly less computer RAM. We have performed varioussimulations to demonstrate the advantages and disadvantages of partitionedPRISM STEM simulations. We find that this new algorithm is particularly usefulfor 4D-STEM simulations of large fields of view. We also provide a referenceimplementation of the multislice, PRISM and partitioned PRISM algorithms.