Real-time sensing of dynamic vector magnetic fields is used in areas ranging from magnetic navigation to biocurrent imaging. Many vector magnetometers measure only a single field component at a time, and may suffer from orientation errors. Employing a frequency-multiplexing technique with an ensemble of nitrogen-vacancy centers in diamond, the authors present a device that measures $all$ components of a dynamic magnetic field at once, operating at high bandwidth and top-notch sensitivity. With low implementation overhead, this technique has applications in fields such as neuroscience, condensed matter physics, and geoscience, as well as in industry.

Current-induced spin-orbit torques (SOTs) in heavy-metal/ferromagnet systems are promising for efficiently manipulating magnetization in nanoscale spintronics, but the energy efficiency of SOT operations remains limited by a combination of material parameters. The authors report very efficient generation of spin current via the spin Hall effect in Au-Pt alloy, which combines a giant internal spin Hall ratio with a relatively low resistivity. This work establishes Au${}_{0.25}$Pt${}_{0.75}$ as a milestone spin-current generator, more energy-efficient than other heavy metals or topological insulators, to benefit the development of fast, efficient SOT-driven magnetic memory and other devices.

Long-term inertial navigation (to keep a satellite on track, for example) is currently limited by accelerometer and gyrometer biases, which cause large position errors. Bias-free sensors based on atom interferometers have been proposed, but they generally lack sufficient bandwidth for navigation. To solve this problem, the authors hybridize an atom interferometer with a classical accelerometer, using an approach based on Kalman filtering that provides optimal, robust estimation of the classical accelerometer’s bias, even in a harsh environment. This approach can readily be extended to other types of atom interferometers, such as gyrometers or gradiometers.

Fast, high-fidelity readout of qubits is crucial in quantum computing. Quantum error correction in particular requires the repeated measurement of subsets of qubits without perturbing any others. Achieving this goal in a multiplexed readout architecture has been challenging, mainly due to the crosstalk of readout signals. In this work, individual Purcell filters are used for each readout resonator to protect the qubits from untargeted readout signals, and from radiative decay. By implementing this scheme, which could find broad use in near-term multiqubit devices, the authors demonstrate the $simultaneous$ readout of up to five qubits.

Where should one place traps? When targets move in a heterogeneous flow environment, the answer is not obvious. The authors formulate flow capture problems involving flows and sinks, and use dynamical-systems techniques to show that blindly positioning traps carries a high risk of failure. Capture efficiency depends on capture rate: Long-term efficiency decreases as the number of traps increases, though short-term efficiency increases. Doubling the number of traps more than doubles the capture rate. This approach will impact engineering solutions ranging from removing atmospheric CO${}_2$ to cleaning up oceanic microplastic pollution.

Generating highly spin-polarized current is one of the main quests in spintronics. The authors design and theoretically benchmark a spin-photovoltaic device based on the intrinsic edge magnetism of a graphene nanoribbon, which creates a spin-polarized current when light is absorbed. The spin photocurrent can be suitably engineered by changing the gate voltage and scale parameters, and in principle a fully polarized current can be attained. This work shows the way to improved design and fabrication of hybrid optoelectronic-spintronic devices.

Beyond the usual, lateral layouts for organic field-effect transistors (OFETs), vertical designs allow for high current densities and fast switching, but charge transport in such a configuration is not well understood. This study integrates experiments on light-emitting transistors with drift-diffusion simulations to investigate the formation of the conduction channel in a vertical OFET. The authors present a model for the lateral extent of the channel, and its scaling behavior. The results will allow us to find better routes to optimize such vertical OFETs, and the methodology extends to other vertical devices as well.

Current-induced spin-orbit torques (SOTs) in heavy-metal/ferromagnet systems are promising for efficiently manipulating magnetization in nanoscale spintronics, but the energy efficiency of SOT operations remains limited by a combination of material parameters. The authors report very efficient generation of spin current via the spin Hall effect in Au-Pt alloy, which combines a giant internal spin Hall ratio with a relatively low resistivity. This work establishes Au${}_{0.25}$Pt${}_{0.75}$ as a milestone spin-current generator, more energy-efficient than other heavy metals or topological insulators, to benefit the development of fast, efficient SOT-driven magnetic memory and other devices.

Magnetic vortices, the simplest topologically nontrivial magnetization configurations, have potential for applications in magnetic recording and microwave and spintronic devices. In the most studied case of an isolated magnetic dot, the vortex state can be achieved only in a relatively large dot. The authors show that dipolar coupling of a dot to a surrounding perpendicularly magnetized antidot supports vortices at reduced diameters and thicknesses, allowing enhanced efficiency and operating frequency, and even additional functionality, in vortex-based spintronic devices.

Reducing the amount of fine particles (fines) in dynamic brittle fragmentation is important within the mining industry, to save energy and reduce environmental hazard. Blasting-round design to achieve this has been held back by a lack of understanding of the fundamental physics, and by the technical complexity of blasting-process measurements. The authors use numerical simulations and experimental data to study the physical origin of fines generated in civil-engineering blasts. Surprisingly, the fragments can be classified according to universal mechanisms. This insight has the potential to generate innovative engineering solutions that may save resources and protect the environment.

Domain-wall conductivity (DWC) in ferroelectrics receives a lot of attention these days, as it involves a confined two-dimensional electron gas at room temperature, which seems promising for devices. This study finds that neighboring domain walls (DWs) in periodically-poled LiNbO${}_3$ exhibit opposite DWC behavior: A head-to-head configuration favors electron transport, while tail-to-tail inhibits it. This asymmetric conduction is quantified using a quantum-mechanical model that also accounts for the observed DW roughness. These findings allow tuning of DW-based electron gases in nanoelectronics, such as logic circuits, memory, or switching devices.

When water floods through porous rock, geological heterogeneities can result in bypassed, undisturbed spaces still containing contaminants, or oil. The fractal-growth model presented in this study allows scientists and engineers to assess the extent of these bypassed regions orders of magnitude more quickly than with conventional simulations, for quick characterization of the effects of geological uncertainties on oil recovery or contaminant remediation in aquifers.

Ruthenium dioxide is an important material, widely used in nanoelectronic devices and interconnects, supercapacitors, and as a catalyst. A good understanding of the origin of the low-frequency $1/f$ noise in RuO${}_2$’s resistivity will advance the design and efficiency of these applications. The authors demonstrate that the flicker noise in RuO${}_2$ films originates from fluctuating oxygen vacancies, which act as dynamical structural defects, and they quantify the activation-energy distribution function of these dynamical defects and calculate the oxygen-vacancy density from the measured temperature dependence of the $1/f$ noise.

Pure spin current is of central importance in spintronics, and typically a sizable spin-orbit coupling (SOC) is needed to generate it. This study proposes using the photogalvanic effect occurring in a two-dimensional material (here phosphorene) $without$ $SOC$ to deliver pure spin current to the outside world at zero bias voltage, with no accompanying charge current. This “spin battery” system harvests photons for its energy source, and thus presents an innovative approach to self-powered flexible spintronics, including wearable devices.

Implementation of accurate quantum gates based on Rydberg interactions is required for scalable quantum computing with ultracold neutral atoms, but has been held back by the difficulty of realizing high-fidelity two-qubit Rydberg gates. This study proposes an easily realizable controlled-$Z$ gate of high intrinsic fidelity, based on spin echo in Rydberg atoms. The ability to attain an accurate entangling Rydberg gate, with neither pulse shaping nor atomic vibrational-ground-state cooling, makes ultracold atoms promising for large-scale quantum computing.

Controlling the individual resonance frequencies in networks of nanostring resonators is typically achieved using electrostatic gate structures, which present an extra layer of fabrication and a potential source of noise. This study demonstrates how the resonance frequencies of individual nanostrings in a network can be tuned simply by employing a global force. This technique enables dynamic control of a nanostring’s resonance frequency, which is experimentally explored using the classical analogy of Landau-Zener transitions, and furthermore is expected to be directly applicable to cantilevers as well.

The coexistence of ferroelectricity and magnetism—for a long time considered mutually exclusive phenomena—now underpins the flourishing field of magnetoelectric multiferroicity. The authors investigate the multiferroic properties induced by spin ordering in a complex oxide that in many respects differs from any magnetoelectric material studied to date. Besides its general interest, this compound would seem to be of direct relevance for exquisitely sensitive measurements of magnetic fields.

Temperature sensing with N-$V$ centers using quantum techniques is very promising, and the fundamental limit of sensitivity with a high-density ensemble has not yet been revealed. The authors estimate how several noise parameters depend on the spin concentration in such a system, and use those estimates to predict the optimal concentration of centers for maximizing temperature sensitivity. The sensitivity limit identified here is much better than in previous reports, indicating the excellent potential of temperature sensing with N-$V$ centers.

Wave guiding is important in acoustic circuits for on-chip devices that are essential for several fields, including telecommunication, sensing, and medical imaging. Designing configurable guiding devices is still in its early stages, however. The authors construct acoustic guides by adding a line of hollow pillars to a forest of solid pillars on a thin plate, and demonstrate evidence of guiding due to the whispering-gallery mode. They also show interesting phenomena, such as the evolution of the guided beam pattern and asymmetric transmission of plate waves. These results should promote the design of more sophisticated waveguides and acoustic circuits.

Time-frequency duality plays a pivotal role in modern optical science and engineering. Conventional duality, which is connected by one-dimensional Fourier transformation, is insufficient for characterizing quantum mechanical correlations in the time-frequency behavior of multiple photons; a higher-dimensional treatment is required. The authors directly measure the two-photon distributions of generated biphotons in both frequency and time domains, and show that they satisfy the Fourier-limited condition in two-dimensional time and frequency space, but not in conventional one-dimensional space. This study helps to pave the way to tomorrow’s quantum optical technologies.

Highly sensitive magnetic field measurements are desirable for medical diagnoses and imaging, as well as research in geomagnetism and fundamental physics. Atomic magnetometers measure the Larmor frequency of an alkali vapor, and their sensitivity depends on generating a high degree of ground-state coherence in the atomic population via optical pumping. This study uses an electro-optic frequency comb to probe a broad spectral region, yielding measurements of the Larmor frequency that are both temporally and spectrally resolved. The techniques developed here allow real-time analysis of the degree of polarization of the atomic ensemble, and the avenues by which this polarization is probed.

Over the last several decades, significant effort has been dedicated to developing memory for energy-efficient cryogenic computers. The proposed memory cells involving magnetic junctions typically have an area far exceeding that of the junction itself, due to the lack of both an area-efficient cell selector within the RAM array, and a compact sensor. The device described here solves both problems: Its multiple terminals allow for implementation of efficient addressing within the RAM array, and its vertically integrated design provides compact sensing (readout) using an $S-I-S^{\prime }$ junction.

Negative differential resistance (NDR, in which current flow $decreases$ with increasing voltage) in van der Waals tunneling devices shows promise for ultrafast electronics. NDR due to momentum-conserving tunneling has been seen in high-quality, well-aligned heterostructures obtained by mechanical exfoliation, but this processing not scalable. The authors propose using moiré superlattice effects to modulate the density of states in the transistor’s drain and source, possibly leading to NDR without momentum-conserving tunneling. The results here suggest that transistors built from two-dimensional materials made by large-scale methods like chemical vapor deposition might also display NDR.

Semiconductor quantum dots enable integration of quantum optoelectronics into information processing with ultralow energy consumption. Here (In,Ga)As quantum-dot–quantum-well (QD-QW) hybrid nanosystems are proposed, where QD excited spin states are laterally tunnel coupled through the adjacent QW. The relaxation time of the spin ensemble’s polarization is 70 times the photoluminescence decay time, showing a high degree of spin polarization. The transient spin polarization in the QDs can be precisely controlled by the QW thickness, paving the way for a dynamical spin network.

Reliably joining two materials of differing stiffness is a ubiquitous problem across the sciences and technology. Taking inspiration from hierarchical structure that appears in the hooves of ungulates (horses, zebras, cows, etc.), the authors demonstrate how the geometry of the interface between the materials dictates the mechanical properties of the larger structure. This work could find application anywhere two different materials are to be joined permanently, with particular importance in medical implants.

Identifying high-performance materials for ferroelectric-memory technology ($e.g.$ FeRAM) is important for increasing the density of information storage. Here the bulk room-temperature ferroelectric semiconductor Sn${}_2$P${}_2$S${}_6$ is studied, and the results demonstrate the possibility of its use as an active material in a multilevel ferroelectric memory cell. In addition, a model is presented to explain the coexistence of ferroelectric and antiferroelectric hysteresis loops in Sn${}_2$P${}_2$S${}_6$.

In recent years, reduced graphene oxides under high humidity have been used as superior solid electrolytes for proton exchange, but the mechanism of proton-exchange enhancement is not fully understood. Combining first-principles and semiempirical approaches, this study predicts the pathways and activation barrier for proton migration in the presence of water on a film of reduced graphene oxide, identifying the water-mediated transport of protons. This insight improves our prospects for developing efficient solid-state proton conductors for applications that include fuel cells, batteries, and chemical filters.

Typically the phase velocity of light propagating through a material is smaller than the speed of light in free space, but in the vicinity of strong, optically active material resonances the opposite can hold true, and the material’s refractive index $n$ drops below 1. This work explores optical phenomena made possible by this reduction of $n$ of dielectric materials near optical-phonon resonances, including frustrated external reflection and direct coupling to surface plasmons. These possibilities will have an impact on optics applications, including cloaking, sensing, angular filtering, and air-core waveguiding.

The orientation of polarization in ferroelectric thin films is important for applications of these materials in nonvolatile memory and field-effect transistors with ferroelectric gates, and in catalysis and photocatalysis. One means of control is to choose a well defined substrate, but the influence of a substrate’s electronic properties on polarization orientation is not well understood. In this investigation of model systems, the experimental findings are integrated with electrostatic models to determine the magnitudes and orientations of the interfacial electric fields that are useful for setting the polarization of the thin ferroelectric epilayer.

Diffuse optical imaging has strong potential for $in$ $vivo$ and deep-tissue applications in medicine, but has been hampered by low spatial resolution. The authors present a superresolution method for diffuse optical imaging that is based on point localization in a diffusion framework, and they demonstrate an improvement in resolution of over two orders of magnitude when imaging fluorescence. This approach will enable optical imaging through centimeters of tissue, at a resolution of tens of microns.

Optoelectronic devices based on intersubband transitions (ISBT) typically require surface texturing after growth, to overcome the selection rules for polarization and operate under normally incident light. The authors demonstrate that $m$-plane ZnO/(Mg,Zn)O multiple quantum wells (QWs) self-assemble in a geometry that allows absorption at normal incidence, depending on the polarization state of the light with respect to the $c$ axis. These multiple QWs behave as a self-assembled metamaterial, a building block for polarization-sensitive optoelectronic devices.

Localized waves are particular solutions of wave equations (including Maxwell’s) that resist diffraction over long distances. Among such solutions are $frozen$ $waves$, beams whose longitudinal intensity patterns can be shaped on demand, allowing $e.g.$ cylindrical beams with intensity only in well-defined spatial intervals—”pieces of light”. The authors show, analytically and experimentally, how to obtain structured light by linking these pieces. Such beams of structured light could find many applications, including optical tweezers, controlling the orbital angular momentum of light, holography, and lithography.

$n$-doped GaAs nanowires are important in optoelectronic devices and quantum technology, but creating good electrical contacts for them is difficult. The authors demonstrate a systematic optimization of nanoscopic Ohmic contacts to such wires by varying the diffusion parameters for contact formation. They find record-low resistances for these contacts, persisting at cryogenic temperatures. The measurements are supported by simulations based on the transfer-matrix method, which can reveal characteristic parameters of the setup once the barrier shape is known.

Controlling the polarization of light at the extreme nanoscale has long been a major scientific and technological goal of nanophotonics. The authors discuss polarization splitting through ultrathin van der Waals heterostructures in the infrared regime, relying on a mechanism that does not resort to the interference effect. Moreover, the predicted phenomenon is insensitive to the angle of incidence. This work thus identifies a promising platform for tailoring light-matter interaction at the nanoscale, and for the design of advanced nanophotonic elements, such as polarization beam splitters and epsilon-near-zero materials.

Huygens’ metasurfaces are interesting, as they can modify transmitted wave fronts with negligible reflection. To achieve perfect broadband transparency of a Huygens’ surface, the authors design a meta-atom based on two shifted split-ring resonators with particular eigenmodes, with the shift canceling magnetoelectric coupling and bringing the electric and magnetic responses into resonance at the same frequency. Broadband transparency with frequency-dependent phase shifting within full 360° coverage is observed, numerically and experimentally, in the microwave range. This meta-atom could be used in high-efficiency polarization converters, beam splitters, and other optically thin devices.

Semiconductor quantum wells based on (In,Ga)N continue to attract interest because of their potential for optoelectronic devices, but key physical aspects remain unclear. For instance, how do alloy microstructure, structural inhomogeneities, and Coulomb effects interact to impact the electronic and optical properties of $c$-plane (In,Ga)N/GaN quantum wells? Here atomistic calculations that go beyond continuum-based models yield microscopic insight to address this question. The results show that reducing interfacial roughness between (In,Ga)N and GaN should lead to improved radiative properties of (In,GaN)/GaN light-emitting diodes.

Quick-response (QR) codes are widely used in modern society, and new approaches to generate them are desirable, to keep pace with the ongoing miniaturization of devices and the daunting increase in the volume of information. This study uses a metasurface approach to hide a QR code in the polarization profile of a light beam, which can then be revealed by a linear polarizer. This technique is promising for anticounterfeiting and encryption efforts, with potential applications in product identification, item tracking, and document management.

While nanostructures exhibit remarkable optical and thermal properties, including super-Planckian emission beyond the black-body limit, they are usually made of isotropic materials; anisotropic nanostructures are rarely considered, despite their potential for applications. Investigating near-field thermal radiation between gratings of natural anisotropic graphite, the authors find that nanopatterning enhances heat flux dramatically, due to the excitation of anisotropic surface modes not possessed by plain graphite. This deepened understanding could open routes to more efficient thermal management and thermophotovoltaic energy conversion.

Long-term inertial navigation (to keep a satellite on track, for example) is currently limited by accelerometer and gyrometer biases, which cause large position errors. Bias-free sensors based on atom interferometers have been proposed, but they generally lack sufficient bandwidth for navigation. To solve this problem, the authors hybridize an atom interferometer with a classical accelerometer, using an approach based on Kalman filtering that provides optimal, robust estimation of the classical accelerometer’s bias, even in a harsh environment. This approach can readily be extended to other types of atom interferometers, such as gyrometers or gradiometers.

Terahertz electromagnetic waves have applications in fields as diverse as biological imaging, materials science, and astrophysics. However, the development of compact, high-power, broadly tunable terahertz sources is challenging. The authors propose a frequency mixer that uses a free-electron beam to drive two cascaded gratings, producing multicolor coherent terahertz radiation via the superradiant Smith-Purcell effect. This device can simultaneously generate several terahertz emissions of high average power, and its frequency can be tuned from 0.8 to 1.8 THz, offering a compact, efficient terahertz source.

The mechanism behind high-$T_c$ superconductivity still is not fully understood, yet is key in the quest for room-temperature superconductors. Recently developed techniques based on color centers in diamond offer fresh possibilities to study magnetism in high-$T_c$ superconductors (HTSs). The authors’ approach to imaging vortices in an HTS using N-$V$ centers allows quantitative measurement of the magnetic field vector at high bandwidth over a wide temperature range, which was not possible before, as the distance between sample and sensor was too large. In principle this approach is also suitable for studying a variety of other systems, including magnetic domains and skyrmions.

The authors demonstrate that passive decoy-state quantum digital signature (QDS) using parametric down-conversion sources can be used effectively for encrypted communication over a distance of 200 km. With this method, the probability of leaking information to the eavesdropper is avoided, which improves security. The superiority of this system is due to its efficient passive decoy-state scheme and low-loss experimental system. The present work can also be extended to the recently proposed measurement-device-independent quantum digital signatures, and thus represents a significant step for QDSs on the path from the laboratory to practical applications.

Charge transfer (CT) states play a key role in the operation of organic solar cells, yet little is known about their energetic distribution under illumination. By measuring a dynamic redshift in the faint photoluminescence of CT states over time, the authors characterize relaxation within the CT density of states, and show that this process is generally $incomplete$ under normal operating conditions. The result is a nonthermal distribution of CT states, characterized by an elevated effective temperature, which overturns a basic assumption implicit in most models of organic solar cells to date.

Double-resonance optically pumped magnetometers are attractive instruments for unshielded magnetic field measurements, such as geophysical surveying and archaeology, due to their wide dynamic range and high sensitivity. However, conventionally these are $scalar$ devices, measuring only the field’s total magnitude. Through modeling and laboratory demonstration, the authors show that the amplitude and phase of first- and second-harmonic signals contain enough information to measure a static magnetic field’s magnitude $and$ $orientation$. This is all achieved through improved signal analysis, allowing the technique to be implemented in practical sensors without additional hardware.

Surface acoustic waves at gigahertz frequencies are well suited to interact resonantly with spin waves in magnetostrictive materials, with the prospect of a wavelike control of magnetization reversal in spintronic devices. Directly observing the resulting spin-wave dynamics in time and space has been challenging. This study presents an elegant approach in which the rf excitation of the waves is synchronized with femtosecond probe pulses of a laser, to reveal the acoustically driven resonant excitation of magnetization dynamics, for deeper insight into the mechanisms of magnon-phonon coupling.

Separating blood plasma in microchannels is of great relevance to microfluidics-based biodetection. However, the physics behind the separation of plasma from whole blood using acoustophoresis (movement driven by sound waves) is not well understood. This study uses experiments and simulations to provide an improved understanding of plasma separation from whole blood using acoustophoresis. It is seen that acoustophoretic focusing of cells depend on two time scales: that of shear-induced diffusion, and that of actual acoustophoresis. These results will help in designing highly efficient blood-plasma separation devices for lab-on-a-chip applications.

Large magnetic-anisotropy energy and high Curie point are key properties in the design of permanent magnets, which are crucial for emerging energy technologies. Based on first-principles calculations, the authors explain why alloying is a viable strategy for optimizing these properties in paramagnetic CeCo${}_3$. Here Mg alloying substantially increases the density of states at the Fermi level, transforming a paramagnet into a ferromagnet with the above properties. These results show a route to discovering ferromagnetic materials, and provide fundamental insight into the magnetic properties of these compounds.

The transfer of orbital angular momentum (OAM) from a vortex wave field to an object is interesting for developing vortex-based acoustic or optical tweezers, particularly for biomedical applications. When the object is not aligned with the vortex’s core, it is unclear how OAM transfer relates to energy transfer. The author finds that here the ratio of torque to absorbed power is not given by—and can even be opposite to—the ratio of vortex topological charge to angular frequency. This suggest a negative radiation torque, associated with energy absorption, that spins an off-core axisymmetric object around its center of mass, in the direction opposite to the wave vortex’s handedness.

Fast, high-fidelity readout of qubits is crucial in quantum computing. Quantum error correction in particular requires the repeated measurement of subsets of qubits without perturbing any others. Achieving this goal in a multiplexed readout architecture has been challenging, mainly due to the crosstalk of readout signals. In this work, individual Purcell filters are used for each readout resonator to protect the qubits from untargeted readout signals, and from radiative decay. By implementing this scheme, which could find broad use in near-term multiqubit devices, the authors demonstrate the $simultaneous$ readout of up to five qubits.

Electronic ink displays based on total internal reflection are promising for video-speed paperlike devices, but the electrophoretic switching behavior and voltage-reflection relation for high particle concentrations remain unclear. This study highlights the importance of countercharges and surface charges. The dynamic switching of electronic ink according to electric field and particle concentration is discussed, and an analytic description of steady-state grey levels is given. The insight from the electrooptical model is relevant to developing next-generation video-speed displays, liquid-toner printers, and other applications of electronic ink.

Metallic antennas have become ubiquitous tools for extracting light from atoms, molecules, and nanocrystals. Their design is generally based on the understanding that such antennas operate through the Purcell effect: the dependence of the fluorescence rate of individual emitters on their environment. The authors experimentally show that this paradigm fails for ensembles of $coupled$ emitters, whose interactions with metallic antennas can be understood with a statistical description of light emission. These findings carry important implications for nano-optics and solution-processed optoelectronics.

Magnetoimpedance (MI) sensors show remarkable linearity and sensitivity to weak magnetic fields, down to the picotesla regime, making them very attractive as high-performance micromagnetic detectors. The authors provide a complete experimental and theoretical description of the MI response in exchange-biased multilayers, and propose a practical method to control the symmetry and the magnitude of the MI response, by tuning the exchange-bias angle and the direction of the applied magnetic field.

Real-time sensing of dynamic vector magnetic fields is used in areas ranging from magnetic navigation to biocurrent imaging. Many vector magnetometers measure only a single field component at a time, and may suffer from orientation errors. Employing a frequency-multiplexing technique with an ensemble of nitrogen-vacancy centers in diamond, the authors present a device that measures $all$ components of a dynamic magnetic field at once, operating at high bandwidth and top-notch sensitivity. With low implementation overhead, this technique has applications in fields such as neuroscience, condensed matter physics, and geoscience, as well as in industry.

In quantum mechanics, $exceptional$ $points$ are non-Hermitian degeneracies with intriguing properties, and the non-Hermiticity gives an extra degree of freedom that allows for innovative applications. Exploiting the unique topological structure of energy surfaces near the exceptional point, the authors propose a terahertz switch in which the system’s transmission can be tuned by changing the chemical potential of monolayer graphene. Their idea can in principle be applied to design a switch for other frequency ranges, and manipulating the energy surfaces of non-Hermitian systems can serve as a useful paradigm to design photonic devices with further functionalities.

Structural modifications to monolayer C${}_3$N (think “mostly graphene, some nitrogen”) might provide a systematic material-design method to manipulate its thermal transport properties, for advanced thermal management. The authors’ calculations show that by adding and removing C atoms, the thermal conductivity of monolayer C${}_3$N can in fact be tuned dramatically, rendering these materials promising for various applications at ambient temperature. The distinct responses of this system are related to localized out-of-plane phonon modes that rely on chemical bonding.

Current-induced magnetization switching by spin-orbit torques has become of technological interest for spintronics, because it provides high-speed operation while preserving device stability. However, conventional spin-orbit-torque switching requires a rather large switching current, and an external in-plane magnetic field for deterministic switching. In this study an alternative switching scheme using a circularly polarized alternating current is investigated, which allows not only a low switching current, but also field-free switching. This achievement provides a useful means to realize energy-efficient spin-orbit-torque devices.

To learn how best to manipulate spin states in low-dimensional structures, the authors use density functional theory to study the magnetism at the edges of nanoribbons of stanene (think graphene, but made of tin, and thus featuring strong spin-orbit coupling). Based on their several findings, they propose that both magnetic anisotropy and exchange coupling can be controlled by carrier doping, for experimentally feasible gate modulation of magnetization in the nanoribbon. Further tuning by a transverse electric field would give full access to spin and valley degrees of freedom. These intriguing results offer a practical approach to energy-efficient spintronic devices.

The three-dimensional (3D) temperature distribution can affect the efficiency, performance, and long-term stability of high-power semiconductor devices. Typical characterization techniques can only probe a surface, in two dimensions—a major limitation. This study uses a “split time window” method for Raman spectroscopy to perform 3D temperature profiling in a working high-power LED, achieving both high spatial resolution and accuracy. This $in$ $operando$ thermometry method will be very useful for revealing often-complicated device performance issues, such as efficiency droop in LEDs or current-induced degradation in lasers or power electronics.

A central challenge in building a scalable quantum computer with superconducting qubits is the execution of high-fidelity two-qubit gates in the presence of many resonant elements. As more elements are added to the architecture, and as the multiplicity of their couplings grows, the design’s frequency space becomes crowded, and performance suffers. The authors present a way to address this difficulty: selective activation of interactions between transmon qubits of fixed frequency and those of tunable frequency. This activation depends on both the amplitude and frequency of modulation, and using the amplitude as an additional condition for resonance alleviates frequency crowding.

Frustrated total internal reflection (FTIR) imaging is an interesting, important optical tool that can be used to measure the pressure exerted beneath soft contacting objects, such as biological materials, polymers, human or animal feet, and the outsoles of sneakers. Wider application of the technique has been prevented by a lack of understanding of the combined optical and mechanical processes that occur when an object contacts a waveguide. To fill this gap, the authors observe light scattering when an elastomer touches a planar waveguide, and also derive a combined theory of light scattering and contact mechanics that agrees quantitatively with the data.

Spin-orbit torque (SOT) allows energy-efficient control of magnetism for nonvolatile digital memory. However, deterministic perpendicular switching requires lateral symmetry breaking, and remains a challenge in SOT devices. Here a mechanism for deterministic perpendicular switching is simulated, showing that uniaxial anisotropies like strain-induced magnetoelastic anisotropy can break a system’s lateral symmetry and yield field-free switching in SOT devices. This method for electric field control of magnetism may inspire next-generation memory devices such as magnetic random-access memory (MRAM).

This study proposes a scheme for ultrasensitive radiation detection that can be used for several applications, ranging from detection of the cosmic microwave background radiation to security imaging. The sensor exploits the giant thermoelectric effect recently discovered in superconductor-ferromagnet hybrid structures, and therefore does not require external driving fields and has a low operating temperature. Taking advantage of this property could have a special impact on the design of multipixel detectors required for next-generation applications.

Controlling the propagation of electromagnetic waves at the subwavelength scale is hampered by diffraction. This study uses a chain of connected subwavelength metamaterial resonators to solve the problem, by elegantly improving the coupling strength of localized surface magnetic modes in the planar system. The metamaterial elements are connected physically, so that conduction current can be used to transfer energy efficiently. Of further interest is that the surface wave can be switched between forward and backward modes. These results are expected to impact the design of integrated photonic devices.

Where should one place traps? When targets move in a heterogeneous flow environment, the answer is not obvious. The authors formulate flow capture problems involving flows and sinks, and use dynamical-systems techniques to show that blindly positioning traps carries a high risk of failure. Capture efficiency depends on capture rate: Long-term efficiency decreases as the number of traps increases, though short-term efficiency increases. Doubling the number of traps more than doubles the capture rate. This approach will impact engineering solutions ranging from removing atmospheric CO${}_2$ to cleaning up oceanic microplastic pollution.

Understanding carrier transport and recombination is the key to improving the efficiency of photovoltaic energy conversion, but it is difficult to directly monitor these processes on their nano- to picosecond time scales in complete devices. This study applies time-resolved photoemission spectroscopy to measure transient photovoltage in an actual solar cell. With temporal resolution as fine as tens of picoseconds, charge separation and recombination are observed respectively as the rise and decay of photovoltage. This approach affords a unique opportunity to characterize the microscopic properties of complete photovoltaic devices.

Offering transport, actuation, and $in$ $situ$ reversal at low power, in the absence of moving parts and noninvasively, magnetohydrodynamic fluid flow is a basis for state-of-the-art microscale devices with improved heat, mass, and momentum transport capabilities. This study explores generating a two-phase microfluidic flow in the presence of a unidirectional or oscillating Lorentz force, revealing the tunable control parameters for producing various flow patterns. It also unveils a unique reciprocating motion under an oscillatory Lorentz force, suggesting efficient micromixers and reciprocating micropumps.

Scintillators based on the oxides known as garnets receive considerable interest, due to their extensive use in medical imaging, monitoring radiation and particle beams in high-energy and nuclear physics, and still other devices. The complex scintillation mechanism in these materials can be hampered, and their performance downgraded, by the localization of charge carriers in the host structure before they can reach the emission centers. This paper provides detailed insight into both the hole and electron localization mechanisms in the garnet structure, thus contributing to our further understanding and optimization of this entire family of scintillators.

Generating highly spin-polarized current is one of the main quests in spintronics. The authors design and theoretically benchmark a spin-photovoltaic device based on the intrinsic edge magnetism of a graphene nanoribbon, which creates a spin-polarized current when light is absorbed. The spin photocurrent can be suitably engineered by changing the gate voltage and scale parameters, and in principle a fully polarized current can be attained. This work shows the way to improved design and fabrication of hybrid optoelectronic-spintronic devices.

Harnessing the effects of spatial dispersion (nonlocality) in metamaterials is a challenge relevant to many applications in optics and photonics, from ultrafast phenomena to chemical and biological sensing. In deeply subwavelength dielectric multilayers these nonlocal effects are generally negligible, but there are critical regimes where they can be amplified, yielding ultrasensitive optical responses. In an unusual approach, the authors interpret these effects in terms of error propagation in dynamical maps, leading to simple, insightful, closed-form solutions that elucidate the roles and effects of the main parameters and aid the identification of additional critical regimes.

The higher-order Poincare (HOP) sphere beam generalizes optical vortices and vector beams, vividly illustrating their higher-order polarization states. However, its doughnutlike intensity profile varies significantly with topological charge, restricting its application in optics. The authors study a perfect HOP sphere, whose annular intensity profile is $independent$ of topological charge. Single- and multiringed beams are produced in anisotropic medium, by exploiting a spiral geometric phase and aptly designed gratings. This work provides an exciting technique for manipulating the angular momentum of light, to improve $e.g.$ optical tweezers, or multiplexing in optical communication.

Photon recycling (emission and reabsorption inside a semiconductor layer) is a fundamental issue for enhancing the performance of hybrid perovskite materials as photovoltaic absorbers. Interpreting photon-recycling effects is not trivial, though, due to complex interplay between carrier transport and recombination and the optical properties of the involved surfaces. The authors present a model of recycling effects based on the concept of photon diffusion, and show that this approach can explain recent experimental observations regarding the optical response of the perovskite layers. These results can guide the improvement of the optoelectronic performance of metal halide perovskites.

The recurrent neural network, a machine-learning approach, is a mathematical model that emulates neuronal function in the human brain. The authors report a quantitative analysis of the figures of merit for reservoir computing, which is a type of recurrent neural network, using the spintronic devices known as magnetic tunnel junctions (MTJs). While MTJs are usually investigated in the context of high-density nonvolatile digital storage, these results show that they are also suitable for advanced computation.

The underlying mechanism of wettability alteration is vital for enhancement of oil recovery (EOR) by water flooding in carbonate reservoirs, such as calcite rock. Based on first-principles molecular dynamics simulations and core-flooding measurements, the authors find that proximal adsorption of ions in brine affects the wettability of calcite: Some ions primarily disturb the interfacial water structure, while others modify the effective surface charge, thus inhibiting and enhancing oil recovery respectively. This study provides needed insight into the physics of wetting at the atomic scale for EOR.

High-resolution lithography for microelectronics demands a powerful, reliable source of light in the extreme-ultraviolet (EUV) range. Today’s sources based on laser-produced plasma already operate near the technological limit, and will be unable to yield the output power needed for tomorrow’s chip production. Thus the authors consider a source based on emission from multiply charged Xe ions, formed and supported in a freely expanding plasma jet by microwave light from a high-power gyrotron. Modeling indicates EUV conversion efficiency potentially exceeding that of a standard device, yet with a simpler overall design, safe operation, and the possibility of continuous-wave operation.

It may look like magic, but it’s science: Using external fields for noncontact particle manipulation offers great advantage in many applications, including lab-on-a-chip technology, materials processing, and containerless transport. In particular, ultrasound wave fields can be used as virtual tweezers to move particles along a chosen trajectory, or to form desired patterns. Typical methods in three dimensions allow manipulation of a single particle, or a static formation of a pattern of particles, but this work presents a method to control the $position$ $and$ $trajectory$ of an entire user-specified pattern, or subsets of the pattern, including individual particles.

Studying resonance modes in exchange-coupled ferromagnetic bilayers is typically accomplished by a magnetic-field-excited ferromagnetic resonance (FMR) technique, in which case it is usually difficult to excite the optical mode, because the out-of-phase resonance cannot efficiently couple to a uniform excitatory magnetic field. This work shows that spin-torque FMR (ST-FMR) can be used to efficiently excite the optical resonance mode, because the current-induced torque naturally does not act uniformly on the two ferromagnetic layers. The results indicate that ST-FMR is an effective probe of interlayer exchange coupling in such systems, to the benefit of research on spintronic devices.

Computation with spin waves is one alternative to CMOS technology, which rapidly approaches its scaling limits. While their propagation is free of Joule heating effects, the generation and manipulation of spin waves often requires current sources, and thus dissipative processes, so methods that rely instead on voltage are sought. The authors demonstrate one such approach, based on surface acoustic waves and the resonant magnetoelastic excitation of bulk and edge modes in ferromagnetic nanowires. The effects are demonstrated using all-optical techniques, and transducer-based analogs should be feasible.

Beyond the usual, lateral layouts for organic field-effect transistors (OFETs), vertical designs allow for high current densities and fast switching, but charge transport in such a configuration is not well understood. This study integrates experiments on light-emitting transistors with drift-diffusion simulations to investigate the formation of the conduction channel in a vertical OFET. The authors present a model for the lateral extent of the channel, and its scaling behavior. The results will allow us to find better routes to optimize such vertical OFETs, and the methodology extends to other vertical devices as well.

Stacks of two-dimensional (2D) materials held together by van der Waals forces could see wide use in nanotechnology. However, a considerable discrepancy is observed when the band alignments in 2D bilayers are calculated using density functional theory, rather than free-carrier electrostatics. The authors show that this is because the tails of the ionic potentials of the boundary atoms extend into the interlayer gap, thereby reducing it, in the electrical sense. Hence the electrostatic model can be corrected by using this reduced equivalent gap, rather than the physical one. This knowledge should facilitate the design of devices based on such 2D stacks.