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Photons do not carry sufficient momentum to induce indirect opticaltransitions in semiconducting materials such as silicon, necessitating theassistance of lattice phonons to conserve momentum. Compared to direct bandgapsemiconductors, this renders silicon a less attractive material for a widevariety of optoelectronic applications. In this work, we introduce analternative strategy to fulfill the momentum-matching requirement in indirectoptical transitions. We demonstrate that when confined to scales below ~3 nm,photons acquire sufficient momentum to allow electronic transitions at the bandedge of Si without the assistance of a phonon. Confined photons allowsimultaneous energy and momentum conservation in two-body photon-electronscattering; in effect, converting silicon into a direct bandgap semiconductor.We show that this less explored concept of light-matter interaction leads to amarked increase of the absorptivity of Si from the UV to the near-IR. Thestrategy provides opportunities for more efficient use of indirectsemiconductors in photovoltaics, energy conversion, light detection andemission.

Low-symmetry van der Waals (vdW) materials have enabled strong confinement of mid-infrared light through hyperbolic phonon polaritons (HPhPs) at the nanoscale. Yet, the bottleneck persists in manipulating the intrinsic polaritonic dispersion to drive further progress in phonon-polaritonics. Here, we present a thermomechanical strategy to manipulate HPhPs in α-MoO 3 using high-pressure and temperature treatment. The hot pressing engineers the stoichiometry of α-MoO 3 by controllably introducing oxygen vacancy defects (OVDs), which cause a semiconductor-to-semimetal transition. Our density functional theory (DFT) and finite-difference time-domain (FDTD) results, combined with experimental studies show that the OVDs induce a metastable metallic state by reducing the bandgap while modifying the intrinsic dielectric permittivity of α-MoO 3 . Photo-induced force microscopy (PiFM) confirms an average dielectric permittivity tunability of |??/?|≈?.?? within a Reststrahlen band of α-MoO 3 , resulting in drastic shifts in the HPhP dispersion. The polariton lifetimes for pristine and hot-pressed flakes were measured as 0.92±0.06 and 0.86±0.11 ps, respectively, exhibiting a loss of only 7%, while the group velocity exhibited an increase of 38.8±0.2%. The OVDs in α-MoO 3 provide a low-loss platform that enables active tuning of mid-infrared HPhPs and have a profound impact on applications in super-resolution imaging, nanoscale thermal manipulation, boosted molecular sensing, and on-chip photonic circuits.

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.

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 present a high-performance coupled electrodynamics–micromagnetics solver for full physical modeling of signals in microelectronic circuitry. The overall strategy couples a finite-difference time-domain approach for Maxwell’s equations to a magnetization model described by the Landau–Lifshitz–Gilbert equation. The algorithm is implemented in the Exascale Computing Project software framework, AMReX, which provides effective scalability on manycore and GPU-based supercomputing architectures. Furthermore, the code leverages ongoing developments of the Exascale Application Code, WarpX, which is primarily being developed for plasma wakefield accelerator modeling. Our temporal coupling scheme provides second-order accuracy in space and time by combining the integration steps for the magnetic field and magnetization into an iterative sub-step that includes a trapezoidal temporal discretization for the magnetization. The performance of the algorithm is demonstrated by the excellent scaling results on NERSC multicore and GPU systems, with a significant (59×) speedup on the GPU using a node-by-node comparison. We demonstrate the utility of our code by performing simulations of an electromagnetic waveguide and a magnetically tunable filter.

High quality-factor (Q) optical resonators are a key component for ultra-narrow linewidth lasers, frequency stabilization, precision spectroscopy and quantum applications. Integration in a photonic waveguide platform is key to reducing cost, size, power and sensitivity to environmental disturbances. However, to date, the Q of all-waveguide resonators has been relegated to below 260 Million. Here, we report a Si3N4 resonator with 422 Million intrinsic and 3.4 Billion absorption-limited Qs. The resonator has 453 kHz intrinsic, 906 kHz loaded, and 57 kHz absorption-limited linewidths and the corresponding 0.060 dB m−1 loss is the lowest reported to date for waveguides with deposited oxide upper cladding. These results are achieved through a careful reduction of scattering and absorption losses that we simulate, quantify and correlate to measurements. This advancement in waveguide resonator technology paves the way to all-waveguide Billion Q cavities for applications including nonlinear optics, atomic clocks, quantum photonics and high-capacity fiber communications.

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.

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.

Various volatile aerosols have been associated with adverse health effects; however, characterization of these aerosols is challenging due to their dynamic nature. Here, we present a method that directly measures the volatility of particulate matter (PM) using computational microscopy and deep learning. This method was applied to aerosols generated by electronic cigarettes (e-cigs), which vaporize a liquid mixture (e-liquid) that mainly consists of propylene glycol (PG), vegetable glycerin (VG), nicotine, and flavoring compounds. E-cig-generated aerosols were recorded by a field-portable computational microscope, using an impaction-based air sampler. A lensless digital holographic microscope inside this mobile device continuously records the inline holograms of the collected particles. A deep learning-based algorithm is used to automatically reconstruct the microscopic images of e-cig-generated particles from their holograms and rapidly quantify their volatility. To evaluate the effects of e-liquid composition on aerosol dynamics, we measured the volatility of the particles generated by flavorless, nicotine-free e-liquids with various PG/VG volumetric ratios, revealing a negative correlation between the particles' volatility and the volumetric ratio of VG in the e-liquid. For a given PG/VG composition, the addition of nicotine dominated the evaporation dynamics of the e-cig aerosol and the aforementioned negative correlation was no longer observed. We also revealed that flavoring additives in e-liquids significantly decrease the volatility of e-cig aerosol. The presented holographic volatility measurement technique and the associated mobile device might provide new insights on the volatility of e-cig-generated particles and can be applied to characterize various volatile PM.

Abstract: While resistors with memory, sometimes called memristive elements (such as ReRAM cells), are often studied under conditions of periodic driving, little attention has been paid to the Fourier features of their memory response (hysteresis). Here we demonstrate experimentally that the hysteresis of memristive systems can be unambiguously distinguished from the linear or non-linear response of systems without hysteresis by the values of certain Fourier series coefficients. We also show that the Fourier series convergence depends on driving conditions, and introduce a measure of hysteresis. These results may be used to quantify the memory content of resistive memories, and tune their Fourier spectrum according to the excitation signal.

An exceptional point of degeneracy (EPD) is induced in a system made of a linear electron beam interacting with an electromagnetic (EM) guided mode in a vacuum tube made of a corrugated circular metallic waveguide with distributed output ports. This scheme enables an exceptional synchronous regime in backward-wave oscillators (BWOs) where the electron beam provides distributed gain to the EM mode with distributed power extraction. Particle-in-cell (PIC) simulation results demonstrate that the proposed EPD BWO has a starting-oscillation current that scales quadratically with BWO length to a nonvanishing value, which does not occur in standard BWOs and demonstrates the occurrence of the EPD and hence the exceptional synchronism operational regime. The degeneracy of two interactive hot modes is also verified by observing the coalescence of their complex-valued wave numbers at the EPD frequency. Observations on the kinetic energy distribution of the electrons along the BWO demonstrate that the proposed EPD-BWO regime is capable of achieving higher power conversion efficiency at higher levels of power generation due to its ability to maintain the synchronism for longer BWO lengths compared to the standard BWO regime of operation.

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.

Supersonic gas jets produced by converging-diverging (C-D) nozzles arecommonly used as targets for laser-plasma acceleration (LPA) experiments. Amajor point of interest for these targets is the gas density at the region ofinteraction where the laser ionizes the gas plume to create a plasma, providingthe acceleration structure. Tuning the density profiles at this interactionregion is crucial to LPA optimization. A "flat-top" density profile is desiredat this line of interaction to control laser propagation and high energyelectron acceleration, while a short high-density profile is often preferredfor acceleration of lower-energy tightly-focused laser-plasma interactions. Aparticular design parameter of interest is the curvature of the nozzle'sdiverging section. We examine three nozzle designs with different curvatures:the concave "bell", straight conical and convex "trumpet" nozzles. Wedemonstrate that, at mm-scale distances from the nozzle exit, the trumpet andstraight nozzles, if optimized, produce "flat-top" density profiles whereas thebell nozzle creates focused regions of gas with higher densities. Anoptimization procedure for the trumpet nozzle is derived and compared to thestraight nozzle optimization process. We find that the trumpet nozzle, byproviding an extra parameter of control through its curvature, is moreversatile for creating flat-top profiles and its optimization procedure is morerefined compared to the straight nozzle and the straight nozzle optimizationprocess. We present results for different nozzle designs from computationalfluid dynamics (CFD) simulations performed with the program ANSYS Fluent andverify them experimentally using neutral density interferometry.

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.

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.

The presence of interface recombination in a complex multilayered thin-filmsolar structure causes a disparity between the internal open-circuit voltage(VOC,in), measured by photoluminescence, and the external open-circuit voltage(VOC,ex) i.e. an additional VOC deficit. Higher VOC,ex value aim require acomprehensive understanding of connection between VOC deficit and interfacerecombination. Here, a deep near-surface defect model at the absorber/bufferinterface is developed for copper indium di-selenide solar cells grown under Cuexcess conditions to explain the disparity between VOC,in and VOC,ex.. Themodel is based on experimental analysis of admittance spectroscopy anddeep-level transient spectroscopy, which show the signature of deep acceptordefect. Further, temperature-dependent current-voltage measurements confirm thepresence of near surface defects as the cause of interface recombination. Thenumerical simulations show strong decrease in the local VOC,in near theabsorber/buffer interface leading to a VOC deficit in the device. This lossmechanism leads to interface recombination without a reduced interface bandgapor Fermi level pinning. Further, these findings demonstrate that the VOC,inmeasurements alone can be inconclusive and might conceal the information oninterface recombination pathways, establishing the need for complementarytechniques like temperature dependent current voltage measurements to identifythe cause of interface recombination in the devices.

We present a new, high-performance coupled electrodynamics-micromagneticssolver for full physical modeling of signals in microelectronic circuitry. Theoverall strategy couples a finite-difference time-domain (FDTD) approach forMaxwell's equations to a magnetization model described by theLandau-Lifshitz-Gilbert (LLG) equation. The algorithm is implemented in theExascale Computing Project software framework, AMReX, which provides effectivescalability on manycore and GPU-based supercomputing architectures.Furthermore, the code leverages ongoing developments of the ExascaleApplication Code, WarpX, primarily developed for plasma wakefield acceleratormodeling. Our novel temporal coupling scheme provides second-order accuracy inspace and time by combining the integration steps for the magnetic field andmagnetization into an iterative sub-step that includes a trapezoidaldiscretization for the magnetization. The performance of the algorithm isdemonstrated by the excellent scaling results on NERSC multicore and GPUsystems, with a significant (59x) speedup on the GPU using a node-by-nodecomparison. We demonstrate the utility of our code by performing simulations ofan electromagnetic waveguide and a magnetically tunable filter.

We have realized different honeycomb lattices for microwave photons in the 4to 8 GHz band using superconducting spiral resonators. Each lattice comprises afew hundred sites. Two designs have been studied, one leading to two bandstouching at the Dirac points and one where a gap opens at the Dirac points.Using a scanning laser technique to image the eigenmodes of this new type ofphotonic lattices, we are able to reconstruct their band structure. Themeasured bands are in excellent agreement with ab initio models that combinenumerical simulations of the electromagnetic properties of the spiral resonatorand analytical calculations.

The development of silicon anodes to replace conventional graphite in effortsto increase energy densities of lithium-ion batteries has been largely impededby poor interfacial stability against liquid electrolytes. Here, stableoperation of 99.9 weight% micro-Si (uSi) anode is enabled by utilizing theinterface passivating properties of sulfide based solid-electrolytes. Bulk tosurface characterization, as well as quantification of interfacial componentsshowed that such an approach eliminates continuous interfacial growth andirreversible lithium losses. In uSi || layered-oxide full cells, high currentdensities at room temperature (5 mA cm 2), wide operating temperature(-20{\deg}C to 80{\deg}C) and high loadings (>11 mAh cm-2) were demonstratedfor both charge and discharge operations. The promising battery performance canbe attributed to both the desirable interfacial property between uSi andsulfide electrolytes, as well as the unique chemo-mechanical behavior of theLi-Si alloys.

Face masks prevent transmission of infectious respiratory diseases by blocking large droplets and aerosols during exhalation or inhalation. While three-layer masks are generally advised, many commonly available or makeshift masks contain single or double layers. Using carefully designed experiments involving high-speed imaging along with physics-based analysis, we show that high-momentum, large-sized (>250 micrometer) surrogate cough droplets can penetrate single- or double-layer mask material to a significant extent. The penetrated droplets can atomize into numerous much smaller (

Novel computational paradigms may provide the blueprint to help solving the time and energy limitations that we face with our modern computers, and provide solutions to complex problems more efficiently (with reduced time, power consumption and/or less device footprint) than is currently possible with standard approaches. Spintronics offers a promising basis for the development of efficient devices and unconventional operations for at least three main reasons: (i) the low-power requirements of spin-based devices, i.e., requiring no standby power for operation and the possibility to write information with small dynamic energy dissipation, (ii) the strong nonlinearity, time nonlocality, and/or stochasticity that spintronic devices can exhibit, and (iii) their compatibility with CMOS logic manufacturing processes. At the same time, the high endurance and speed of spintronic devices means that they can be rewritten or reconfigured frequently over the lifetime of a circuit, a feature that is essential in many emerging computing concepts. In this perspective, we will discuss how spintronics may aid in the realization of efficient devices, primarily focusing on magnetic tunnel junctions. We then provide a perspective on how these devices can impact the development of three unconventional computing paradigms, namely, reservoir computing, probabilistic computing and memcomputing. These paradigms may be used to address some limitations of modern computers, providing a realistic path to intelligent hybrid CMOS-spintronic systems.

We propose a new class of oscillators by engineering the dispersion of two-coupled periodic waveguides to exhibit a degenerate band edge (DBE). The DBE is an exceptional point of degeneracy (EPD) of order four, i.e., representing the coalescence of four eigenmodes of a waveguide system without loss and gain. We present a distributed DBE oscillator realized in periodic coupled transmission lines with a unique mode selection scheme that leads to a stable single-frequency oscillation, even in the presence of load variation. The DBE oscillator potentially leads to a boost of the efficiency and performance of radio frequency (RF) sources, due to the unique features associated with the EPD concept. This class of oscillators is promising for improving discrete-distributed coherent sources and can be extended to radiating structures to achieve a new class of active integrated antenna arrays.

Novel computational paradigms may provide the blueprint to help solving the time and energy limitations that we face with our modern computers, and provide solutions to complex problems more efficiently (with reduced time, power consumption and/or less device footprint) than is currently possible with standard approaches. Spintronics offers a promising basis for the development of efficient devices and unconventional operations for at least three main reasons: (i) the low-power requirements of spin-based devices, i.e., requiring no standby power for operation and the possibility to write information with small dynamic energy dissipation, (ii) the strong nonlinearity, time nonlocality, and/or stochasticity that spintronic devices can exhibit, and (iii) their compatibility with CMOS logic manufacturing processes. At the same time, the high endurance and speed of spintronic devices means that they can be rewritten or reconfigured frequently over the lifetime of a circuit, a feature that is essential in many emerging computing concepts. In this perspective, we will discuss how spintronics may aid in the realization of efficient devices, primarily focusing on magnetic tunnel junctions. We then provide a perspective on how these devices can impact the development of three unconventional computing paradigms, namely, reservoir computing, probabilistic computing and memcomputing. These paradigms may be used to address some limitations of modern computers, providing a realistic path to intelligent hybrid CMOS-spintronic systems.

Precision frequency and phase synchronization between distinct fiberinterconnected nodes is critical for a wide range of applications, includingatomic timekeeping, quantum networking, database synchronization,ultra-high-capacity coherent optical communications and hyper-scale datacenters. Today, many of these applications utilize precision, tabletop lasersystems, and would benefit from integration in terms of reduced size, power,cost, and reliability. In this paper we report a record low 3x10^-4 rad^2residual phase error variance for synchronization based on independent,spectrally pure, ultra-high mutual coherence, photonic integrated lasers. Thisperformance is achieved with stimulated Brillouin scattering lasers that arestabilized to independent microcavity references, realizing sources with 30 Hzintegral linewidth and a fractional frequency instability less than or equal to2x10^-13 at 50 ms. This level of low phase noise and carrier stability enablesa new type of optical-frequency-stabilized phase-locked loop (OFS-PLL) thatoperates with a less than 800 kHz loop bandwidth, eliminating traditional powerconsuming high bandwidth electronics and digital signal processors used tophase lock optical carriers. Additionally, we measure the residual phase errordown to a received carrier power of -34 dBm, removing the need to transmitin-band or out-of-band synchronized carriers. These results highlight thepromise for a path to spectrally pure, ultra-stable, integrated lasers fornetwork synchronization, precision time distribution protocols, quantum-clocknetworks, and multiple-Terabit per second coherent DSP-free fiber-opticinterconnects.

Excitonic coupling, electronic coupling, and cooperative interactions in self-assembled lead halide perovskite nanocrystals were reported to give rise to a red-shifted collective emission peak with accelerated dynamics. Here we report that similar spectroscopic features could appear as a result of the nanocrystal reactivity within the self-assembled superlattices. This is demonstrated by studying CsPbBr3 nanocrystal superlattices over time with room-temperature and cryogenic micro-photoluminescence spectroscopy, X-ray diffraction, and electron microscopy. It is shown that a gradual contraction of the superlattices and subsequent coalescence of the nanocrystals occurs over several days of keeping such structures under vacuum. As a result, a narrow, low-energy emission peak is observed at 4 K with a concomitant shortening of the photoluminescence lifetime due to the energy transfer between nanocrystals. When exposed to air, self-assembled CsPbBr3 nanocrystals develop bulk-like CsPbBr3 particles on top of the superlattices. At 4 K, these particles produce a distribution of narrow, low-energy emission peaks with short lifetimes and excitation fluence-dependent, oscillatory decays. Overall, the aging of CsPbBr3 nanocrystal assemblies dramatically alters their emission properties and that should not be overlooked when studying collective optoelectronic phenomena nor confused with superfluorescence effects.

The rise of flexible electronics calls for cost-effective and scalablebatteries with good mechanical and electrochemical performance. In this work,we developed printable, polymer-based AgO-Zn batteries that featureflexibility, rechargeability, high areal capacity, and low impedance. Usingelastomeric substrate and binders, the current collectors, electrodes, andseparators can be easily screen-printed layer-by-layer and vacuum-sealed in astacked configuration. The batteries are customizable in sizes and capacities,with the highest obtained areal capacity of 54 mAh/cm2 for primaryapplications. Advanced micro-CT and EIS were used to characterize the battery,whose mechanical stability was tested with repeated twisting and bending. Thebatteries were used to power a flexible E-ink display system that requires ahigh-current drain and exhibited superior performance than commercial coin-cellbatteries. The developed battery presents a practical solution for powering awide range of electronics and holds major implications for the futuredevelopment of practical and high-performance flexible batteries.

Dual phase xenon detectors are widely used in experimental searches forgalactic darkmatter particles. The origin of single electron backgroundsfollowing prompt scintillation and proportional scintillation signals in thesedetectors is not fully understood, although there has been progress in recentyears. In this paper, we describe single electron backgrounds in ${}^{83m}Kr$calibration events and their correlation with drift and extraction fields,using the Particle Identification in Xenon at Yale (PIXeY) dual-phase xenontime projection chamber. The single electron background induced by theFowler-Nordheim (FN) effect is measured, and its electric field dependence isquantified. The photoionization of grids and impurities by prompt scintillationand proportional scintillation also contributes to the single electronbackground.

We derive a general lower bound on distributions of entropy production in interacting active matter systems. The bound is tight in the limit that interparticle correlations are small and short-ranged, which we explore in four canonical active matter models. In all models studied, the bound is weak where collective fluctuations result in long-ranged correlations, which subsequently links the locations of phase transitions to enhanced entropy production fluctuations. We develop a theory for the onset of enhanced fluctuations and relate it to specific phase transitions in active Brownian particles. We also derive optimal control forces that realize the dynamics necessary to tune dissipation and manipulate the system between phases. In so doing, we uncover a general relationship between entropy production and pattern formation in active matter, as well as ways of controlling it.

Ti$_3$C$_2$T$_X$ nano-sheets (MXenes) are an emerging class oftwo-dimensional materials with outstanding potential to be employed in energystorage, catalysis, and triboelectric applications, on length scales rangingfrom the nano- to macroscopic. Despite rapidly accelerating interest in thisnew class of materials, their nanoscale frictional properties and inparticular, their potential for solid lubrication on the nanoscale, have notbeen explored in detail yet. In this short communication, we present the firstresults on the nanoscale frictional characteristics of MXenes, in the form of afriction map obtained on an isolated Ti$_3$C$_2$T$_x$ nano-sheet deposited on asilicon dioxide substrate via friction force microscopy. Our experiments revealthat few-layer Ti$_3$C$_2$T$_x$ nano-sheets indeed act as solid lubricants onthe nanoscale, reducing friction on the silicon dioxide substrates, althoughnot as effectively as few-layer graphene. The results reported here pave theway for further studies focusing on nanoscale solid lubrication achieved byTi$_3$C$_2$T$_X$ (MXene) nano-sheets.

There is renewed interest in using advanced techniques to characterize ancient Roman concrete due to its exceptional durability and low-carbon footprint. In the present work, samples were drilled from the “Hospitium” in Pompeii and were analyzed by synchrotron microtomography (μCT) and neutron radiography to study how the microstructure, including the presence of induced cracks, affects their water adsorption. The water distribution and absorptivity were quantified by neutron radiography. The 3D crack propagation, pore size distribution and orientation, tortuosity, and connectivity were analyzed from μCT results using advanced imaging methods. Porosity was also measured by mercury intrusion porosimetry (MIP) as a reference. Ductile fracture patterns were observed once cracks were introduced. Compared to Portland cement mortar/concrete, the Pompeii samples had relatively high porosity, low connectivity, and a similar coefficient of capillary penetration. In addition, permeability was predicted from models based on percolation theory and pore structure data to evaluate the fluid transport properties. Understanding the microstructure of ancient Pompeii concrete is important because it could inspire the development of modern concrete with high durability.

There is renewed interest in using advanced techniques to characterize ancient Roman concrete due to its exceptional durability and low-carbon footprint. In the present work, samples were drilled from the “Hospitium” in Pompeii and were analyzed by synchrotron microtomography (μCT) and neutron radiography to study how the microstructure, including the presence of induced cracks, affects their water adsorption. The water distribution and absorptivity were quantified by neutron radiography. The 3D crack propagation, pore size distribution and orientation, tortuosity, and connectivity were analyzed from μCT results using advanced imaging methods. Porosity was also measured by mercury intrusion porosimetry (MIP) as a reference. Ductile fracture patterns were observed once cracks were introduced. Compared to Portland cement mortar/concrete, the Pompeii samples had relatively high porosity, low connectivity, and a similar coefficient of capillary penetration. In addition, permeability was predicted from models based on percolation theory and pore structure data to evaluate the fluid transport properties. Understanding the microstructure of ancient Pompeii concrete is important because it could inspire the development of modern concrete with high durability.

Integration of semiconducting transition metal dichalcogenides (TMDs) into functional optoelectronic circuitries requires an understanding of the charge transfer across the interface between the TMD and the contacting material. Here, we use spatially resolved photocurrent microscopy to demonstrate electronic uniformity at the epitaxial graphene/molybdenum disulfide (EG/MoS2) interface. A 10× larger photocurrent is extracted at the EG/MoS2 interface when compared to the metal (Ti/Au)/MoS2 interface. This is supported by semi-local density functional theory (DFT), which predicts the Schottky barrier at the EG/MoS2 interface to be ∼2× lower than that at Ti/MoS2. We provide a direct visualization of a 2D material Schottky barrier through combination of angle-resolved photoemission spectroscopy with spatial resolution selected to be ∼300 nm (nano-ARPES) and DFT calculations. A bending of ∼500 meV over a length scale of ∼2-3 μm in the valence band maximum of MoS2 is observed via nano-ARPES. We explicate a correlation between experimental demonstration and theoretical predictions of barriers at graphene/TMD interfaces. Spatially resolved photocurrent mapping allows for directly visualizing the uniformity of built-in electric fields at heterostructure interfaces, providing a guide for microscopic engineering of charge transport across heterointerfaces. This simple probe-based technique also speaks directly to the 2D synthesis community to elucidate electronic uniformity at domain boundaries alongside morphological uniformity over large areas.

The previously developed bistable amphoteric native defect (BAND) model is used for a comprehensive explanation of the unique photophysical properties and for understanding the remarkable performance of perovskites as photovoltaic materials. It is shown that the amphoteric defects in the donor (acceptor) configuration capture a fraction of photoexcited electrons (holes) dividing them into two groups: higher-energy bright and lower-energy dark electrons (holes). The spatial separation of the dark electrons and dark holes and the k-space separation of the bright and dark charge carriers reduce electron-hole recombination rates, emulating the properties of an ideal photovoltaic material with a balanced, spatially separated transport of electrons and holes. The BAND model also offers a straightforward explanation for the exceptional insensitivity of the photovoltaic performance of polycrystalline perovskite films to structural and optical inhomogeneities. The blue-shifted radiative recombination of bright electrons and holes results in a large anti-Stokes effect that provides a quantitative explanation for the spectral dependence of the laser cooling effect measured in perovskite platelets.

Grazing-incidence X-ray scattering is a common technique to elucidate nanostructural information for thin-film samples, but depth-resolving this nanostructure is difficult using a single or few images. An in situ method to extract film thickness, the index of refraction and depth information using scattering images taken across a range of incident angles is presented here. The technique is described within the multilayer distorted-wave Born approximation and validated using two sets of polymer thin films. Angular divergence and energy resolution effects are considered, and implementation of the technique as a general beamline procedure is discussed. Electric-field-intensity-modulated scattering is a general technique applicable to myriad materials and enables the acquisition of depth-sensitive information in situ at any grazing-incidence-capable beamline.

Spin transfer torque magnetic random access memory (STT-MRAM) is a promising candidate for next generation memory as it is non-volatile, fast, and has unlimited endurance. Another important aspect of STT-MRAM is that its core component, the nanoscale magnetic tunneling junction (MTJ), is thought to be radiation hard, making it attractive for space and nuclear technology applications. However, studies on the effects of ionizing radiation on the STT-MRAM writing process are lacking for MTJs with perpendicular magnetic anisotropy (pMTJs) required for scalable applications. Particularly, the question of the impact of extreme total ionizing dose on perpendicular magnetic anisotropy, which plays a crucial role on thermal stability and critical writing current, remains open. Here we report measurements of the impact of high doses of gamma and neutron radiation on nanoscale pMTJs used in STT-MRAM. We characterize the tunneling magnetoresistance, the magnetic field switching, and the current-induced switching before and after irradiation. Our results demonstrate that all these key properties of nanoscale MTJs relevant to STT-MRAM applications are robust against ionizing radiation. Additionally, we perform experiments on thermally driven stochastic switching in the gamma ray environment. These results indicate that nanoscale MTJs are promising building blocks for radiation-hard non-von Neumann computing.

Combined microwave-optical pump-probe methods are emerging to study thequantum state of spin qubit centers and the charge dynamics in semiconductors.A major hindrance is the limited bandwidth of microwave irradiation/detectioncircuitry which could be overcome with the use of broadband coplanar waveguides(CPW). We present the development and performance characterization of twospectrometers: an optically detected magnetic resonance spectrometer (ODMR) anda microwave detected photoconductivity measurement. In the first method lightserves as detection and microwaves excite the investigated medium, while in thesecond the roles are interchanged. The performance is demonstrated by measuringODMR maps on the nitrogen-vacancy center in diamond and time resolvedphotoconductivity in p-doped silicon. The results demonstrate both an efficientcoupling of the microwave irradiation to the samples as well as an excellentsensitivity for minute changes in sample conductivity.

Polarized light microscopy provides high contrast to birefringent specimen and is widely used as a diagnostic tool in pathology. However, polarization microscopy systems typically operate by analyzing images collected from two or more light paths in different states of polarization, which lead to relatively complex optical designs, high system costs, or experienced technicians being required. Here, we present a deep learning-based holographic polarization microscope that is capable of obtaining quantitative birefringence retardance and orientation information of specimen from a phase-recovered hologram, while only requiring the addition of one polarizer/analyzer pair to an inline lensfree holographic imaging system. Using a deep neural network, the reconstructed holographic images from a single state of polarization can be transformed into images equivalent to those captured using a single-shot computational polarized light microscope (SCPLM). Our analysis shows that a trained deep neural network can extract the birefringence information using both the sample specific morphological features as well as the holographic amplitude and phase distribution. To demonstrate the efficacy of this method, we tested it by imaging various birefringent samples including, for example, monosodium urate and triamcinolone acetonide crystals. Our method achieves similar results to SCPLM both qualitatively and quantitatively, and due to its simpler optical design and significantly larger field-of-view this method has the potential to expand the access to polarization microscopy and its use for medical diagnosis in resource limited settings.

The impedance behavior of pre-oxidized iron in liquid lead-bismuth eutectic (LBE) at 200 °C is studied using electrochemical impedance spectroscopy. The structures and resistance of oxide grown on iron oxidized in air at different temperatures and durations are compared. The results show that the resistance of the oxide film increases with increasing oxidizing temperature, due to the formation of a thicker scale and fewer defects. At the same temperature (600 °C), increasing the oxidation time can also reduce the defect concentration in the oxide film and improve the impedance of the oxide scale in LBE.

The impedance behavior of pre-oxidized iron in liquid lead-bismuth eutectic (LBE) at 200 °C is studied using electrochemical impedance spectroscopy. The structures and resistance of oxide grown on iron oxidized in air at different temperatures and durations are compared. The results show that the resistance of the oxide film increases with increasing oxidizing temperature, due to the formation of a thicker scale and fewer defects. At the same temperature (600 °C), increasing the oxidation time can also reduce the defect concentration in the oxide film and improve the impedance of the oxide scale in LBE.

Perovskite solar cells have drawn much attention in recent years, owing toits world-record setting photovoltaic performances. Despite its promising usein tandem applications and flexible devices, its practicality is still limitedby its structural instability often arising from ion migration and defectformation. While it is generally understood that ion instability is a primarycause for degradation, there is still a lack of direct evidence of structuraltransformation at the atomistic scale. Such an understanding is crucial toevaluate and pin-point how such instabilities are induced relative to externalperturbations such as illumination or electrical bias with time, allowingresearchers to devise effective strategies to mitigate them. Here, we designedan in-situ TEM setup to enable real-time observation of amorphization in doublecation mixed perovskite materials under electrical biasing at 1 V. It is foundthat amorphization occurs along the (001) and (002) planes, which representsthe observation of in-situ facet-dependent amorphization of a perovskitecrystal. To reverse the degradation, the samples were heated at 50 oC and wasfound to recrystallize, effectively regaining its performance losses. This workis vital toward understanding fundamental ion-migration phenomena and addressinstability challenges of perovskite optoelectronics.

Understanding heat transport in semiconductors and insulators is of fundamental importance because of its technological impact in electronics and renewable energy harvesting and conversion. Anharmonic lattice dynamics provides a powerful framework for the description of heat transport at the nanoscale. One of the advantages of this method is that it naturally includes quantum effects due to atoms vibrations, which are needed to compute the thermal properties of semiconductors widely used in nanotechnology, like silicon and carbon, even at room temperature. While the heat transport picture substantially differs between amorphous and crystalline semiconductors from a microscopic standpoint, a unified approach to simulate both crystals and glasses has been devised. Here, we introduce a unified workflow, which implements both the Boltzmann Transport equation and the quasi-harmonic Green-Kubo methods. We discuss how the theory can be optimized to exploit modern parallel architectures, and how it is implemented in κALD o: a versatile and scalable open-source software to compute phonon transport in solids. This approach is applied to crystalline and partially disordered silicon-based systems, including bulk silicon and clathrates, and on silicon-germanium alloy clathrates with largely reduced thermal conductivity.

The selective excitation of localized surface wave modes remains a challenge in the design of both leaky-wave and bound-wave devices. In this Letter, we show how the truncation of a metasurface can play an important role in breaking the spatial inversion symmetry in the excitation of surface waves supported by the structure. This is done by combining a large anisotropy in the dispersion relation and the presence of an edge that also serves as a coupling mechanism between the plane wave excitation and the induced surface waves. By resorting to the exact solution to the scattering problem based on a discrete Wiener-Hopf technique, we show that by inverting the component of the impinging wavevector parallel to the truncation, two distinct surface waves are excited.

Optimally chiral electromagnetic fields with maximized helicity density, recently introduced by Hanifeh et al. [M. Hanifeh, M. Albooyeh, and F. Capolino, ACS Photonics 7, 2682 (2020)10.1021/acsphotonics.0c00304], enable chirality characterization of optically small nanoparticles. Here we demonstrate a technique to obtain optimally chiral near fields that leads to the maximization of helicity density under the constraint of constant energy density, beyond the diffraction limit. We show how optimally chiral illumination induces balanced electric and magnetic dipole moments in an achiral dielectric nanoantenna, which leads to generating optimally chiral scattered and total near fields. In particular, we explore helicity and energy densities in the near field of a spherical dielectric nanoantenna illuminated by an optimally chiral combination of azimuthally and radially polarized beams. This beam combination generates parallel induced electric and magnetic dipole moments in the nanoantenna that in turn generate an optimally chiral scattered field with the same helicity sign of the incident field. The application of helicity maximization to near fields results in helicity enhancement at the nanoscale, which is of great advantage in the detection of nanoscale chiral samples, microscopy, and optical manipulation of chiral nanoparticles.

Methods of optical dynamic nuclear polarization open the door to the replenishable hyperpolarization of nuclear spins, boosting their nuclear magnetic resonance/imaging signatures by orders of magnitude. Nanodiamond powder rich in negatively charged nitrogen vacancy defect centers has recently emerged as one such promising platform, wherein 13C nuclei can be hyperpolarized through the optically pumped defects completely at room temperature. Given the compelling possibility of relaying this 13C polarization to nuclei in external liquids, there is an urgent need for the engineered production of highly “hyperpolarizable” diamond particles. Here, a systematic study of various material dimensions affecting optical 13C hyperpolarization in diamond particles is reported on. It is discovered surprisingly that diamond annealing at elevated temperatures ∼1720 °C has remarkable effects on the hyperpolarization levels enhancing them by above an order of magnitude over materials annealed through conventional means. It is demonstrated these gains arise from a simultaneous improvement in NV− electron relaxation/coherence times, as well as the reduction of paramagnetic content, and an increase in 13C relaxation lifetimes. This work suggests methods for the guided materials production of fluorescent, 13C hyperpolarized, nanodiamonds and pathways for their use as multimodal (optical and magnetic resonance) imaging and hyperpolarization agents.

The exciton-phonon coupling in highly oriented pyrolytic graphite is studied using resonant inelastic x-ray scattering (RIXS) spectroscopy. With ∼70  meV energy resolution, multiple low energy excitations associated with coupling to phonons can be clearly resolved in the RIXS spectra. Using resonance dependence and the closed form for RIXS cross section without considering the intermediate state mixing of phonon modes, the dimensionless coupling constant g is determined to be 5 and 0.35, corresponding to the coupling strength of 0.42  eV+/-20  meV and 0.20  eV+/-20  meV, for zone center and boundary phonons, respectively. The reduced g value for the zone-boundary phonon may be related to its double resonance nature.

In the field of beam physics, two frontier topics have taken center stage due to their potential to enable new approaches to discovery in a wide swath of science. These areas are: advanced, high gradient acceleration techniques, and x-ray free electron lasers (XFELs). Further, there is intense interest in the marriage of these two fields, with the goal of producing a very compact XFEL. In this context, recent advances in high gradient radio-frequency cryogenic copper structure research have opened the door to the use of surface electric fields between 250 and 500 MV m-1. Such an approach is foreseen to enable a new generation of photoinjectors with six-dimensional beam brightness beyond the current state-of-the-art by well over an order of magnitude. This advance is an essential ingredient enabling an ultra-compact XFEL (UC-XFEL). In addition, one may accelerate these bright beams to GeV scale in less than 10 m. Such an injector, when combined with inverse free electron laser-based bunching techniques can produce multi-kA beams with unprecedented beam quality, quantified by 50 nm-rad normalized emittances. The emittance, we note, is the effective area in transverse phase space (x, px/mec) or (y, py/mec) occupied by the beam distribution, and it is relevant to achievable beam sizes as well as setting a limit on FEL wavelength. These beams, when injected into innovative, short-period (1–10 mm) undulators uniquely enable UC-XFELs having footprints consistent with university-scale laboratories. We describe the architecture and predicted performance of this novel light source, which promises photon production per pulse of a few percent of existing XFEL sources. We review implementation issues including collective beam effects, compact x-ray optics systems, and other relevant technical challenges. To illustrate the potential of such a light source to fundamentally change the current paradigm of XFELs with their limited access, we examine possible applications in biology, chemistry, materials, atomic physics, industry, and medicine—including the imaging of virus particles—which may profit from this new model of performing XFEL science.

III-Nitride light emitting diodes (LEDs) are the backbone of ubiquitouslighting and display applications. Imparting directional emission is anessential requirement for many LED implementations. Although optical packaging,nano-patterning and surface roughening techniques can enhance LED extraction,directing the emitted light requires bulky optical components. Opticalmetasurfaces provide precise control over transmitted and reflected waveforms,suggesting a new route for directing light emission. However, it is difficultto adapt metasurface concepts for incoherent light emission, due to the lack ofa phase-locking incident wave. In this Letter, we demonstrate metasurface-baseddesign of InGaN/GaN quantum-well structures that generate narrow,unidirectional transmission and emission lobes at arbitrary engineered angles.We show that the directions and polarization of emission differ significantlyfrom transmission, in agreement with an analytical Local Density of OpticalStates (LDOS) model. The results presented in this Letter open a new paradigmfor exploiting metasurface functionality in light emitting devices.

AbstractTheory predicts that two-dimensional (2D) materials may only exist in the presence of out-of-plane deformations on atomic length scales, frequently referred to as ripples. While such ripples can be detected via electron microscopy, their direct observation via surface-based techniques and characterization in terms of interaction forces and energies remain limited, preventing an unambiguous study of their effect on mechanical characteristics, including but not limited to friction anisotropy. Here, we employ high-resolution atomic force microscopy to demonstrate the presence of atomic-scale ripples on supported samples of few-layer molybdenum disulfide (MoS2). Three-dimensional force/energy spectroscopy is utilized to study the effect of ripples on the interaction landscape. Friction force microscopy reveals multiple symmetries for friction anisotropy, explained by studying rippled sample areas as a function of scan size. Our experiments contribute to the continuing development of a rigorous understanding of the nanoscale mechanics of 2D materials.