Why be passive? This study points to the difference between thermoelectric refrigeration and the use of thermoelectrics in active cooling of $e.g.$ electronics, lasers, or batteries. Unlike in refrigeration, here heat has to be drained from a temperature $above$ that of the heat sink (which will happen naturally, but perhaps not quickly enough). In refrigeration, the most important criterion is the thermoelectric figure of merit $ZT$, but in active cooling a device needs to have high thermal conductivity plus a high power factor. Thus high-$ZT$ Peltier modules are actually counterindicated for active cooling…even though they are sold commercially for it.

Practical control of polarization vortices in ferroelectric nanostructures could plays an important role in next-generation nanoscale electronic devices. However, switching vortex polarization is quite challenging, never mind via mechanical methods. This study identifies deterministic switching of vortex chirality in a compositionally graded ferroelectric nanoplate under compressive stress, using phase-field simulations and ferroelectric instability analysis. In addition, the underlying mechanism for such vortex control is explored. These results are tantalizing for nonvolatile memory and oxide electronics.

Efficient electrical switching of antiferromagnets (AFMs) is key to their use in high-density, ultrafast, nonvolatile spintronic memory. Mn${}_2$Au, an AFM with opposite spin sublattices, is a unique metallic material, in that fieldlike spin torque can switch its AFM moments. However, switching induced by antidamping torque remains to be verified in metallic AFMs. Here the authors demonstrate current-induced switching of AFM moment in both a (103)-oriented Mn${}_2$Au single layer and a Mn${}_2$Au/Pt heterojunction by fieldlike torque and antidamping torque respectively. The simultaneous realization of both torque types in metallic Mn2Au makes it a promising candidate for AFM spintronics.

Electronic emulation of the biological synapse (the memory and learning element of the brain) is an important step toward realizing brain-inspired computing systems. The authors demonstrate a voltage-controlled magnetic device, based on oxygen migration, that emulates major functionalities of a biological synapse such as potentiation; depression; plasticity that depends on spike magnitude, rate, and timing; and short- to long-term memory formation. Additionally, there is separation of read and write paths, and an ability to be programmed with “negative weighting”, which helps to overcome the disadvantages of memristor synapses.

Bianisotropic all-dielectric metasurfaces offer a broad range of functionalities in optics, but so far have been designed case by case, and their generalization to different forms of polarization control is not straightforward. Moreover, their demonstrated range of bianisotropic properties has been limited, due to the single-layer topologies used. The authors propose multilayered metasurfaces built from high-contrast subwavelength gratings of varying orientations. Such multilayered all-dielectric metasurfaces can yield polarization conversion with broadband, multiband, and multifunctional responses, and can be designed in a systematic manner to realize all three types of responses.

Nanomagnetic logic is promising for low-power computing, and for integration of data processing and memory in the same architecture. In this work the thermal relaxation paths associated with logic operations in artificial spin ice are considered, which involve switching of individual nanomagnets toward a low-energy state. Both monotonic and intermittent paths have been identified, which lend themselves to deterministic and probabilistic computing respectively. Furthermore, the balance of competing relaxation paths can be shifted, which is essential for implementing probabilistic computation—for example, in artificial neural networks where the outcome can be tuned via a feedback loop.

Imbalanced charge transport in organic solar cells is a common, yet often overlooked, issue. The authors combine experiments, simulations, and analytical theory to clarify the role of mismatched electron and hole mobilities in surface recombination at a solar cell’s electrical contacts. Importantly, they find that the open-circuit voltage depends not only on the bulk recombination rate and injection-barrier heights, but also on the mobility ratio in the active layer.

Microsphere optical cavities are of great interest, since they can be used as efficient light-emitting devices or highly sensitive biosensors. Our understanding of the roles of various physical mechanisms in emission is hindered, though, by the lack of a model to integrate all mechanisms in a simulation. The present study fills this need by considering the contributions of both low-$Q$ leaky modes and high-$Q$ resonant modes, and evaluating the Purcell factor for all of these modes rigorously. Both spontaneous and stimulated emission are included in this model, and the lineshapes of over 30 calculated resonance peaks agree very well with experiment.

Practical control of polarization vortices in ferroelectric nanostructures could plays an important role in next-generation nanoscale electronic devices. However, switching vortex polarization is quite challenging, never mind via mechanical methods. This study identifies deterministic switching of vortex chirality in a compositionally graded ferroelectric nanoplate under compressive stress, using phase-field simulations and ferroelectric instability analysis. In addition, the underlying mechanism for such vortex control is explored. These results are tantalizing for nonvolatile memory and oxide electronics.

The authors show that, counterintuitively, adding extremely small, fully $absorbing$ metal nanoparticles to a material can notably enhance its light emission. The cause of the unexpected, huge enhancement seen in surface-enhanced Raman spectroscopy (SERS) is the tunneling of photons out of the strongly absorbing metal surface. This quantum phenomenon imposes finite limits on the dissipation of emitted photons by proximal absorbing matter, and presents an interesting analogy to Hawking radiation near a black hole. The effect could be further exploited in light generation, optical sensing, and radiative heat transfer.

Magnonic quasicrystals can be used to manipulate spin waves, offering possibilities beyond those of periodic magnonic crystals. The authors investigate one-dimensional magnonic Fibonacci quasicrystals and demonstrate the existence of collective spin waves over a broad range of wave vectors. The spin-wave spectra here are tunable by changing magnetic field amplitude (for continuous band-structure adjustment), magnetization configuration (for reprogrammability), or the dimensions of the elements (for scalability). Beyond being fundamentally interesting, these properties show that magnonic quasicrystals are promising for tomorrow’s spintronic, microwave, and magnonic technologies.

One of the most challenging goals of microfluidics is the design of devices that can efficiently and precisely guide the movement of fluid at the nano- and microscales. This work proposes channels with elongated tilted obstacles in between, which, under the effect of a transverse gradient in temperature or concentration, can generate net fluid motion along the channels. Simulations show that flow properties are determined not only by the applied gradient and obstacle orientation, but also by the obstacle’s intrinsic surface characteristics. Applications would include fluidic mixers and alternators, also with the potential to harvest waste heat or chemical energy.

Measurements of nonlinear electrical transport at high magnetic fields provide critical insight into the physics of superconductors and other exotic electronic materials, with direct implications for magnet technology. However, such measurements in pulsed very high fields (the only option to exceed 45 T) have been thwarted by the short pulse durations and complex in-field dynamics. Using a rapid “smart” technique, the authors surmount the barriers to measuring superconducting critical current in very high magnetic fields. Exploring this uncharted region, they reveal the particular vortex dynamics induced by a rapidly varying field.

In the quest for materials for solid-state refrigeration, proper characterization of magnetocaloric properties is crucial to optimization. In particular, it is important to properly study the irreversible change in the magnetocaloric effect due to the presence of thermal hysteresis, which is commonly overlooked in the literature. This report investigates in detail how the observed adiabatic temperature change in a class of shape-memory Heusler alloys depends on the measurement protocol, by analyzing the temperature- and field-dependent irreversibility of the effect. The results illustrate that the thermal hysteresis should always be taken into account for such compounds.

Dynamic cantilever magnetometry (DCM) offers unique capabilities and sensitivity to probe magnetization processes in individual microscopic samples. Assembling the samples and a cantilever and the sensitivity of DCM are important for applications in nanomagnetism research; so far, DCM has been used primarily on wire-shaped samples, because no universal method for sample assembly has been available. To fill that gap, this study presents a method, using dual focused ion beams, a scanning electron microscope, and a nanomanipulator, that allows the extension of DCM to study the magnetic properties of individual nanostructures of arbitrary shape.

Why be passive? This study points to the difference between thermoelectric refrigeration and the use of thermoelectrics in active cooling of $e.g.$ electronics, lasers, or batteries. Unlike in refrigeration, here heat has to be drained from a temperature $above$ that of the heat sink (which will happen naturally, but perhaps not quickly enough). In refrigeration, the most important criterion is the thermoelectric figure of merit $ZT$, but in active cooling a device needs to have high thermal conductivity plus a high power factor. Thus high-$ZT$ Peltier modules are actually counterindicated for active cooling…even though they are sold commercially for it.

This research on a fully voltage-driven spin-based computing device, the “vTOPSS”, could promote significant gains in energy efficiency, beyond the state of the art. Thus far the challenge in spin-based devices has been related to the large electric current required, and the associated Joule heating. A vTOPSS overcomes such limitations by driving a magnetic insulator via spin accumulation, generated by applying an electric field across a proximal topological insulator. Current flows perpendicular to the surface of the topological insulator and perpendicular to the applied electric field, making it nondissipative. For binary switching operations, a vTOPSS could consume mere attojoules.

Quantum sensors have attained unrivaled sensitivities that enable fundamental discoveries and quantum technologies, but they are prone to instability and decoherence via external influences. Exploiting feedback between quantum sensors, the authors present a combinatorial device in which a second quantum system more sensitive to external perturbations is used to stabilize a primary sensor. This approach would use $e.g.$ both nuclear and electronic spins in diamond to build a stable quantum gyroscope, allowing navigation without GPS. More broadly, this work demonstrates the advantage of colocating a quantum sensor to steady other quantum devices, rather than using bulky classical gear.

Utilization of magnons (collective spin excitations) as information carriers is tempting, but remains a challenging research topic. The authors study thin-film two-component magnonic crystals (MCs) and report the mechanism behind the opening of complete magnonic band gaps. This mechanism originates from the demagnetizing field tailored by in-plane squeezing of the MC and nonuniform mode softening. For properly chosen squeezing, a tiny change in external magnetic field further tunes the band gap. Such reversible, fine control of band-gap opening makes this variety of MCs very promising for spin-wave devices that are tunable $in$ $operando$.

Metasurfaces provide a compact scheme for generating Airy beams (which accelerate, bend, and do not diffract), but usually suffer from either lack of amplitude modulation or low transmission efficiency. To address these issues, this study proposes high-efficiency Huygens’ meta-atoms that allow independent control of electric and magnetic responses, allowing nearly arbitrary transmission amplitude and phase. Hence the Airy-beam generators assembled from them present high transmission efficiency. This result can stimulate production of efficient devices for wave-front manipulation, with diverse functionalities in different frequency domains, including the terahertz.

Cu(In,Ga)Se${}_2$ (CIGS) yields really promising solar cells, but defects and local variations in the composition of this complex alloy also cause modifications to its electronic structure. Understanding the influence of fluctuating potentials on performance is of utmost importance for CIGS-based solar cells. A detailed theoretical study of these fluctuating potentials is developed, and compared to experimental results. The results show unequivocally that losses in open-circuit voltage are correlated with the existence of fluctuating potentials in the CIGS layer, particularly for electrostatic fluctuations as opposed to band-gap variations, and are present at room temperature.

Driving donor-bound-exciton transitions in silicon devices is useful for photoconductive detection of ESR, and for generating donor-spin hyperpolarization. The authors demonstrate the use of such optical excitation with ${}^{28}$Si:P samples in a parallel-plate geometry that eliminates the need to deposit contacts. They develop a detailed, consistent model for the origin of the observed signal, and use this detection scheme to follow ESR in samples with very low P concentrations, leading to P-electron-spin coherence times approaching seconds. This noncontact method could be used to study other donors in Si, or spin-active defects in other materials with an associated bound exciton.

With their natural randomness, polycrystalline materials as gain elements have opened an avenue for the development of ultrafast laser sources. The authors’ model, based on the rigorous orientation of crystalline domains, lays the foundation for ultrafast three-wave interactions in a material, in terms of randomized polarizations and intensities. This work clarifies the potential of random phase matching in ultrafast applications, as well as the standard for nonlinear gain in this process, by comparison to the well-developed technology of quasi-phase matching.

This article describes the active control of terahertz waves using hybrid metamaterials containing embedded vanadium dioxide. VO${}_2$ undergoes an insulator-metal phase transition at around 68 °C, causing its conductivity to change by about 5 orders of magnitude, which yields mode switching in the metamaterial. This mode switching, which is analogous to the resonant mode switching seen in plasmonics, can be realized experimentally using thermal, electrical, or optical stimuli. Such a diverse range of stimuli should be very useful for manipulating terahertz waves in practical applications, with a particular eye toward next-generation wireless communication.

Graphene field-effect transistors with grating gates are considered to be the main building blocks for plasmon-enhanced terahertz detectors, promising high responsivity and voltage tuning of the resonant frequency. Nevertheless, the frequencies and lifetimes of plasmons in these structures are poorly understood. The authors use Fourier-transform spectroscopy to reveal the scaling relations between plasmon resonance frequency, carrier density, and grating geometry. It is shown that plasmon lifetime in large-area chemical-vapor-deposited (CVD) graphene can exceed the scattering rate determined from dc transport, confirming the prospects of CVD graphene for resonant terahertz detection.

Exchange bias (EB) continues to attract much attention for applications in today’s and tomorrow’s technologies, including spin valves and magnetic tunnel junctions, magnetoelectric switching, and magnetic recording. This study describes an antiferromagnetic-antiferromagnetic EB system—with no ferromagnet at all—that features two anomalous EB behaviors, and explains the mechanism behind them in terms of a field-induced dynamic magnetic transformation. This result is expected to provide fresh ideas and opportunities for the application of the EB effect in devices.

Making transistors with high on-off ratio out of pristine graphene (without a band gap) would be a major development in quantum electronics and flexible electronics. The authors show theoretically how to achieve ballistic graphene transistors by combining uniaxial strain and device engineering. Substantial quantitative improvements to prior idealized models result from including realistic contact effects and strain fields. A design to independently control strain and charge density in quantum transport strain engineering (QTSE) experiments is also offered. These results can guide and accelerate the experimental realization of recent proposals for the QTSE of two-dimensional materials.

Graphene provides an ideal platform for active modulation of near-field radiative heat transfer. While much research here focuses on tuning the chemical potential by doping or gating, this work proposes using a magnetic field to control near-field heat flux between graphene sheets. The authors predict giant thermal magnetoresistance and negative thermal magnetoresistance of the near-field radiative heat flux for different Fermi energies, and Shubnikov-de-Haas-like oscillations in the spectral heat flux. In a static magnetic field, coupling of excited magnetoplasmon-polariton modes of graphene boosts radiative heat transfer beyond the blackbody limit by several orders of magnitude.

Doping fluorine into the Al${}_x$Ga${}_{1-x}$N layer is critical to the performance of enhancement-mode Al${}_x$Ga${}_{1-x}$N/GaN high-mobility transistors, but the understanding of the role of F in these devices is rather limited. This study starts from the defect properties of F in Al-Ga-N alloy, investigating its interaction with native defects and impurities, as well as its effect on the alloy’s Fermi energy. The insight obtained nicely explains the mechanism for the change in a device’s electronic performance after doping with F. It also suggests a way to modulate electronic performance, by optimizing the F-incorporation process.

Increasing the resolution of jet-based printing and deposition techniques is important for many industrial applications. This study couples flow focusing to a laser direct-write technique and uses surface-tension effects to reduce the size of the ejected droplet. Using experiments and simulations, the authors show that generating a transient meniscus-shaped liquid-air interface allows for the ejection of smaller droplets from liquid films. This approach is expected to impact the development of deposition techniques that incorporate multiple forms of energy to enhance droplet resolution.

Hybrid circuits uniting charges, spins, and photons in integrated solid-state devices are seen as crucial to the development of information processing. This work reports an on-chip hybrid device based on the strong coupling between a yttrium iron garnet sphere and microwaves in an interferometric setup, which enables the control of both level repulsion and level attraction. In modeling the system, hybrid circuits are generalized by introducing both mutual capacitance/inductance and mutual resistance. The realization of both coherent and dissipative couplings in a planar cavity may open avenues for the design of dissipatively coupled systems for information processing.

Producing better organic solar cells requires deeper knowledge of carrier recombination in polymer-fullerene systems, and a refined understanding of how to measure it. The authors analyze different methods based on impedance spectroscopy to study the dependence of recombination current on charge-carrier density, and identify which approach will yield correct results under given circumstances. Their discussion of assumptions and limitations in such measurements, and a method to extract recombination parameters from impedance data under reverse bias, should be of practical interest to the many engineers and physicists working on thin-film solar cells.

The orbital angular momentum (OAM) of light has become a central topic in quantum information and optical communication. In applications OAM is typically probed by observing a beam’s diffraction through binary amplitude and phase masks. Distinguishing the different OAM states of the beam is usually difficult, though, so here the authors program a digital micromirror array to shift the setup’s slit transversely, to exploit the unique phase of an OAM beam. By observing how diffraction varies as the slit changes, one can probe higher OAM values than usual, allowing access to theoretically unlimited OAM states for high-dimensional quantum systems, or for multiplexing in communication.

In a chip-based waveguide, the confined nature of the optical field and proximity of the waveguide’s surface must not be ignored. The short transit time of warm atoms through highly confined light fields can strongly influence the observed atomic absorption spectra. Most theoretical studies have focused on fields with Gaussian intensity profiles, but is that realistic? Here the authors model the transit-time-limited interaction between atomic gas and an exponentially decaying evanescent field, and find spectral lineshapes unlike those for a Gaussian field. These simulations compare well to experiments on Rb vapor and the evanescent guided modes of an exposed-core optical fiber.

Exploiting the orbital angular momentum (OAM) of light in applications has emerged as a key subject in optics and photonics. Here the authors discuss a holographic technique in the context of generating radio waves that carry OAM. Using suitable reflectarray patterns, a linearly polarized plane wave can be converted to a vortex beam with a spiral phase profile over a wide frequency range—or to multiple OAM beams with flexible beam directions, topological charges, and polarization states. These results are rather interesting for $e.g.$ expanding the capacity of wireless communication systems.

More and more attention is turned to using particular physical systems for innovative computational schemes. For example, an array of synchronized oscillators can be used to solve unconventional, computationally hard problems. Here the authors demonstrate synchronization of spin-transfer-torque nano-oscillators via long-range Oersted magnetic field coupling. This coupling scheme presents the advantage of engineering asymmetric coupling between any two oscillators, which would mimic different biological neural networks. Also, the phase as well as the strength of the coupling can be varied, allowing access to the nonlinear dynamical regime.

Polymer-electrolyte-membrane (PEM) electrolyzers are used to store renewable-source energy in the chemical bonds, of H${}_2$, and understanding gas transport in the liquid-saturated porous transport layer is crucial to improving their efficiency. While incompressible fluid-fluid displacement in porous media has been well studied, here the authors investigate the impact of compressibility on gas transport, via experiments in patterned micromodels. Including a thin layer of small pore throats at the system’s inlet enhances viscous fingering, for lower gas saturation in the bulk. This insight will help to design next-generation PEM electrolyzers for tomorrow’s energy systems.

Efficient electrical switching of antiferromagnets (AFMs) is key to their use in high-density, ultrafast, nonvolatile spintronic memory. Mn${}_2$Au, an AFM with opposite spin sublattices, is a unique metallic material, in that fieldlike spin torque can switch its AFM moments. However, switching induced by antidamping torque remains to be verified in metallic AFMs. Here the authors demonstrate current-induced switching of AFM moment in both a (103)-oriented Mn${}_2$Au single layer and a Mn${}_2$Au/Pt heterojunction by fieldlike torque and antidamping torque respectively. The simultaneous realization of both torque types in metallic Mn2Au makes it a promising candidate for AFM spintronics.

Waveguide-based devices are essential for highly efficient nonlinear optical conversion. However, the nonlinearity associated with the conversion in such devices is not self-evident. The authors present a method using optical frequency metrology to reveal the nonlinearity inside a waveguide, and establish that the spectral broadening in a periodically poled lithium niobate waveguide is due to quadratic nonlinearity. They also demonstrate absolute frequency measurement using the generated ultrabroadband comb. This study will contribute to the design of chip-scale, fully integrated devices for efficient nonlinear optical conversion.

Gilbert damping is of great importance for ultrahigh-speed spintronic devices, as it determines the magnetization dynamics of a ferromagnet. However, $ab$ $initio$ determination of Gilbert damping is challenging, due to the many different mechanisms behind the phenomenon. The authors use scattering theory and extend the powerful torque method to calculate Gilbert damping, and then study several examples of a monolayer of $3d$ transition-metal ferromagnet atop a topological insulator. Surprisingly, Gilbert damping in such a system is boosted by about an order of magnitude, thanks to the spin-orbit coupling in the topological insulator.

Using the excitation of surface plasmon polaritons (SPPs) for optical analog computing of spatial differentiation could find widespread application in $e.g.$ edge detection and image processing, but the inevitable backscattering of SPPs propagating around defects or discontinuities would in general introduce noise to the output signal. The authors design a backscattering-immune spatial differentiator based on a nonreciprocal plasmonic platform, where balance between the rates of asymmetric leakage and intrinsic absorption is required. Here a topologically protected, one-way leaky mode in the terahertz region is just the ticket.

The authors theoretically investigate a cyclic superconducting quantum refrigerator for on-chip cooling. This work predicts substantial cooling of the cold reservoir (Cu) through repeated cycles of adiabatic magnetization and demagnetization of the working substance (Ta), with the excess heat moving to a hot reservoir of superconductor (Nb) having a large gap. The proposed refrigerator is particularly well suited for additional cooling from the base temperature of a dilution refrigerator, about 10 mK, down to about 1 mK. This device could be very effective for cooling superconducting circuits, single-photon detectors, or on-chip sensors.

Switching and routing of broadband optical signals is important for a number of emerging applications involving reprogrammable optical processors and microwave photonic signal processing. Conventional optical switches, based on static refractive-index modulation, are fundamentally limited in their switching bandwidth by disperse phase shifts. The authors show that $dynamic$ refractive-index modulation can lead to achromatic phase shift, and thus an optical switch with a far broader bandwidth, overcoming the limitations of conventional switches. This has the potential to open up opportunities for on-chip processing of ultrabroadband optical pulses.

High-efficiency magnetic materials are important for a wide variety of applications, including digital memory and data storage technology, and microwave devices. Obtaining low loss in ferromagnetic metals is especially challenging, due to electron-magnon coupling and nonlocal relaxation channels in the multilayered stacks required for technology integration. This study reveals that scalable, industry-standard deposition techniques can be employed to achieve significant reduction in the loss of Co-Fe alloy by varying the growth conditions and multilayer design. This result paves the way to next-generation devices coupling magnon propagation with spin-dependent transport.

Electromechanical imaging by scanning probe microscopy (SPM) in a conductive liquid environment is interesting for investigating the physics of electronic, electrochemical, or biological underwater applications. It remains challenging, though, because a conductive liquid obscures what is really going on in SPM detection. This study presents a nanoelectrode SPM probe for such environments. The experimental and theoretical insights gained in this work could provide a powerful method for $in$ $vivo$ or $in$ $operando$ characterization of the electromechanical functionality of materials, or the performance of underwater electronics, biological piezoelectric systems, or energy converters.

Injecting an insoluble gas into a liquid results in subcooling of the liquid, due to forced evaporation into the bubble. While this “bubble cooling” effect causes losses of energy and mass in fluidized-bed reactors, it also can be exploited as an alternative, simple strategy for removing heat from a liquid. Previous studies have assumed the rate of evaporation of the liquid into the bubbles to be independent of the degree of subcooling, but in this study the authors quantify bubble growth directly by high-speed imaging, and disprove this hypothesis. The bubble expansion ratio is a strong function of the liquid’s temperature, as described well by the authors’ model of bubble growth.

Increasing the free-carrier concentration in silicon is a pressing issue in modern electronics. The common shallow-level donors like P and As only permit electrically active doping up to 5×10${}^{20}$ cm${}^{-3}$. The authors discover that, surprisingly, doping with deep-level Te donors can actually exceed this limit. Density-functional calculations unveil the microscopic mechanism behind this behavior: Substitutional Te dimers occupying adjacent Si lattice sites provide free electrons. This work offers an alternative route to realizing the ultrahigh $n$-type doping required for Si-based next-generation electronics, as well as infrared optoelectronics.

Spin waves propagating in nanoscale ferromagnetic waveguides are considered promising for a new generation of digital and analog magnonic signal-processing devices, where the signal is encoded in a spin wave’s amplitude or phase, or both. Stable, error-free operation of spin-wave devices requires well-defined spin-wave phase, which can be disrupted by technological imperfections or thermal noise, leading to the accumulation of phase errors in a magnonic circuit. The authors demonstrate that such phase errors can be corrected by the application of parametric pumping, via voltage-controlled magnetic anisotropy, to local gates placed in the spin wave’s path.

The motion of magnetic domain walls (DWs) in heavy-metal/Co-Fe-B/MgO wires is interesting for developing innovative data-storage devices, but pushing around DWs in these systems using only spin-polarized current has turned out to be much more difficult than expected. This work measures the DW velocity induced by simultaneous application of spin-polarized current and magnetic field in Ta/Co-Fe-B/MgO. The spin-polarization ratio is found to be as low as 0.26, which explains why it has been so difficult to move DWs using just current. This reduced ratio is attributed to spin-flip scattering from the Ta layer; therefore, in such a device use a thinner heavy-metal layer to realize DW motion.

The use of two-dimensional materials could thin down electronic devices to atomic layers, a regime where the precise control of electronic transport becomes a great challenge. A single layer of the ferromagnetic semiconductor CrI${}_3$ has an electronic structure that is significantly modulated by noncollinear magnetization. By introducing a ferromagnetic domain wall, a one-dimensional conduction channel just a few nanometers wide can be formed in the insulating monolayer. These conducting wires can be spatially relocated, “rewritten”, by applying a magnetic field or injecting spin waves, suggesting reprogrammable electronic nanodevices.

Individual epitaxial semiconductor quantum dots (QDs) have been extensively considered as an “artificial atom” platform for quantum optics and photonics applications. The QD carrier dynamics responsible for ultimate device performance is indeed complex, due in part to rich interaction with the wetting layer’s two-dimensional carrier reservoir. The authors investigate this interaction with time-resolved experiments and rate-equation modeling, showing that these analyses are important for understanding the limitations of single-photon photoluminescence emission, improving lasers and fast optical modulators, and developing next-generation ultrafast all-optical switches.

By means of the acoustic radiation force, standing waves of ultrasound can form a two-dimensional microarray to hold dispersed particles or cells in solution. This phenomenon can be used in lab-on-a-chip technology for single-cell analysis and analytical chemistry, though the physics behind ultrasound patterning is not fully understood. This study provides an analytical expression for the acoustic potential energy that traps cells, and reveals that the trapping points at pressure nodes, antinodes, and internode midpoints depend on a cell’s size and mechanical properties. This theoretical approach should help to engineer acoustic-patterning devices, especially for “one cell per well” analysis.

The converse magnetoelectric effect (CME, $i.e.$ the magnetic response to a static electric field) in ferrite-ferroelectric composites is important for an innovative class of voltage-tunable, high-frequency magnetic devices. Technical challenges persist, though, due to weak CME coupling, large ferromagnetic-resonance (FMR) linewidth, and the need for a large bias magnetic field. This study aims to overcome those challenges by using single-crystal-like nickel ferrite films exhibiting high magnetostriction and narrow FMR linewidth. These films present a very high growth-induced magnetic anisotropy field that eliminates the need for a bias field, and facilitates device integration.

Cut the cord: Technology for wireless power transfer is important for conveniently charging electronic devices at a distance. Here metasurfaces are typically employed for wavefront shaping, to improve long-range wireless power transfer via far-field coupling. The authors present a “smart table” for wireless charging, with a metasurface incorporated as an intermediary between transmitter and receiver resonators, to substantially enhance the $near-field$ coupling. Power-transfer efficiency of over 80% is experimentally obtained, at distances up to 1 m between the resonators.

Proper estimation of the efficiency of spin-orbit torque (SOT) in ferromagnetic multilayers with small coercive force is not trivial, yet is in high demand for implementing current-induced magnetization reversal in memory cells. This study proposes a method for estimating the effective field of SOT based on analyzing the position of a domain wall displaced parallel to the current, combined with the spatial distribution of the Oersted field. This advanced method is more reliable than the extensively used second-harmonic and current-induced-magnetization-curve techniques, for a wide spectrum of materials, including Pt-based systems.

Antiferromagnetic (AFM) oscillators are of interest because of their potential to operate at THz frequencies, and thus to fill the “terahertz gap”. Previous studies on AFM oscillators have focused on using the spin Hall effect to generate spin current, but other mechanisms exist. Here the authors investigate using spin current with an arbitrary polarization direction that is not simply due to the spin Hall effect. Their results will be useful for application of such oscillators, and more broadly for understanding AFM spin dynamics.

Two-dimensional (2D) materials such as topological insulators (TIs) are promising components for the design of spintronic devices, and this work offers a way to quantify their advantage. This article presents a study of the conversions between charge and spin currents based on the regular and inverse spin Hall effects in heavy metals, and based on the regular and inverse Edelstein effects in a 2D electron gas. The authors propose an approach for comparing the efficiency of conversion in 3D and 2D systems; this comparison is not straightforward, as those different physical effects are responsible at different dimensionalities. Spoiler: TIs turn out to be far more effective converters.

Separating the contributions of modulus and damping to elastic nonlinearity is important for understanding the physics behind observations, for applications in ultrasonic imaging and nonlinear nondestructive testing, and more generally for the characterization of materials. This study offers an experimental approach which, besides being simple to implement, allows one to separate nonlinearities in velocity and attenuation from a single monochromatic signal. The proposed method should enable monitoring of the very early stages of slow nonlinear dynamical phenomena, such as conditioning or relaxation, which is difficult with other experimental techniques.

Power amplifiers display some interesting physical phenomena, such as nonreciprocity, but their characteristics have not been fully explored. In this study of the detailed relationship between supply power and amplification, the authors combine a coding metasurface with active amplifiers to realize arbitrary editing of the spatial distribution of a propagating wave’s energy. With this metasurface, the energy in space of linearly polarized microwaves can be amplified or reduced at will by controlling the voltage.

Cu(In,Ga)S${}_2$ is a promising material for multijunction solar cells due to its tunable band gap, but still suffers from low efficiency of unknown origin. The authors study its structural, optical, and electrical properties with respect to annealing temperature, and find that higher temperature reduces the density of deep defects, leading to the higher open-circuit voltage that is responsible for the recently observed improvements in efficiency. This study could lead to further optimization of this material, and better design of multijunction solar cells.

Mechanically guided assembly through compressive buckling emerges as a viable approach to fabricate complex three-dimensional (3D) mesostructures and functional devices. However, it remains a challenge to efficiently and accurately find the 2D precursor of a desired 3D structure. The authors propose a systematic computational method based on an adaptive genetic algorithm, and solve this inverse design problem for ribbon-shaped 3D structures. This work has a broad range of applications in the design optimization of ribbon-shaped 3D microelectronic devices, such as tunable antennas and energy harvesters.

Gradient metasurfaces have enabled the control of electromagnetic waves for arbitrary wavefront shaping in both optical near and far fields. So far the responses of such a structure in transmission and reception have been identical, due to time-reversal symmetry. This work shows that time-modulated gradient metasurfaces can efficiently convert between only two desired frequencies, allowing one to break and manipulate reciprocity by imposing drastically different phase gradients in frequency up- and down-conversion. Thus a wide variety of functionalities may be implemented, including beam steering and focusing, with strong and angle-insensitive nonreciprocal responses.

Reflective objectives are widely used in many imaging systems, but can hardly be compact, due to their curved mirrors. To address this problem, the authors planarize the mirrors in a Cassegrain-style objective by substituting geometric-phase-based flat metasurfaces for conventional curved mirrors. These reflective metasurface objectives are intrinsically free of residual light, and thus less demanding in terms of precision in illumination and nanofabrication, compared to refractive metalenses. This planar metasurface design can potentially reshape and simplify the core components of many microscopes and telescopes, and opens an avenue to ultracompact optics.

Telecom-wavelength single photons are the leading carriers of quantum information for practical photonic quantum technologies, and there has been significant progress in developing compatible components like detectors and single-photon sources. However, an on-chip, broadband quantum memory for storage and reemission of single telecom photons—indispensable for future quantum repeaters—has been lacking. The authors demonstrate a light-matter interface for quantum storage of 1532-nm photons: a cryogenically cooled lithium niobate crystal waveguide. The memory hinges on a broadband atomic frequency comb created by persistent spectral hole burning through long-lived superhyperfine levels.

Strong electric field enhancement is important for increasing the sensitivity of spectroscopy-based biomolecule detection. Although decreasing the gap between plasmonic nanoparticles can enhance the field, there is a limit for the maximum enhancement that can be achieved with this method. This study combines structure at two different length scales, micro- and nano-, to boost the electric field by utilizing two well-known phenomena: the Rayleigh anomaly, and plasmonic resonances. In this way surface-enhanced Raman scattering can be boosted an order of magnitude, for ultrasensitive detection.

Dilute magnetic semiconductors (DMSs) constitute a class of interesting and important materials for potential applications in spintronics. The recent discovery of isovalent Fe-doped $III-V$ semiconductors with very high Curie temperature $T_C$ has opened an avenue for DMS development—and also a challenge to the existing theory of magnetic coupling in semiconductors. This study presents a mechanism for the ferromagnetism of isovalent Fe-doped DMSs and explains the origin of their high $T_C$, which hopefully will accelerate their development. In these materials the key is a particular $p-d$ orbital coupling with charge transfer to the iron.

Heat flow at the nanoscale is still a topic full of unsolved questions. This lack of understanding is one of the main reasons for the persistence of the “thermal wall” that impedes the downsizing of electronic devices. This work shows that phonon hydrodynamics could be behind this unexplained behavior. Generalizing Fourier’s law by incorporating hydrodynamic terms reveals that phenomena like viscosity and vorticity appear as important aspects for describing heat at this scale, such that agreement between theory and simulations improves significantly.

Quantum-limited Josephson parametric amplifiers are crucial components in readout chains for circuit quantum electrodynamics. The power handling of state-of-the-art parametric amplifiers is limited by signal-induced Stark shifts. The authors use an innovative circuit element with a Stark-shift-free sweet spot in parameter space to boost the power handling of such an amplifier by an order of magnitude, which is quite promising for the implementation of bilinear Hamiltonians with high dynamic range in quantum information processing.

Magnonic crystals (magnetic metamaterials for manipulating spin waves) are important building blocks for rf filters and logic. However, regular crystals pose significant constraints for complex designs. This study combines time-resolved x-ray microscopy and numerical modeling to show that quasiperiodic Fibonacci crystals provide advanced control of the magnonic band structure. Furthermore, x-ray microscopy yields insight into the quasicrystal’s inner workings, and a simple analytical model is derived to facilitate application of these systems.

Multimode microwave resonators that can be strongly coupled to superconducting qubits allow exploration of large-scale entanglement, or quantum simulations of many-body phenomena. Typically, long superconducting transmission lines or large arrays of transmission-line resonators are required. This work demonstrates superconducting metamaterial resonators with a dense mode spectrum above a low-frequency band gap. Imaging of the microwave fields shows that these devices exhibit a left-handed dispersion relation, consistent with modeling and simulations. This compact design is compatible with superconducting-qubit integration for applications in quantum information and communication.

The authors predict multiterminal rectification arising in a spin-orbit material when it is coupled to a ferromagnet with a low energy barrier. The proposed all-metallic device can rectify arbitrarily small signals within a frequency band dictated by an angular-momentum conservation principle, which points to applications in “passive” microwave or rf detection and energy harvesting from weak ambient signals.

The phenomenon of flux-flow instability is important for applications in photon detection and quantum information processing based on superconductors, because of the breakdown of superconductivity at high vortex velocities. This study exploits a collective dynamical ordering of magnetic flux quanta in decorated Nb films, in which all vortices move along stripes of Co and reach significantly higher velocities than in plain films. The observed suppression of flux-flow instability by the vortex-guiding effect could improve the performance of superconducting photodetectors and transmission lines.

Electronic emulation of the biological synapse (the memory and learning element of the brain) is an important step toward realizing brain-inspired computing systems. The authors demonstrate a voltage-controlled magnetic device, based on oxygen migration, that emulates major functionalities of a biological synapse such as potentiation; depression; plasticity that depends on spike magnitude, rate, and timing; and short- to long-term memory formation. Additionally, there is separation of read and write paths, and an ability to be programmed with “negative weighting”, which helps to overcome the disadvantages of memristor synapses.

Bianisotropic all-dielectric metasurfaces offer a broad range of functionalities in optics, but so far have been designed case by case, and their generalization to different forms of polarization control is not straightforward. Moreover, their demonstrated range of bianisotropic properties has been limited, due to the single-layer topologies used. The authors propose multilayered metasurfaces built from high-contrast subwavelength gratings of varying orientations. Such multilayered all-dielectric metasurfaces can yield polarization conversion with broadband, multiband, and multifunctional responses, and can be designed in a systematic manner to realize all three types of responses.

Lasers with ultrafast response are crucial elements for many optoelectronics applications. The response time of dielectric surface emitting lasers is limited to hundreds of picoseconds, while using metallic plasmonic structures can significantly reduce that time. The authors study the behavior of a plasmonic distributed-feedback laser in the regime of large signal modulation, showing that the response time depends strongly and nonmonotonically on the size of the pump beam. For typical experimental parameters, there is an optimal size of the pump beam for which the response time can be reduced to a picosecond.

The development of compact optical frequency references plays an important role in future optical timing systems, on the ground and in space, including next-generation global-navigation satellite systems. Realization and qualification of such systems is held back by technical challenges in creating robust, reliable devices for optical metrology under demanding conditions. Here researchers demonstrate the autonomous operation of an I${}_2$ frequency reference and the measurement of its optical frequency using a dual frequency comb on a sounding rocket, as a milestone on the way to routine application of such optical technologies in space.

On-chip microwave oscillators are ubiquitous in embedded communication systems found in $e.g.$ mobile phones, medically implanted devices, and defense electronics. They are usually built with operational amplifiers consisting of several transistors and other circuit elements that consume much of a chip’s area, and dissipate excessive energy. This work demonstrates a fresh approach for implementing a microwave oscillator that requires only a single magnetotunneling junction and a passive resistor. Its output oscillation shows exceptional spectral purity, with nearly a single frequency component.

Measurements of resistivity switching in Josephson junctions and nanowires are widely used in the experimental superconductivity community, to attain insight into decay processes of metastable states, to test qubits, or to detect single photons. Such measurements are commonplace, but not easy. The authors formulate and experimentally verify the criterion for clean, independent switching measurement. They study the correlated regime, where the result of the test for one pulse affects the outcome for the following pulses, showing how to introduce thermal correlation into the “flipping coin” experiment in a controlled manner, and analyzing its implications for the switching statistics.

All-purpose (or adaptive) materials are of great significance for practical applications. However, typical materials have fixed inherent properties, and hence cannot adapt to environmental changes. This study designs and fabricates a class of passive metamaterial shells that exhibit adaptive thermal responses to changes in their surroundings—chameleonlike behavior, in other words, for $e.g.$ cloaking objects against radiant heat,. Such chameleonlike thermal metashells will have an impact because they can meet different thermal requirements under various conditions, without foreknowledge of those conditions.

The authors design a single-charge electrometer with sensitivity improved by an order of magnitude, compared to the state of the art. This improvement is obtained by taking full advantage of the quantum nature of the measuring circuit. Importantly, such an electrometer could improve by one or two orders of magnitude the sensitivity of astroparticle experiments such as bolometric direct dark-matter detection, or coherent neutrino scattering. This quantum sensor is also potentially useful in mesoscopic physics, in particular to unveil the physics of Majorana fermions.

The optical pumping of atoms is a central process in many applications of atomic physics, such as laser cooling and the precise determination of physical constants. Popular methods for producing metastable helium atoms in supersonic expansions are discharge and electron-impact excitation. However, both approaches yield helium atoms in two metastable states, $2^1{S}_0$ and $2^3{S}_1$, which limits the applications of these highly energetic species. This study presents an experimental scheme in which the $2^1{S}_0$ state population is depleted via excitation to the $4^1{P}_1$ state, using diode-laser radiation. The results suggest nearly 100% depletion efficiency at all beam velocities studied.

Researchers discuss a single-photon detector based on a Josephson tunnel junction, formed by electrodes made of different superconductors residing at different temperatures. This radiation sensor takes advantage of the steplike behavior of the critical current $I_c$ in the dissipationless regime: When a photon is absorbed, it produces a temperature rise that can cause $I_c$ to suddenly jump. The proposal is to detect the absorption via a fast, noninvasive readout scheme with multiplexing capabilities, based on the change in Josephson kinetic inductance associated with the change in $I_c$. This detector can also discern the photon’s frequency, from midinfrared to ultraviolet.

Nonlinear magneto-optical rotation (NMOR) is a sensitive technique for measuring magnetic fields, though the polarization self-rotation (PSR) effect is a fundamental source of excess quantum noise that limits its sensitivity. This study uses a phase shifter to solve the limitation due to the PSR effect. Implementation of the phase shifter improves the signal-to-noise ratio up to 10 dB in the range of Earth’s magnetic field. This approach should be useful for high-sensitivity magnetic field measurements in a geomagnetic environment.

This work establishes a fruitful connection between the fields of “stealth materials” in soft matter and acoustics, pushing the field of stealth materials in a more general direction, based on the principles of wave physics. The positions of scatterers in an air-filled acoustic waveguide are chosen such that, in the Born approximation, suppression of scattering over a broad range of frequencies is achieved, and thus broadband transparency. Experimental results are in excellent agreement with theory, despite the presence of losses and the finite size of the system.

Metasurfaces for wavefront transformations are fundamentally limited in terms of efficiency, if based purely on local responses, by which unitary reflection can only be achieved with active and lossy elements. The authors explore $nonlocal$ phenomena in acoustic metasurfaces, by considering transverse energy tunneling between neighboring constituent elements. Using this form of acoustic nonlocality, they achieve unitary anomalous reflection using fully $passive$ metasurface elements. While this paper explicitly addresses acoustics, similar approaches may be explored in electromagnetics and optics, and for elastic waves, with interesting opportunities for beam steering and more.

So-called “exchange-spring magnets” are important for synthesizing rare-earth-free permanent magnets with a high energy product, but the latter is limited by the critical thickness of the soft magnetic phase, above which exchange coupling deteriorates. Thus understanding the physics of exchange coupling is necessary to overcome that technical difficulty. This study uses experiment and theory to build up a model system that allows systematic analysis of exchange interactions in a MnGa/FeCo bilayer exchange system. The significant roles of epitaxial growth and hard/soft interfacial properties offer engineering solutions for multilayered exchange-spring magnets in $e.g.$ electric motors.

Mobility measurements using single-carrier devices are one of the most common means for probing charge transport in intrinsic semiconductors; however, the built-in voltage must be known in advance, for precise analysis of the current-voltage curves obtained from measuring space-charge-limited current. The author presents an easy, direct analytical model for determining the built-in voltage of a single-carrier device directly from the $J-V$ curves, without any knowledge of the charge-carrier mobility. This approach is expected to be immediately and widely useful to the large community working on semiconductor physics and devices.

In photoemission, control of the carrier-envelope phase (CEP) is highly desirable for manipulating strong-field light-matter interactions on the attosecond timescale. However, artifacts due to fluctuations in laser intensity obscure weak, intrinsic CEP effects when using single-shot CEP tagging. The authors demonstrate that, by further tagging the laser-pulse energy and applying appropriate intensity corrections, weak CEP-dependent photoemission with modulation depth as low as ~1% from a tungsten surface can be detected—a significant improvement in sensitivity. This approach can facilitate ultrafast waveform control in atoms, molecules, surfaces, and plasmonic nanostructures.

Photoelectron yield spectroscopy (PYS) is a standard technique for probing the ionization potentials of various materials, including the organic films relevant to electronics. While these spectra contain much information about a material’s electronic structure, the lack of an underlying theory has left it inaccessible. Thus the authors develop a general theory and simple method of analysis to obtain the peak position and width for the highest occupied molecular orbital (HOMO) of organic matter, directly from the PYS spectrum. This crucial information for the chemistry and physics of organic compounds can now be readily obtained, using academic or standard industrial instrumentation.

Rare-earth ions (REIs) in solid hosts have important applications in the storage and conversion of quantum information at optical and microwave frequencies. Yttrium orthosilicate is a widely used host for REIs, with excellent optical properties, but its compatibility with superconducting resonators and circuits for quantum technology is not known. This study uses Y${}_2$SiO${}_5$ directly as a substrate for superconducting devices, revealing dielectric losses comparable to those of sapphire. This enables fabrication of high-quality microwave devices coupled to optically accessible rare-earth spins, pointing to applications in quantum storage and microwave-to-optical conversion.

The emission of terahertz radiation from optically excited ferromagnet/metal structures points to innovative spintronic THz sources. To optimize such technology, more thorough models and simulations are needed. The authors combine efficient solutions for light- and THz-wave propagation with the Boltzmann transport equation, for the dynamics of the optically excited electrons, to gain insight into broadband THz emission from Fe/Pt structures. The model provides a numerically efficient connection between intrinsic material properties and experimental parameters on the one hand, and the efficiency of the emitter structure and details of the emitted spectrum on the other.

Calculating the fields around an antenna (for example) is one thing, but handily measuring them is another. The authors experimentally demonstrate imaging of the electromagnetic near field in the visible range. Thermofluorescent absorbers in thin films (which possibly could be sprayed or painted on) are a cheap, fast way to test and characterize radiative sources on a large scale. This approach is not only less expensive than methods requiring infrared cameras, but also offers better spatial resolution.

Permanent-magnet applications such as electric motors demand a high energy product (simultaneously high saturation magnetization and uniaxial magnetic anisotropy) and Curie temperature, preferably in $3d$ metals without any rare-earth elements. The authors’ first-principles calculations highlight a promising pathway to achieving these properties in $B2$-ordered FeCo alloy, by interstitial doping with $2p$-electron nonmetal elements, such as N. This prediction could have real impact on the manufacturing of practical, rare-earth-free, high-performance permanent magnets.

Nanomagnetic logic is promising for low-power computing, and for integration of data processing and memory in the same architecture. In this work the thermal relaxation paths associated with logic operations in artificial spin ice are considered, which involve switching of individual nanomagnets toward a low-energy state. Both monotonic and intermittent paths have been identified, which lend themselves to deterministic and probabilistic computing respectively. Furthermore, the balance of competing relaxation paths can be shifted, which is essential for implementing probabilistic computation—for example, in artificial neural networks where the outcome can be tuned via a feedback loop.

Microwave kinetic inductance detectors (MKIDs) are central equipment in radioastronomy, thanks to their ease of fabrication and potential for multiplexing, which allow for millimeter-wavelength cameras with thousands of pixels. However, increasing their kinetic inductance to obtain maximum responsivity degrades their performance, due to the onset of nonlinearities. This work determines the optimum working point of MKIDs by taking into account the interplay between these two competing factors. It is also found that a lower-gapped ground plane can act as a phonon trap, providing lower noise-equivalent power in MKIDs limited by quasiparticle noise.

Li${}_2$TiO${}_3$ is a most promising breeder-blanket material for a nuclear fusion reactor. Knowing the structures and stabilities of Li${}_2$TiO${}_3$ surfaces can be critical to better understanding the physical and chemical processes that occur there, but an atomistic understanding from $ab$ $initio$ calculations is still lacking, due to the complexity of terminations in this three-component system. This work provides stability analyses for both perfect and defective Li${}_2$TiO${}_3$ (001) by calculating the surface energies of its different terminations, and consequently obtaining the ternary surface phase diagram. The method used here can also be applied to similar three-component systems.

Driven by huge demand for spin-based devices, research in spintronics has revealed spin-dependent electrical transport in nanostructures of conducting magnetic nanoparticles (NPs). This work investigates self-assembled structures of core-shell NPs exhibiting tunneling magnetoresistance with a tunable switching field. Here the core provides the spin-polarized carriers, while the shell is a tunnel barrier, and also modulates the switching field via magnetic anisotropy that varies with shell composition. This approach allows simplified fabrication and versatility by combining functionalities in a single nanoparticle, providing a promising basis for the design of future NP devices.

Imbalanced charge transport in organic solar cells is a common, yet often overlooked, issue. The authors combine experiments, simulations, and analytical theory to clarify the role of mismatched electron and hole mobilities in surface recombination at a solar cell’s electrical contacts. Importantly, they find that the open-circuit voltage depends not only on the bulk recombination rate and injection-barrier heights, but also on the mobility ratio in the active layer.