The emerging field of $magnonics$ utilizes propagating coherent magnons (collective excitations of electron spins) as carriers of information. Scaling down magnonic devices requires finding new approaches to the efficient excitation of magnons at nanoscale. This study demonstrates an approach to excite coherent GHz-frequency magnons in magnetic insulators by means of dc electric currents. The proposed method opens a route for implementing highly efficient nanomagnonic computing systems.

Efficient characterization and modeling of gate-defined potential barriers is key to engineering tomorrow’s quantum-dot-based computing devices. Although a number of models exist, many factors are still not fully understood, such as the dependence of the barrier on gate voltage for a wide range of bias, and the effect of neighboring electrodes. The authors use transport spectroscopy to characterize a MOS tunnel barrier, and analyze the barrier using a quasianalytic model that includes cryogenic and quantum confinement effects. The barrier shows different regimes of voltage dependence, and this result provides a path toward compact modeling of tunnel junctions in quantum devices.

Magnetic random-access memory (MRAM) using spin-transfer torque for write operations has been intensively developed as a technology for saving energy. The authors’ recently presented voltage-controlled spintronic memory (VoCSM), which instead employs the spin Hall effect for writing, is here refined. High writing efficiency in VoCSM is achieved by means of an $a$-TaB/$\beta$-Ta spin Hall electrode, which features reduced write-current density, low write-error rate, strong durability, and high breakdown voltage. This improved VoCSM is seen as a path to high-density, high-speed nonvolatile memory with low power consumption.

Cesium lead halides in the perovskite crystal structure are promising absorbers to enable cheap, high-performance photovoltaics or light-emitting devices. Using first-principles calculations plus Boltzmann transport theory, the authors report an intrinsic limit on the room-temperature carrier mobility of CsPb$X_3$ that is due to scattering via electron-phonon coupling. Using different halides $X$ in the compound can change the mobility by a factor of 3—5, because of the change in electronic effective mass, as well as the scattering rate. This insight should impact the engineering of optoelectronic devices based on these perovskites.

Phase estimation has applications from quantum imaging to gravitational-wave detection. In areas such as biological-system sampling or quantum metrology, it is crucial to optimally acquire information from a very limited number of probes. To address this need, the authors describe and experimentally verify a machine-learning method for optimal adaptive single-photon phase estimation based on a small number of trials. This approach could be used to optimize quantum metrology protocols, and can be extended to general multiparameter scenarios.

Electrooptomechanical hybrid systems are garnering interest as candidate quantum transducers, to link microwave and optical fields in a future quantum Internet, for example. Achieving quantum-level operation in such systems is a challenge, though. This work discusses equivalent circuits as a unifying framework for designing and analyzing such hybrid quantum transducers, while also including quantum noise in a straightforward manner. By providing a common diagrammatical language for the electronic, optical, and mechanical elements involved, this approach may facilitate a joint effort in electrical engineering and quantum optomechanics to realize hybrid quantum networks.

Understanding the conversion efficiency limit under $nonequilibrium$ conditions is important for developing high-efficiency solar cells that go beyond the Shockley-Queisser (SQ) limit. The authors present such a theory, to clarify the impact of charge-carrier extraction and thermalization dynamics on the limit. Simulation of a simple, planar solar cell is used to address the parameter regime (in terms of carrier extraction time) where the standard SQ theory applies, and to determine the conversion efficiency limit outside that regime. This theory could also help to achieve high efficiencies in other types of nonequilibrium solar cells.

Lead-free halide double perovskites could be useful in optoelectronic applications, such solar cells and photocatalysis. However, a comprehensive understanding of intrinsic defect properties is needed, as defects strongly affect carrier density and transport. First-principles energy calculations are used to investigate representative lead-free halide double perovskites and identify deep defect levels. The authors show how chemical potentials of the constituent elements can be modified, to suppress unwanted defects or to tune the Fermi level. These guidelines will further the exploration of these compounds for use in high-performance optoelectronic devices.

The emerging field of $magnonics$ utilizes propagating coherent magnons (collective excitations of electron spins) as carriers of information. Scaling down magnonic devices requires finding new approaches to the efficient excitation of magnons at nanoscale. This study demonstrates an approach to excite coherent GHz-frequency magnons in magnetic insulators by means of dc electric currents. The proposed method opens a route for implementing highly efficient nanomagnonic computing systems.

Solid solutions of the wide-band-gap semiconductors ZnO and GaN hold great promise for diverse optoelectronic applications, including photocatalysis, photovoltaics, and light-emitting diodes. However, the difficulty of synthesizing single-crystalline samples of these alloys thwarts a deeper understanding of their physics. The authors develop a technique for low-temperature epitaxy to solve this problem, and present a systematic investigation of the optical properties and electronic structure of the system. This epitaxy technique could be used to fabricate high-quality alloys or superlattices of various oxynitride systems for advanced optoelectronics.

In photonic systems, exploring the valley degree of freedom (due to the presence of a local extremum in band structure in momentum space) is interesting for application of the photonic Hall effect, robust delay lines, and perfect outcoupling refraction. This work demonstrates that electromagnetic flow in a photonic-crystal waveguide can be tuned using this topological degree of freedom. Both tunable excitation of valley bulk states and tunable valley-dependent edge-state dispersion are shown. These results may impact the dynamic modulation of light in devices.

Efficient characterization and modeling of gate-defined potential barriers is key to engineering tomorrow’s quantum-dot-based computing devices. Although a number of models exist, many factors are still not fully understood, such as the dependence of the barrier on gate voltage for a wide range of bias, and the effect of neighboring electrodes. The authors use transport spectroscopy to characterize a MOS tunnel barrier, and analyze the barrier using a quasianalytic model that includes cryogenic and quantum confinement effects. The barrier shows different regimes of voltage dependence, and this result provides a path toward compact modeling of tunnel junctions in quantum devices.

Ferroelectric BiFeO${}_3$ is otherwise promising for photovoltaic applications, but its large band gap limits efficient absorption of sunlight. The authors show that, through structural engineering, ferroelectric (BiCrO${}_3$)${}_m$/(BiFeO${}_3$)${}_n$ superlattices can exhibit unexpectedly low band gaps that approach the ideal Shockley-Queisser value. These results indicate that superlattice structuring can be an effective strategy for designing oxide semiconductors with fundamental band gaps much smaller than in either parent material, which points the way to ferroelectric photovoltaic applications.

Micro- and nanochannels are promising for state-of-the-art sensing technologies in biomedical devices, but the accurate and selective control of nanomaterials near the sensing elements is a challenge. This paper rises to the challenge by introducing an unconventional manipulation technique, combining laser-induced thermophoresis with a sudden narrowing of the channel. The strength of thermophoresis, compared to other transport mechanisms, lies in its selectivity: Targets can be guided to the sudden contraction, while unwanted particles are repelled. Also, the nearby flow field leads to dynamical concentration of target materials.

Superresolved imaging is strongly desired for acoustic nondestructive testing, but normally is available only in a single frequency band. This study presents an elegant design for a multiband double-negative ($i.e.$ both bulk modulus and effective mass density are negative) unit cell, using the coupling of single-negative resonators, where double negativity in multiple bands is due to overlap of dipolar and monopolar modes within the unit cell. The number of double-negative bands depends on the number of optimally coupled resonator pairs in the unit cell, and thus is easily manipulated. This approach is appealing for subwavelength imaging devices that span many frequencies.

Quadratic terahertz optical nonlinearities are difficult to explore in time-domain spectroscopy when a strong linear optical response is present, as the quadratic and much stronger linear responses may overlap significantly. This study shows how to separate the responses, enabling experimental study of terahertz second-harmonic generation in a multitude of materials for photonic applications in communication, security screening, and medical and biological imaging. The method also is easily extended to applications at other wavelengths (from far-infrared to ultraviolet), and to higher-order nonlinearities.

$Optical$ $vortices$ (corkscrews of light spinning about zeros in the electric field) offer many applications in $e.g.$ optical communication, superresolution microscopy, and astronomical imaging, so creating them controllably is of considerable interest. The authors reveal that an electric-field-induced, self-assembled grid of topological defects can generate optical vortices at different length scales, via two mechanisms: directly by transmission through an individual defect, and by diffraction from a dislocation in the grid pattern. The efficiency of generating optical vortices here can approach 100%, even for different wavelengths of light, and can be tuned by an applied voltage.

Self-assembled quantum dots (QDs) can be used as on-demand sources of polarization-entangled photon pairs, but it turns out that their fullest utility is spoiled by a subtlety of electronic structure that cannot be ironed out simply. Investigating a little-studied symmetry-breaking effect between excitons and biexcitons in QDs, the authors derive the relations between lifetime asymmetries, polarization angles, and fine-structure splittings, then verify them using large-scale atomistic simulations. The complete description of excitons and biexcitons in QDs presented here could also be useful in understanding the optical properties of other semiconductor nanostructures.

When an array of thin ferromagnetic nandots is magnetized by an in-plane magnetic field, the dots have a panoply of magnetic states to occupy, due to the interplay of the internal magnetic field, interelement interaction, and external field. Ferromagnetic nanoscale crosses present a unique spin configuration and spin-wave properties, which are easily tunable by subtle reorientation of an in-plane external field. This technique could be used in variety of magnetic or spintronic applications, such as magnetic storage, frequency-dependent couplers, directional couplers, or logic devices.

Magnetic random-access memory (MRAM) using spin-transfer torque for write operations has been intensively developed as a technology for saving energy. The authors’ recently presented voltage-controlled spintronic memory (VoCSM), which instead employs the spin Hall effect for writing, is here refined. High writing efficiency in VoCSM is achieved by means of an $a$-TaB/$\beta$-Ta spin Hall electrode, which features reduced write-current density, low write-error rate, strong durability, and high breakdown voltage. This improved VoCSM is seen as a path to high-density, high-speed nonvolatile memory with low power consumption.

Trapped ions have achieved the highest fidelity in local processing of quantum information, but their short emission wavelengths are incompatible with long-distance distribution of information at telecom wavelengths, which prevents their use in a large-scale quantum network. We present an interface capable of two-way frequency conversion of single photons between a Sr${}^{+}$ node-compatible wavelength and the telecom $C$ band, forming part of a critical pathway for future hybrid light-matter quantum networks. This scheme offers bidirectional translation of widely separated frequencies in a single stage, with noise levels low enough for high-fidelity interconnects.

Cesium lead halides in the perovskite crystal structure are promising absorbers to enable cheap, high-performance photovoltaics or light-emitting devices. Using first-principles calculations plus Boltzmann transport theory, the authors report an intrinsic limit on the room-temperature carrier mobility of CsPb$X_3$ that is due to scattering via electron-phonon coupling. Using different halides $X$ in the compound can change the mobility by a factor of 3—5, because of the change in electronic effective mass, as well as the scattering rate. This insight should impact the engineering of optoelectronic devices based on these perovskites.

In principle the rotational Doppler effect could be used to detect the turning of a distant object, but the approach has not progressed beyond the laboratory, due to low photon-collection efficiency and orbital angular momentum (OAM) mode crosstalk. The authors address these challenges and successfully observe the effect over a 120-m free-space link between two rooftops. By analyzing the photon counts for the relevant OAM components, they deduce the speed and symmetry of rotating bodies, even at the photon-counting level. This work is a significant step toward remote sensing in a realistic environment based on the rotational Doppler effect of “twisted light”.

Optically pumped continuous-wave (cw) semiconductor disk lasers (SDLs) have an established commercial impact, and recent progress in ultrashort-pulse operation makes them attractive for applications in frequency metrology. There is, however, a tradeoff between femtosecond pulse lengths and average output power. Quantum-dot (QD) materials could potentially solve this problem—but not just any dots. The authors show that SDLs based on active submonolayer QDs produce record-high cw output power, but without stable mode locking, which is due to fundamental physical reasons.

Replacing enough carbon with boron turns diamond into a superconductor, making it even more interesting for applications in electronics, quantum computing, and photon detection. Currently the best samples only superconduct below 10 K, but theory predicts that this threshold could be pushed much higher. Taking an unconventional approach, the authors systematically compensate charge carriers in a single sample using high-energy ion implantation, revealing a strong link between hole concentration and superconductivity. The demonstrated ability to directly alter diamond’s superconducting properties through ion implantation and annealing is promising for device engineering.

Spin-based quantum-dot qubits in semiconductors are of interest in large-scale quantum computing, because they are naturally compatible with mature semiconductor manufacturing technologies. Electromagnetic noise spoils spin coherence, which sets the number of operations possible before the quantum information is lost over time, so a good understanding of a qubit’s noise environment is required. This work uses a quantum-dot qubit metrologically, to study the environment of a silicon quantum chip, demonstrating that this technique is valuable in the quest to mitigate noise in tomorrow’s quantum architectures.

Nanopores can efficiently reduce the thermal conductivity $\kappa$ of a crystalline material, but experimentally it is difficult to find optimal structures, due to the complex geometrical parameters involved, and their distributions. Instead the authors use ray-tracing Monte Carlo simulation and show that, for spherical pores, $\kappa$ reduction is insensitive to the size distribution. It is found that heat conduction in nanoporous systems can be reasonably described by a phonon-scattering model with a single length scale. This practical guidance for designing thermal conductivity should promote heat management at the nanoscale, for better device performance.

The generation and distribution of entanglement is a central topic in quantum information science, enabling important applications in quantum communication and computing. This has created great interest in creating “flying” entangled states. The authors present a parametric cavity that generates multimode states of microwave photons with programmable entanglement structure. This advance will facilitate progress in a range of fields, including microwave cluster states, error-correctable logical qubits for quantum communication, and the quantum simulation of relativistic quantum information processing systems.

While the metal-graphene interaction exhibits exciting potential for optoelectronic applications, the physics of such an interface is poorly understood. The authors explain how the simultaneous combination of Raman spectroscopy and photoresponse acts as a uniquely capable diagnostic for this system. Applying this technique to a semimetal/graphene interface, they demonstrate that the doping of graphene on bismuth is very large, compared to the case of a metal or semiconductor. This important result may have significant impact on graphene-based optoelectronics.

Dispersion of propagating wave pulses and the spreading of the energy carried by them are observed in many applications. While dispersion is sometimes considered unfavorable, the authors show that it can be used to focus waves and the energy they carry at arbitrary points in space and time. This is demonstrated experimentally by focusing flexural acoustic waves to induce dynamic fracture of a glass beam, which suggests a methodology for dynamic bending tests, and moreover verifies a concept that extends beyond structural acoustics and fracture mechanics to other fields working on dispersive waves.

In acoustics, the method of $Marchenko$ $redatuming$, which can focus wave fields in an unknown lossless medium accessible from one side only, is powerful in helping to characterize a medium’s properties accurately and efficiently. Ubiquitous dissipation of wave energy challenges the method’s practical application, though. The authors develop the dissipative Marchenko method, which overcomes the lossless-medium assumption by numerically constructing a corresponding effectual medium from the double-sided scattering data measured from the actual lossy medium. This method will impact resource exploration, medical imaging, and nondestructive testing, among other fields.

Organic thin-film transistors are especially apt for mobile and wearable electronics, in which low-voltage operation is a critical prerequisite. The charge transport in organic semiconductors is significantly governed by the density and energetic distribution of trap states. The authors extend an established method for determining the density of trap states to transistors with low operating voltage, and thus thin layers of gate dielectric. This approach helps to study the fundamental physics of organic semiconductors, and to improve the performance of devices based on them.

Highly mismatched alloys such as Ga(N,Bi,As) offer promising prospects for photonic and photovoltaic devices with tailored functionalities. The authors present a theoretical analysis of the evolution of electronic structure with composition in Ga(N,Bi,As) alloys and heterostructures. Large-scale atomistic calculations reveal that N and Bi perturb the conduction- and valence-band structure respectively, and that these perturbations are independent, even with significant alloy disorder. This allows strain engineering to target near- to midinfrared wavelengths with lattice-matched growth on GaAs or Ge substrates in photonic or photovoltaic devices.

Leaky-wave antennas (LWAs) are useful in acoustic applications requiring directional sound-beam forming, but realizing a compact apparatus is challenging, and there are limitations when independent control of phase contrast and leakage rate is required. The authors propose an LWA that overcomes these problems by using an engineered surface with admittance that varies like a sine wave, formed by artificial open guiding structures. This method could find use in ultrasonic sensors, ultrasonic imaging, or underwater sonar.

Usually we discuss a metamaterial in terms of just its optical behavior on a solid substrate in air. What about the behavior of a metamaterial operating in a viscous low-loss liquid? This study uses the theories of classical electromagnetism and fluid dynamics to solve for the coupling between the electromagnetic force and the liquid-resistance force, and the resulting dynamics of a double split-ring resonator (a common form of meta-atom in a complete metamaterial). Experiments match the predictions well. These results help to guide and extend our consideration of metamaterials to applications in liquid environments.

Outside of Hogwarts, experimental verifications of invisibility cloaking have been limited to single-frequency operation for relatively small objects. The authors propose a multifrequency metamaterial cloak for large objects, based on the intrinsic ability of an optimized medium to adjust optical phase delay by the cloak according to the size of the object. This innovative approach to designing multiband systems could have real impact on stealth technology.

Unidirectional light propagation, as in the emerging field of topological photonics, offers robust transport of light in devices despite the presence of fabrication flaws. This highly nontrivial effect, however, can only be achieved in correspondingly nontrivial structures. The authors present a significantly simplified paradigm in which dynamic modulation of a photonic waveguide conducts light in one direction, even through structural defects. This approach opens an avenue to exploring topological photonics, and to slow-light waveguides unhindered by their imperfections, which is exciting for applications in integrated optics.

The advent of low-loss dielectric metasurfaces offers a paradigm of miniaturization for free-space optical elements like lenses and deflectors, but these are limited to static functionality. This study illustrates the possibility to tune the focal length of a metalens through spatially uniform modulation of refractive index. Using traditional thermo-optic effects, the authors demonstrate large refractive index changes ($\mathrm{\Delta }n$ = 0.15) in InSb Mie resonators. They furthermore show how to form a metasurface from a single material, enabling easier wafer-scale production.

The unconditional and fast reset of fixed-frequency superconducting qubits is crucial for the operation of near-term quantum computers at elevated trigger rates. Many reset schemes are either conditional (depend on the qubit’s state) or require flux tunability. This study presents an all-microwave pulsed reset sequence that uses the readout resonator to unconditionally empty a fixed-frequency transmon qubit, and that also reduces the qubit thermal population. This method could be used to increase the repetition rate of a superconducting quantum computer running, for example, variational eigensolvers requiring many measurements.

Defects in hexagonal boron nitride ($h$-BN) are sources of single photons, even at room temperature, and creating $h$-BN films by molecular beam epitaxy (MBE) is promising for applications, as this allows the deposition of $h$-BN on various substrates. The authors show that $h$-BN films grown by MBE do contain defects that emit in both the ultraviolet and visible spectral ranges, and are localized within multilayer islands that form at the nucleation centers of the film. These luminescent defects have real potential as quantum light sources in optoelectronic devices based on epitaxial combination of dissimilar two-dimensional materials.

A Bloch surface wave (BSW) lens is the low-loss counterpart of a plasmonic lens, designed to convert far-field light into a subwavelength near-field optical spot, and vice versa. Propagation of a BSW is greatly extended in the absence of a metal’s intrinsic loss; therefore, analysis of a BSW passing through multiple-ring slots is critical for efficient design. The authors find that reflectance loss of the ring slot is nontrivial for a multiring BSW lens, and they present an optimized design that balances increasing ring number with reflective loss. This work could find use in near-field imaging, sensing, lighting, integrated photonic circuits, and optical trapping.

Phase estimation has applications from quantum imaging to gravitational-wave detection. In areas such as biological-system sampling or quantum metrology, it is crucial to optimally acquire information from a very limited number of probes. To address this need, the authors describe and experimentally verify a machine-learning method for optimal adaptive single-photon phase estimation based on a small number of trials. This approach could be used to optimize quantum metrology protocols, and can be extended to general multiparameter scenarios.

Hollow-core optical fibers loaded with cold atoms show great promise as platforms for quantum information processing and highly nonlinear optics, but their limitations have remained poorly understood. Using detailed Monte Carlo simulation, the authors model the loading dynamics and predict the sensitivity of the system to various experimental parameters. Thus guided, they develop an experimental apparatus that can load an order of magnitude more atoms than usual, to yield the extremely high optical depths needed for high-efficiency quantum memories.

Producing and controlling light at terahertz frequencies continues to be a major research theme. Being able to tune a continuous narrow-linewidth THz wave over a wide frequency range enables important applications, such as high-resolution spectroscopy and ultrafast optical communication. The authors achieve such tuning of the THz wave generated by difference frequency mixing using excitons confined in a semiconductor quantum well. Using exciton optical nonlinearity enables wide frequency tunability without the intensity drop caused by phonon scattering. This approach is bound to impact the engineering of terahertz wave fields.

Electrooptomechanical hybrid systems are garnering interest as candidate quantum transducers, to link microwave and optical fields in a future quantum Internet, for example. Achieving quantum-level operation in such systems is a challenge, though. This work discusses equivalent circuits as a unifying framework for designing and analyzing such hybrid quantum transducers, while also including quantum noise in a straightforward manner. By providing a common diagrammatical language for the electronic, optical, and mechanical elements involved, this approach may facilitate a joint effort in electrical engineering and quantum optomechanics to realize hybrid quantum networks.

The cobalt-based Heusler alloys are promising for spintronic applications, thanks to their high spin polarization and half-metallic character. This work investigates the spin-dependent transport and thermoelectric effects in epitaxial Co${}_2$TiSn thin films, with and without V doping. The anomalous Nernst angle and anomalous Hall angle are both determined, and the relationship between these two important parameters is discussed. These experimental results are a step toward realizing spin-caloritronic devices for efficient on-chip energy harvesting of waste heat.

Current-induced spin-orbit torque (SOT) can be used to control magnetization in magnetic random-access memory (MRAM), but typically requires an applied in-plane magnetic field. To eliminate the need for this applied field, for better on-chip memory designs, the authors deposit the $4d$ transition metal Mo in a canted shape as the SOT source. Although the spin-orbit interaction in Mo is weaker than in $5d$ metals like Pt, W, or Ta, deterministic switching can still be achieved. This result suggests that growth configuration can play a more important role than material selection in some cases, which could really impact the engineering of next-generation field-free SOT-MRAM.

Microwave devices are at the heart of many classical and emerging quantum technologies, but development can be hindered by the techniques available for characterization and debugging. The authors present a microscope that uses fluorescent, atomlike defects in diamond (N-$V$ centers, which themselves have arisen as a major research topic) to image the microwave fields a few micrometers above microwave circuitry. This enables $in$ $situ$, noninvasive, real-time exploration of device function and failure. Studies of test structures reveal surprising features, which will inspire further engagement with the microwave-engineering community.

$III-V$ semiconductor nanowires (NWs) with embedded quantum dots (QDs) are promising building blocks for advanced electronic and optoelectronic devices. The authors investigate the properties of the recently discovered QDs that spontaneously form inside GaAs/Ga(N,As) core/shell NWs. The valence-state character of these QDs ranges from pure heavy-hole to mixed heavy- and light-hole, which is attributed to a combination of core/shell-induced strain and local strain due to alloy fluctuations. This finding provides not just insight into the electronic structure of the QDs, but also a means to probe the local strain within NWs, by examining the fine structure of their embedded QDs.

In spintronics, electrical control of a ferromagnet could be achieved by integrating it onto a piezoelectrically active substrate. The authors describe theoretically such strain-mediated electric-field control of ferromagnetic resonance (FMR) in ultrathin-film nanomagnets. By calculating maps of FMR frequency and its electrical tunability, they show that the latter increases drastically at most spin-reorientation transitions driven by substrate-induced lattice strains. This feature provides efficient electrical control of FMR, which is necessary for developing signal-processing microwave devices with high tuning speed and low energy consumption.

Divacancy defects in SiC have emerged as a promising platform for optically active spin-based quantum technologies, yet little is known about their spin properties at high temperatures. The authors demonstrate coherent control of these spins above 550 K, finding a polynomial temperature dependence of zero-field splitting, and a spin coherence time that decreases with increasing temperature. They further demonstrate thermal sensing at about 450 K. This understanding of defect-based spin dynamics in SiC promotes its use in broad-temperature-range quantum sensing.

In semiconductor physics, one step beyond the exciton is the $trion$, a conglomerate of three charged particles (such as two electrons and a hole) bearing a net unpaired spin in the ground state. Using simple, sensitive photocapacitance spectroscopy, the authors study and electrically control trions and the Fermi edge singularity in heterostructures of $III-V$ semiconductors, at a fairly high temperature of 100 K. This is an important step toward optoelectronic devices based on such many-body complexes, possibly operating at room temperature, or featuring materials with larger excitonic binding energies, such as transition-metal dichalcogenides.

Recent progress in the development of intensive neutron sources and the required peripheral equipment has enabled high-spatial-resolution neutron imaging that is simultaneously energy-resolved. The authors demonstrate a high-speed neutron-imaging system with a cutting-edge superconducting detector. This detector has the potential not just for applications requiring high spatial resolution, but also for two-dimensional detectors for other kinds of beams, by adopting an adequate conversion layer.

Strain-coupled composite multiferroics, comprised of magnetostrictive and piezoelectric materials, promise tunable control of magnetism in applications such as rf resonators, sensors, and energy harvesters. Using polarized neutron reflectometry, the authors demonstrate that magnetostrictive and weakly magnetostrictive materials in a multilayer can be strain-coupled to an electric field over large length scales that enable designer functionality across a range of device applications.

Antiferromagnetic materials are gaining importance in spintronic applications, because of their stability against external magnetic field perturbations. This work investigates a negative exchange interaction between two insulating ferrimagnets, yttrium iron garnet and gadolinium iron garnet. The authors demonstrate that a heterostructure based on these insulating garnets behaves as a synthetic antiferromagnet, which could be further engineered to optimize the functionalities exploited in insulating spintronic devices. As proof of concept, they present a memory element with orthogonal magnetization switching that can be read via spin Hall magnetoresistance.

In terms of band-gap engineering for devices, despite silicene’s great promise, it has received considerably less attention than its carbon cousin graphene. Taking advantage of its tunable band gap, the authors use an effective Hamiltonian formalism to examine the formation of silicene superlattices. They theoretically consider engineering of the local band structure of silicene through application of an out-of-plane electric field, together with control over the spin and valley degrees of freedom, and finally show how to optimize negative differential resistance. This design principle could be extended to stacks of other two-dimensional materials as well.

Reconfigurable integrated circuits draw the attention of the quantum optics community, because of their remarkable capability to set up different experiments on a single device. While universal reconfigurable integrated circuits are usually fabricated lithographically, this work demonstrates a less expensive, faster femtosecond-laser-writing technology for creating programmable photonic circuitry, thus opening up their usage to a wider audience.

The spin photodiode is essential for decoding circularly polarized light for future applications in optical telecommunication. However, most spin photodiodes only work with a large applied magnetic field, because of the in-plane magnetization of the spin detector. The authors investigate the growth and spin-polarized photocurrent of a perpendicularly magnetized Co-Fe-B/MgO spin detector on germanium. Without using any magnetic field, they obtain asymmetry in photocurrent helicity that is detectable at room temperature, for 1310-nm light. This work will impact the development of devices for the optical transport of spin information.

Despite significant experimental and theoretical studies of spin transport in graphene, the absorption of spins by a nanomagnet deposited on the graphene is not yet well understood. This study describes an $in$ $situ$ measurement as an Fe island is deposited on a graphene spin valve, and proposes a mechanism to quantitatively explain the effect of the Fe thickness in terms of the absorbed spin current, including the effect of a nonmagnetic interlayer. Its findings provide an improved understanding of the flow of spin current out of a graphene-based nonlocal spin valve, as necessary for spin-transfer-torque switching.

The thermal infrared near-field spectroscope (TINS) allows nanoscale thermal imaging of a metallic or dielectric surface, without external laser illumination of the surface itself. The measured signal from a TINS is not well understood, though. In this study the full dipole model, including radiation correction, for describing TINS signals is revisited, and implications for the measured spectra are discussed. This model provides the basis for an improved theoretical description of TINS and other near-field scanning thermal microscopes.

The interface of tensile-strained Ge grown on (In,Al)As is currently being considered for use in TFETs and optoelectronic applications, so it is important to understand the nature of the band offset across the heterovalent Ge/(In,Al)As (001) interface. Using a range of first-principles calculations, the authors show that the valence- and conduction-band offsets here are highly sensitive to interfacial details, even in a very narrow surrounding region. Transitions between type-I and -II offsets are observed as a function of interface stoichiometry and interdiffusion, indicating potential routes to controlling the Ge/(In,Al)As band alignment, and hence a device’s properties.

One of the key properties of graphene for spintronics is its ability to transport spins unaltered over long distances. However, the injection of a spin-polarized current is challenging, because of conductivity mismatch. The authors implement dynamical spin pumping to inject a pure spin current into a single layer of graphene, using an on-chip wideband microwave excitation scheme. This paves the way for future low-power studies of the spin properties of graphene, and for high-frequency spintronics applications.

Proton radiotherapy uses a narrow proton beam to deliver a highly localized dose of radiation to a tumor, but verifying this dose is difficult, due to the relatively small number of protons in each beam. The authors compute the electric field produced by a proton beam moving inside five different biological tissues, and show that the magnitude and spatial profiles of the field lend themselves to measurement. These results will aid in the design and construction of electric field detectors capable of measuring the proton range, which will be a major step toward improving this therapy.

Thermal-piezoresistive pumping is a thermoelectromechanical feedback mechanism used to tune micro- and nanomechanical resonators, simplifying signal amplification in resonant force sensors and oscillators. The authors present a generalizable model that predicts the effective quality-factor tuning due to pumping for micromechanical resonators of varying device geometry, orientation, and material composition. They also find that current models for silicon’s piezoresistivity diverge from experiment as doping increases. Their work will help to optimize piezoresistive devices for different operating temperatures, reduced power usage, and higher oscillation frequency.

A good understanding of how lithium-ion battery electrodes charge at the microscopic level is key to improving performance. Cathodes in these batteries often consist of ensembles of small particles, and the number of particles that are changing their lithium content during charge or discharge affects performance. The authors present a simple model that assumes the number of active particles is determined by a random process. It predicts that the number of actively charging particles is highest just before the first particles become completely charged, which agrees with experimental observations.

Command over fundamental magnetic properties, such as magnetic anisotropy and Gilbert damping, could lead to a universal magnetic memory with long data retention and ultralow power consumption. The authors study these properties in film heterostructures fabricated on industry-friendly platforms, with subnanometer deposition control. By tweaking the ferromagnet’s thickness by one monolayer, a 25% reduction in power consumption is projected for memory devices with the proposed film stack, by virtue of a 50% reduction in the damping parameter. These discoveries have immediate implications for memory applications, and will accelerate their development.

Electron spins confined to quantum dots are currently of interest as a scalable platform for constructing a quantum computer, although here implementing a fast, high-fidelity gate to entangle qubits remains a challenge. The authors demonstrate a device architecture that enables coupling of qubits via a mediating quantum state, realized using a many-electron quantum dot. This platform potentially enables medium-range entangling gates in spin-based qubit architectures, and offers the prospect of scaling spin qubits beyond linear arrays.

As a student, James Clerk Maxwell invented a hypothetical optical device, $Maxwel{l}^{\prime }s$ $fisheye$ (MFE), in which light goes in circles and every point is focused. 164 years later, researchers succeed in making a high-quality optical version of the MFE on a silicon chip for telecommunication wavelengths, and demonstrate its unusual imaging properties. Key to the quality of their device is a lithographic technique in which smooth and sharp structures are made in a single step. This fabrication technique may also find use in the manufacture of other optical devices.

Energy-efficient excitation of spin waves is a fundamental building block—and also a major limiting factor—for a promising class of spintronic devices. Here researchers investigate spin-wave generation using a spin-orbit-torque (SOT) antenna, based on a heavy-metal/ferromagnet waveguide. Broadband excitation of propagating spin waves in a Ta/Co-Fe-B stack is demonstrated, while modeling and simulations reveal the details of the excitation, propagation, and mode formation. Finally, a simple circuit model shows the scaling properties of SOT antennas with respect to conventional inductive antennas. These results are a key step toward employing SOTs in spin-wave transducers.

Accurate measurements of absorptance during high-power laser irradiation of metals is vital to understanding the physical phenomena that underpin important practical applications, such as laser welding and additive manufacturing. These measurements are difficult because at high irradiance a metal substrate is rapidly changing temperature, phase, and shape. The authors employ an integrating-sphere approach to make accurate dynamic measurements. Besides the practical importance of the data for modeling laser interactions, several interesting physical phenomena are identified, including keyhole formation.

As a possible solution to the problem of waste heat generated in miniaturized computer chips, one may construct logic circuits that are operated with heat instead of charge. An important missing ingredient for this emerging field of phase-coherent caloritronics is a heat circulator, analogous to a microwave circulator in electronics. This work proposes a clever way to realize a phase-coherent thermal circulator, based on a multiterminal Josephson junction. The device can operate almost ideally with just tiny magnetic fields, thus enabling practical applications.

The appearance of half-metallicity, modulated by a transverse electric field, in stanene nanoribbons could be exploited to generate highly spin-polarized currents in tunnel-field-effect transistors. In particular, the sensitivity of interband tunneling to small modulations of stanene’s band gap, its reduced dependence on temperature, and its robustness against the presence of defects, lead to calculations suggesting that one can obtain tunable, 98% spin-polarized current with a small applied voltage on the control electrode. The proposed device could be useful in exploring innovative concepts of spin injectors or filters, which are fundamental building blocks of spintronics.

$Oscillate$ $boiling$ is superior to the more familiar nucleate boiling, in terms of heat-transfer rate and working limit. However, the common technique to generate it, based on optical heating, is not feasible for applications. The authors introduce a compact electrical heating device that can induce oscillate boiling in a well-controlled, scalable fashion. The device includes an integrated temperature probe, to provide insight on the thermal profile of the system. Thermal rates of 10${}^8$ K/s are attained, indicating the potential of oscillate boiling for rapid heat transfer from microelectrical devices in applications.

The ultrafast injection of spins and manipulation of magnetism by laser light offers a revolution in the speed of magnetic data storage. Here fully $ab$ $initio$ calculation show that the flow of spin current from a ferromagnet to a nonmagnetic substrate in a spintronic device plays a crucial role in controlling laser-induced femtosecond spin dynamics. This highlights that substrate engineering can be used for precise control of early-time magnetization dynamics, thereby opening a path to material design for efficient spin injection in magnetic memory devices.

Transmission electron microscopy excels at determining a sample’s physical structure—the locations and identities of its constituent atoms—, but is typically blind to electronic structure. A combination of electron-beam-induced current (EBIC) imaging with scanning transmission electron microscopy (STEM), however, does reveal electronic properties by detecting the emission of both secondary electrons and the corresponding holes, for differential contrast that is otherwise inaccessible. Thus STEM EBIC can provide high-resolution images of the properties that govern the function of nanoelectronic devices.

The ferromagnetic resonance induced by spin-transfer torque (ST-FMR) is important for microwave detectors, as it can convert an ac microwave signal directly into a dc signal that can be easily measured. However, so far only one resonance mode could be detected for a given external magnetic field, resulting in a relatively low efficiency. This study considers ST-FMR in geometrically confined nanoring magnetic tunnel junctions (NR-MTJs), in which $two$ resonance modes are observed at a given magnetic field. This result might significantly improve the efficiency of such microwave detectors, by means of a dual-mode detection configuration.

A feverish rate of change: Thermometry is key to thermodynamics, but measuring temperature changes at the nanoscale is challenging. In this investigation, a superconducting weak link (Josephson junction) is used to measure rapidly changing electron temperature in a long superconducting wire, with nanosecond resolution. The approach used here extends the range of existing experiments to faster time scales, paving the way for cutting-edge work on the thermodynamics of low-temperature quantum circuits and the development of ultralow-energy calorimetry and bolometry.

Understanding the conversion efficiency limit under $nonequilibrium$ conditions is important for developing high-efficiency solar cells that go beyond the Shockley-Queisser (SQ) limit. The authors present such a theory, to clarify the impact of charge-carrier extraction and thermalization dynamics on the limit. Simulation of a simple, planar solar cell is used to address the parameter regime (in terms of carrier extraction time) where the standard SQ theory applies, and to determine the conversion efficiency limit outside that regime. This theory could also help to achieve high efficiencies in other types of nonequilibrium solar cells.

Propagating magnons, due to their rich physics, could enable the development of compact microwave devices for low-temperature applications, such as quantum computing and communication. The authors study the low-temperature behavior of propagating magnetostatic surface spin waves. The change in shape of magnon pulses is found to be consistent with numerical simulations based on the dispersion relation. These results enable the engineering of low-temperature magnonic systems, as well as the use of spin waves in combination with superconducting circuits in hybrid quantum devices.

The authors discuss a fundamental concept and pragmatic approach for negative refraction of elastic waves by a parity-time ($PT$) -symmetrical metamaterial, to improve acoustic control at wavelength and subwavelength scales. Unlike strategies based on complex configurations of metamaterials or phononic/photonic crystals, this work shows how to achieve $tunable$ negative refraction easily, using a combination of piezoelectric materials and external electrical circuits. Applications such as elastic-guided-wave superfocusing, superresolution wave devices, and elastic-wave structural health monitoring can be realistically pursued, based on such a $PT$-symmetrical metamaterial.

Nuclear magnetic resonance (NMR) is among the most useful methods for chemical analysis and $in$ $vivo$ determination of tissue structure, with tremendous impact across many fields. At the nanoscale, atomic-scale detectors such as N-$V$ centers in diamond operate in the quantum regime, and typically at magnetic fields below 1 T. Extending this approach to high fields is of keen interest, for improved resolution. To this end, the authors present energy-efficient control schemes for high-field NMR at the nanoscale. The low intensities of the microwave control fields also facilitate applications in biological settings, where heating effects need to be avoided.

Electrical control of spin transport at room temperature is crucial for spintronic applications. The authors’ calculations show that spin currents can be guided quite efficiently by employing carrier drift in Y-shaped graphene channels that act as spin-current demultiplexers. Introducing drift can reduce transport times, compared to diffusive transport, making this approach suitable for high-frequency operation. Exploration of different geometries confirms the scalability of the proposed architecture. These results indicate that spin drift can help to improve the functionality of spintronic devices, combining multiplexing with $in$ $situ$ memory capability.

$Nonreciprocity$ has emerged as a fundamental concept in modern science and technology. Recent advances in magnet-free nonreciprocal systems have spurred a resurgence of interest in this subject, but have also led to questions and confusion. The authors review nonreciprocity from a global, first-principles perspective, with the aim of establishing a solid foundation for the topic, to resolve previous issues and promote forthcoming research in the field.

Design methodologies for high-performance photocathodes become increasingly important, driven by the needs of advanced x-ray light sources and ultrafast electron diffraction and microscopy. Cathode design has been hampered by interdependent performance metrics, where improving one compromises another. Rapid developments in modeling and nanomaterial fabrication allow tailoring of electronic structure in ways that influence the fundamental mechanisms of photoemission: absorption, transport, and emission. This survey of recent efforts and results in each of these areas proposes research priorities that are likely to yield significant gains in cathode design.