The weakest link: Flexible electronics are an exciting prospect, but reliability in their connections is crucial to real-world deployment. Using a dispersion of conductive particles in an insulating fluid, researchers discuss the physics and engineering behind self-healing interconnects in which repair is automatically triggered upon the development of an open-circuit fault. Healed spots possessing metallic conductivity and nearly plastic stretchability are demonstrated. This work promises self-healing stretchable interconnects to improve circuit reliability.

Due to their large impact on health and environmental safety, fine and ultrafine particles are of special importance. While infrared (IR) spectroscopy can yield important material-specific information, direct investigation of small samples or single nano-objects is impossible, due to the great mismatch between particle size and IR wavelengths. Fortunately, surface-enhanced infrared absorption spectroscopy allows detection of $individual$ ultrafine dust particles a few tens of nanometers in diameter. The extraordinary near-field enhancement of the IR vibrational signal is achieved using plasmonically resonant Au “bowtie” apertures, in which the ultrafine specks are easily trapped.

$Magnetic$ $skyrmions$ are topologically protected chiral spin textures that are promising for memory and logic applications. Low-energy manipulation of skyrmions is crucial, especially in pushing them around; to date, a large driving current has been required for high-speed skyrmion motion, resulting in significant Joule heating and device instability. The authors simulate a simple structure, featuring identical, equidistant electrodes on a nanowire of uniform thickness, that allows voltage-driven high-speed skyrmion motion at low power—a promising development in skyrmion nanodevices.

Theory indicates that a quantum computer manipulating quantum information by means of geometric phases in Hilbert space ($holonomic$ $quantum$ $computing$) could be resilient to certain forms of noise. Two-qubit nonadiabatic holonomies are important for computing architectures based on fixed-frequency superconducting qubits, as they provide the means to directly realize an exchange-type operation. Here researchers implement a nonadiabatic holonomic operation between two such qubits connected by a microwave resonator, to create entangled states. As proof of principle, this operation is used to calculate the ground state of molecular hydrogen.

To obtain a desired profile of transmitted wave modes, what must a diffraction grating look like? This work presents a generalized, systematic method to design devices for control of the flow of wave energy. This control is achieved by engineering the diffraction properties of gratings, and simplified configurations are found, compared to similar gradient-metasurface devices. Although focused on flexural waves in thin elastic plates, this approach can be easily applied to other domains, such as optics, electronics, or fluid acoustics, as the diffraction principles are the same in all of these contexts.

Magnetic damping is one of the most important, yet least controllable, parameters that determine the performance of a spintronic device. The authors find that magnetic damping can be drastically tuned in exchange-biased-ferromagnet/antiferromagnet (FM/AFM) multilayers. Damping is reduced by half through “choking” of the nonlocal spin interaction between FM and AFM. These results provide a physical understanding of the spin dissipation in AFM/FM multilayers, and suggest a way to drastically control the magnetic damping of ferromagnetic materials, for progress in spintronics.

Optical clocks based on trapped ions can achieve systematic frequency uncertainties below 10${}^{-18}$, enabling relativistic geodesy and precise tests of fundamental physics. However, today’s single-ion clocks require averaging times of over a week to resolve frequencies at this level, and no multi-ion clock has been implemented, due to difficulties in controlling ion motion. The authors present an ion-trap array that confines up to 100 clock ions with time-dilation shifts in the low-10${}^{-19}$ range, and measure the driven motion of individual ions within an ion crystal. These results point to the next generation of ion clocks, and the realization of clock schemes that have languished.

High kinetic inductance and low microwave losses in disordered superconductors offer unique opportunities for microwave engineering at low temperatures. To explore the dissipation mechanisms in these systems, the authors study microwave coplanar resonators fabricated from strongly disordered aluminum films. Because of the ultraslow wave propagation in these high-impedance structures, they can be shrunk to 1% of conventional size, and their intrinsic quality factors are high enough for a wide range of applications, from microwave photon detectors to quantum computing circuitry.

The authors demonstrate, both theoretically and experimentally, that engineered diffraction gratings can modulate the acoustic wavefront more efficiently than can traditional devices based on complex metamaterials and metasurfaces. To demonstrate the feasibility of this approach, a three-dimensional conical ground cloak is fabricated and tested, with remarkable results, compared to other cloaks based on different physical mechanisms. This achievement shows a path to advanced control of mechanical waves by means of simpler, more efficient principles.

The use of circuit quantum electrodynamics with single-atom electron spins could lead to scalable, highly coherent qubit schemes for quantum information processing. Reaching the regime of strong magnetic coupling of a photon to a single spin is challenging, though, because of the relatively small spin magnetic moment, and the relatively weak magnetic field of a typical superconducting circuit resonator. The authors show that using a lumped-element resonator with a nanoscale spiral inductor can greatly enhance the photon’s magnetic field in the vicinity of the spin trapped in a donor potential. This enables strong magnetic coupling of the resonator to the spin.

As the search continues for ever-higher densities in digital data storage, current technologies are reaching the physical limits of scalability, necessitating an alternative approach, such as utilizing the third dimension. The authors propose a three-dimensional memory device in which the information is stored in magnetic constrictions in a nanowire on a medium with perpendicular magnetic anisotropy. Transverse data transfer is achieved by heat-assisted stray-field-induced magnetization and lateral transfer to neighboring bits—hence lateral domain duplication—via spin-transfer torque.

Quantum cascade lasers are finicky, in terms of the structural quality of the active region, especially for strain-mismatched heterostructures like Ge/Si-Ge quantum wells. Accurate control of electron-state coupling between well and barriers in the laser’s different stages is crucial. This study shows how to produce high-quality asymmetric coupled $n$-type Ge/Si-Ge multiple quantum wells and carefully control resonant tunneling through barriers of different thicknesses, and thus interwell coupling and wave-function hybridization. The set of robust material parameters provided here is a must-have on the route to Si-based THz devices.

$Magnetic$ $skyrmions$ are topologically protected chiral spin textures that are promising for memory and logic applications. Low-energy manipulation of skyrmions is crucial, especially in pushing them around; to date, a large driving current has been required for high-speed skyrmion motion, resulting in significant Joule heating and device instability. The authors simulate a simple structure, featuring identical, equidistant electrodes on a nanowire of uniform thickness, that allows voltage-driven high-speed skyrmion motion at low power—a promising development in skyrmion nanodevices.

Silicon solar cells have dominated the photovoltaics industry for decades, but the quest for lower cost, higher efficiency, thinner, and more flexible systems has shifted research to a variety of other materials for harvesting solar energy. The research in this article, exploiting the wave nature of sunlight, suggests that thin-film silicon could leapfrog past competing technologies. Using wave-interference-based light trapping in a thin, flexible sheet of silicon photonic crystal, the authors discuss the possibility of achieving power-conversion efficiency beyond the world record for any single material.

Tunable resonators with high quality factors underpin the storage and retrieval of microwave-domain quantum information in spin ensembles used as long-lived quantum memories, and can enable multifrequency high-sensitivity electron spin resonance (ESR). The authors develop a single-layer technology, based on embedding nanoSQUIDs in superconducting niobium resonators, to realize high-quality frequency-tunable devices that are resilient to moderate magnetic fields. These devices will enable tunable-resonator-enhanced ESR protocols to be performed at specific fields and frequencies, such as storing quantum information in spins at low-decoherence “clock transitions”.

In systems with non-Hermitian Hamiltonians, multiple eigenmodes can coalesce at exceptional points of degeneracy (EPDs), yielding frequency splitting, which is useful in building sensors. EPDs are usually obtained in $PT$-symmetric systems of at least two coupled resonators with precise gain and loss values. However, the authors show that passive, lossless single resonators also support EPDs, when time-periodic variation of a system element is introduced. Thus EPDs can be realized in simple time-periodic systems solely by tuning a modulation frequency, which should facilitate the design of ultrasensitive detectors and modulators, and power-efficient oscillators and amplifiers.

Physicists constantly look for symmetries to understand phenomena, yet sometimes it’s asymmetry that ends up being revealing. Here the authors demonstrate two types of acoustic monopoles: When excited by a sound wave, the pressure field near a pressure monopole is symmetric, but the air’s velocity is not. On the other hand, the velocity field near a velocity monopole is symmetric, but the pressure field is not. It is also possible to construct a hybrid monopole with neither type of symmetry. These results are expected to promote the design of acoustic metamaterials and related applications, such as higher absorption in subwavelength-scale devices.

Superconducting qubits are conventionally controlled with shaped microwave pulses from a microwave carrier tone. This works well for small systems, but is difficult to scale up to the millions of qubits needed for a general-purpose quantum computer. Instead, the authors suggest irradiating a qubit with trains of quantized flux pulses derived from single flux quantum (SFQ) digital logic. The pulses are generated by an SFQ driver circuit cofabricated on the qubit chip. This work opens the door to tight integration of a quantum array with a classical coprocessor, to lower wiring heat load, latency, and overall system footprint.

For better motors in electric vehicles, we had better understand magnetism at the microscale. Domain-wall motions and magnetization-reversal processes during the initial magnetization process in a hot-deformed Nd${}_2$Fe${}_{14}$B permanent magnet are simulated by solving the Landau-Lifshitz-Gilbert equation for a model ensemble of 5302 grains. The initial-magnetization curve reproduces the two-step structure seen in experiment. In the first step, the domain walls move inside the multidomain grains, and the magnetization reversal of the single domains occurs in the second step. The effects of networks among the grains through dipolar interaction are also estimated.

The valley degree of freedom in a compound’s electronic structure has enormous potential for use in information storage and processing. Electrical switching of the valley index is most practical for real-world applications, but has not yet emerged. This work develops a route toward electrically controlled valley polarization by utilizing a multiferroic substrate as an intermediate. The proposed ferroelectricity-valley coupling and tunable magnetic behavior are both achieved in antiferromagnetic monolayer MnPSe${}_3$ on insulating ferroelectric YMnO${}_3$ substrate, raising the prospects of electrically controlled valleytronics and boosting the development of high-performance memory devices.

This study showcases the Loewner equation in evaluating the field-enhancement factor (FEF) of quasi-one-dimensional structures by means of conformal mapping. This is relevant to many applications based on the field-emission properties of nanoelectromechanical systems (NEMS), including nanoscale radios and other types of detectors. The approach here allows for both analytical and numerical first-principles evaluation of the FEF for emitters with high aspect ratio—a difficult task that is often neglected. The results thus are expected to impact our understanding of many devices, especially those involving amplifiers based on field emission of electrons by carbon nanotubes.

Voltage-driven magnetization switching is a promising solution for realizing ultralow-power magnetic memories with energy efficiency well beyond that of devices driven by electric current. For such applications, though, a substantial reduction of the write-error rate is strongly needed. The authors show that the precision of voltage-driven magnetization switching can be improved by precise control of a voltage pulse’s shape. Using a proper fall time in the write pulse effectively cancels magnetization drift, due to intrinsic damping, and helps to reduce write error. This insight points the way for developing reliable voltage-driven spintronic devices.

Several recent studies have reported experiments on acoustophoresis (“acoustic levitation”; suspension of particles in a fluid using sound waves) in acoustically soft polymer devices. However, due in part to a lack of theoretical understanding, it has proven difficult to design proper devices for such experiments. This work provides a theoretical framework for designing all-polymer microfluidic devices for acoustophoretic applications. The authors identify and study the physics of whole-system ultrasound resonances in all-polymer devices, and show that they enable acoustophoresis comparable to that in conventional, acoustically hard silicon-glass devices.

Applying principles of transformation optics to acoustic systems continues to gain traction. It has been shown that transformation elastics, or $illusion$ $elastics$, is challenging, as the elastodynamic wave equation will change its form after coordinate transformation. Here the authors use transformation acoustics based on Fabry-Pérot (FP) resonances, to obtain an illusion such that an arbitrary solid object sounds like a different object in a fluid background. This elastic illusion is confirmed analytically using Mie theory, for both two and three dimensions. This approach could be important in underwater acoustic communication, $e.g.$ sonar.

The results of this theoretical study are important for applications dealing with superconducting nanowire single-photon detectors (SNSPDs), which include quantum information processing. The author shows that the variation in delay time ${\tau }_d$ (timing jitter) of the voltage response after absorption of a single photon, connected to either position-dependent response or variation in the energy deposited to electrons, could be as small as 1 ps when the current in the superconducting strip approaches the depairing current. Reducing this jitter is a main topic of SNSPD research, and understanding the origins of these variations is necessary to that end.

Theory indicates that a quantum computer manipulating quantum information by means of geometric phases in Hilbert space ($holonomic$ $quantum$ $computing$) could be resilient to certain forms of noise. Two-qubit nonadiabatic holonomies are important for computing architectures based on fixed-frequency superconducting qubits, as they provide the means to directly realize an exchange-type operation. Here researchers implement a nonadiabatic holonomic operation between two such qubits connected by a microwave resonator, to create entangled states. As proof of principle, this operation is used to calculate the ground state of molecular hydrogen.

The Fourier-domain mode-locked (FDML) laser is an important light source for optical coherence tomography (OCT), due to its MHz repetition rate, but its imaging range is limited due to technical difficulties in compensating for high-order cavity dispersion and high detection bandwidths. This study attacks the problem using a frequency-comb FDML and circular ranging. The depth ambiguity associated with such a frequency comb is handled by using an acousto-optic frequency shifter. This approach will have an impact on solutions for long-range, high-speed OCT and its applications, particularly in advanced medical imaging.

The availability of wide-band-gap oxides with bipolar conductivity is critical to developing transparent optoelectronic technologies. Unfortunately, most metal oxides show a strong propensity for $n$-type conductivity, while $p$-type doping remains extremely challenging. Focusing on CdO and NiO, which have type-III band offset, this study reveals that their alloy Ni${}_x$Cd${}_{1-x}$O${}_{1+\delta }$ can exhibit bipolar conductivity. Electronic band structure and native defects are manipulated by varying the oxygen stoichiometry. The results strongly suggest that these alloys have great technological potential for applications requiring transparent conducting oxides.

Neuromorphic computing is an efficient solution for large-scale associative learning problems such as pattern recognition, but its hardware implementation is stymied by the need for low-power, scalable faux neurons, typically built using relaxation oscillators. This work proposes relaxation oscillators using VO${}_2$-based heterostructures with optimized thermal time constants for low-power operation at microwave frequencies. Synchronization behavior between two coupled oscillators is also investigated. This study offers a theoretical foundation for the use of such oscillators in neuromorphic computing.

Cutting and surface deformation of metals typically involve large strains and one of four distinct modes of plastic flow. The conditions under which each of these modes occur, and their relative stabilities, have been the subject of much debate. Using controlled deformation experiments and high-resolution $in$ $situ$ observations, the authors reveal a fundamental connection between the ambient chemical environment and the stability of flow modes. Huge gains in surface quality and reduction of deformation forces can be attained by controlling the chemical interaction with the flow, suggesting routes to enhanced metal processing via suitably tailored mechanochemical interactions.

A broadband spintronic diode would be important for electromagnetic energy harvesting, with the advantages of nanoscale size and low output resistance. This study gives experimental proof of broadband microwave detection in a magnetic tunnel junction, with a spintronic diode providing sufficient dc voltage to supply a low-power nanodevice. Such diodes also work as rectifiers below 1 nW, outperforming traditional Schottky diodes. These results point to spintronic diodes as building blocks in self-powered systems such as implantable biomedical devices, wireless sensors, and portable electronics, plus ultralow-power detectors for aerospace applications and the “Internet of Things”.

To obtain a desired profile of transmitted wave modes, what must a diffraction grating look like? This work presents a generalized, systematic method to design devices for control of the flow of wave energy. This control is achieved by engineering the diffraction properties of gratings, and simplified configurations are found, compared to similar gradient-metasurface devices. Although focused on flexural waves in thin elastic plates, this approach can be easily applied to other domains, such as optics, electronics, or fluid acoustics, as the diffraction principles are the same in all of these contexts.

In contrast to previous studies on time-harmonic modulation of electromagnetic systems, the authors investigate possibilities when the modulation function is arbitrary. With arbitrary time variations, one may overcome the inherent limitations of conventional harmonically modulated systems. For example, it is found that a time-varying lossless load of a transmission line can accumulate electromagnetic energy continuously, with no theoretical limit. This discovery is technologically significant, pointing to lossless batteries that accumulate huge amounts of energy from low-amplitude time-varying sources.

Evaluation of microstructure in metallic alloys following solidification in laser powder-bed fusion, an additive-manufacturing process, is challenging due to high growth velocities, typically on the order of cm/s. This theoretical analysis of experiments and phase-field simulations reveals a strongly out-of-equilibrium solidification regime in which the three length scales describing the solidification front become comparable, which rationalizes the features of the dendrites seen in both experiment and simulation. The identification of this regime is an important step in developing optimization schemes for solidification microstructure in 3D printing of metals.

Engineered thermal emitters that operate close to room temperature are emerging as an enabling technology for $e.g.$ passive radiative cooling, and infrared identification. Direct characterization of such low-temperature emitters using conventional Fourier-transform infrared (FTIR) spectroscopy, however, can yield pathological results due to background emission from the apparatus itself: Increasing the temperature may cause an apparent $decrease$ in thermal emission—even for sources with constant emissivity. This study carefully examines these contributions, demonstrates how to properly calibrate FTIR-based measurements of thermal emission, and quantifies the relevant errors.

In semiconductor science, Mg is used as a dopant in as-grown $n$-type GaN, not only to compensate the native electrons, but also to attain a reasonable hole concentration. However, Mg doping leads to defects that emit blue light, and the precise origin of this emission process needs to be understood, so that it can be eliminated. Here a detailed experimental and theoretical analysis identifies the formation of defect complexes consisting of interstitial and substitutional Mg in GaN. This study, performed on a large-surface-area network of GaN nanowalls exhibiting enhanced band-edge emission, also suggests growth conditions for fabricating $p$-type GaN with a high hole concentration.

Electrical switching of the tunneling electrons’ spin polarization recently observed in Co/PbZr${}_{0.2}$Ti${}_{0.8}$O${}_3$/La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$ (Co/PZT/LSMO) multiferroic tunnel junctions could radically change the way spin transport is controlled in spintronic devices. This work uses first-principles calculations to identify a microscopic mechanism that explains the observed effect, based on the presence of an O-related defect complex at the Co/PZT interface. Inversion of ferroelectric polarization causes striking changes in the reactivity of interfacial O, leading to on-off switching of O-Co state hybridization and swapping of the PZT spin polarization, consistent with experiment.

Differential techniques help to measure small quantities in the presence of large disturbances, and prior reduction of external disturbances, performed with an active compensation system based on feedback and control loops, improves such differential measurement. Control loops generally include a filter, which requires accurate design and tuning. In this study, a self-optimized loop filter implemented on a field-programmable gate array enables effective noise cancellation over a broad frequency range, while preventing instabilities, to improve a magnetometer’s performance. Differential signals five orders of magnitude smaller than the environmental fluctuations can be measured.

Strongly driven superconducting Josephson circuits are instrumental in emulating a variety of Hamiltonians that are useful for quantum information processing. However, such a time-dependent, nonlinear, open quantum system can display complex dynamics that lead to degradation of coherence times, instabilities, and chaotic behavior. Here the authors observe an instability that results in the escape of a circuit’s mode into states that are not confined by the Josephson cosine potential. This work guides us toward circuit designs that prevent instabilities, and thus are suitable for parametric pumping and quantum information processing.

Improving qubit coherence times is fundamental to the development of quantum computing technology. In most experiments in circuit quantum electrodynamics, dephasing induced by residual thermal photons in the readout resonator limits the coherence times of superconducting qubits. The authors design and test a type of microwave cavity attenuator that can be well thermalized to the base stage of a dilution refrigerator. With these cavity attenuators, they reproducibly measure enhanced qubit coherence times and report very low thermal-photon populations in the readout mode. This invention is a useful addition to the toolbox for state-of-the-art quantum circuitry.

Despite the excellent performance of magnetic materials in conventional memory devices, inefficient switching and optical losses inhibit their use in photonic computing memories. This work proposes an engineering solution to those troubles: the insertion of thin-film bilayers of a magnet and a material with large spin-orbit coupling near the core of a metal-insulator-metal-like plasmonic waveguide. Each magnet can be electrically switched between two stable, nonvolatile states via spin torque while interacting with the surface plasmon, without imparting strong losses. Placing $n$ of these four-state memory elements in series creates a magnetoplasmonic memory register with $4n$ states.

In the fields of spintronics and magnonics, the forward volume spin wave, though less studied, is most suitable for creating integrated spin-wave circuits. This study demonstrates a magnonic band gap for forward volume spin waves, by introducing metal stripes on a film of yttrium iron garnet (YIG). The development of this magnonic crystal was challenging, due to strong spectral oscillations caused by edge reflections, as well as process difficulties associated with these YIG structures. By introducing a noise-suppression technique, the authors realize a clear magnonic band gap in good agreement with calculations.

Spin-orbit torque (SOT) is an emerging means of magnetization switching, which is the key to the operation of magnetic random-access memory (MRAM). SOT can be generated by applying in-plane current to a heavy-metal/ferromagnet/oxide structure. While SOT is often considered to be isotropic (independent of magnetization direction), this study of Pt/Co/MgO stacks reveals it to be quite $anisotropic$. Furthermore, the anisotropy and strength of the SOT are greatly influenced by processing parameters, such as the sputtering power during MgO deposition, and the postannealing temperature. These findings add to our understanding of SOT, thus promoting MRAM applications.

In condensed matter physics, knowing the lower critical magnetic field $H_{c1}$ is necessary to evaluate the superfluid density, which can provide insight into the gap structure gap and mechanism of superconductivity in a type-II superconductor. Measuring $H_{c1}$ is complicated by the distortion of a magnetic field around the sample, and by the sensitivity and spatial resolution of the chosen technique. This work demonstrates minimally invasive vector magnetometry based on optically detected magnetic resonance of N-$V$ centers in diamond, which, with the help of revised demagnetization corrections for real-world nonellipsoidal samples, provides a sensitive method to measure $H_{c1}$.

Due to their large impact on health and environmental safety, fine and ultrafine particles are of special importance. While infrared (IR) spectroscopy can yield important material-specific information, direct investigation of small samples or single nano-objects is impossible, due to the great mismatch between particle size and IR wavelengths. Fortunately, surface-enhanced infrared absorption spectroscopy allows detection of $individual$ ultrafine dust particles a few tens of nanometers in diameter. The extraordinary near-field enhancement of the IR vibrational signal is achieved using plasmonically resonant Au “bowtie” apertures, in which the ultrafine specks are easily trapped.

Latte, or electronic skin? To make either, you may already have what you need, right in your kitchen. To improve the sensitivity of elastomer-based piezocapacitive pressure sensors, the thickness of the dielectric layer must vary with exposure to a weak force, and the mechanical modulus must be low. The authors propose a simple method for trapping air bubbles in an elastomer to reduce its modulus. Sensors fabricated from elastomer with bubbles are approximately 10 times more responsive than those without, and the response to external pressure is quite linear. These results are promising for the next generation of flexible electronics.

Laguerre-Gaussian (LG) optical modes have been extensively investigated for decades, as they are promising for $e.g.$ optical communication, superresolution imaging, precision measurement, and quantum information processing. These applications inevitably require a high-quality LG laser mode, the production of which is a great challenge for current techniques. This work demonstrates a compact solid-state LG-mode laser with low threshold, high efficiency, high purity, and flexible controllability. This laser’s excellent performance makes it a practical tool for various advanced applications.

Digital holographic tomography (DHT) can provide the complex-valued refractive-index map of a specimen in a three-dimensional format. Conventional DHT methods require the acquisition of a multitude of projection images sequentially, which imposes a fundamental limitation on speed. Here the authors present a snapshot DHT method (“SHOT”) that can acquire all of the projection images instantaneously. SHOT is expected to open doors for high-speed, label-free optical microscopy in three dimensions, even for moving specimens.

Inspired by recent discoveries of topological phases of matter in quantum physics, there has been a rapidly growing research effort to create their analogues in classical wave systems, especially to realize nonscattering waveguides in the presence of disorder. This study presents a robust elastic waveguide based on the acoustic analogue of the quantum valley Hall effect in a nonresonant phononic elastic structure. Elastic waves travel around sharp corners in the waveguide without reflection, and can also be unidirectionally excited. These features break the limits of conventional waveguides and have great potential in acoustic signal processing and vibration control.

Metamaterials characterized by long-range forces introduce a breakthrough in the elastic dynamics. In this study, long-range interactions among particles in an elastic continuum are considered as an effect, for example, of the inclusion of electrical or electromagnetic forces, linking two different worlds: elasticity and electromagnetism. Three universal regimes are revealed and demonstrated in general, depending on the range, intensity and repulsion or attraction nature of the long-range force. Furthermore, the dynamics of populations, traffic models, and social behavior share interesting aspects related to interaction range, extending the reach of this work.

Mobilization of a viscous phase by a less viscous fluid occurs often in nature and engineering. In particular, fluid-fluid-solid interfaces emerge in subsurface flow systems for oil recovery, CO${}_2$ sequestration, and soil remediation. Interface evolution is further affected when surface-active materials or suspended particles are present. Here three-dimensional observations of immiscible displacement indicate the differences in the mechanisms underlying viscous oil mobilization by nanofluids and by surfactant solutions. A tuned mixture of nanoparticles and surfactants in the displacing fluid effectively controls the wetting phase distribution.

Metasurfaces continue to drawn significant attention for manipulating light waves, but most to date have controlled either the phase profile or the amplitude profile of the output light—not both. This study proposes a strategy to control the transmitted phase and amplitude, over a wide band. A compound meta-atom can control the transmitted phase by modulating the gap size of a modified I-shaped structure, and can set the transmitted amplitude by rotating the structure between outer gratings. A proof-of-concept experiment points to the possibility of realizing arbitrary beam shapes.

Photonic interactions between different types of quantum memories will be crucial in the implementation of long-range hybrid quantum networks. Unfortunately, such interactions are typically impossible, due to the differing resonance frequencies of different systems. To overcome this hurdle, the authors collect photons at 493 nm from trapped ${}^{138}$Ba${}^{+}$ and convert them to 780 nm, a wavelength resonant with ${}^{87}$Rb, while preserving the quantum statistics of the photons after frequency conversion. This result increases the networking range of ions and enables hybrid networking experiments between trapped ions and neutral atoms in quantum information processing.

Surface acoustic waves (SAWs) at gigahertz frequencies are well suited to interact resonantly with spin waves in magnetostrictive materials, with the prospect of wavelike control of magnetization reversal in spintronic devices. The authors demonstrate SAW-driven all-acoustic switching over millimeter distances, and toggling between two equilibrium magnetic states for over 20 consecutive acoustic pulses. This proof-of-concept experiment confirms an alternative path to manipulating magnetic bits.

Domain structure accounts for the robust ferroelectricity of some organic compounds, but the limits of instrumentation have prevented detailed domain characterization. Using a state-of-the-art image sensor, the authors develop a sensitive, rapid visualization technique based on the first-order electro-optic effect, enabling mm-scale visualization of ferroelectric domain structures with sub-$\mu$m spatial resolution. The ability to probe depth allows unambiguous discrimination of a domain wall’s charge state, and thus its topological nature and motion, to promote applications of organic ferroelectrics in high-performance, low-cost devices.

Just find a way to stay cool: The confluence of high-power chip heat dissipation and localized hot spots expose the limitations of current cooling methods in electronics. To overcome these limitations it is essential to rapidly spread the on-chip sites heat generation and dissipate the extracted heat. The authors investigate the impact of the combination of a diamond heat-spreading layer and microfluidic convection on the performance of a model electronic chip with localized heating. It is shown experimentally and numerically that this combination can significantly reduce the maximum chip temperature.

Exciton gates, with an asymmetry in forward and reverse rates of energy transfer, are a means to overcome the diffusive and subdiffusive nature of exciton transport in organic semiconductors. The impact of multiple gating interfaces on exciton transport must be studied, though, to inform application of such gates in devices. Here a molecular site imbalance is engineered to establish gates, and the impact on exciton transport is examined by optically injecting excitons and detecting those that migrate through the structure. This work offers insight into the design of organic optoelectronic devices based on exciton gates, for $e.g.$ planar heterojunction photovoltaic cells.

Optical third-harmonic generation (THG) is important for various applications in nanomedicine, photocatalysis, and biosensing, but advances are held back due to small third-order nonlinear susceptibility ${\chi }^{(3)}$ and phase mismatch between the fundamental and third-harmonic waves. This work uses a graphene plasmonic coupler to achieve tunable, highly efficient THG through combining phase matching with resonant three-photon transitions. The study not only offers understanding and guidance for the problem at hand, but also points to further exploration of tunable, subwavelength coherent sources and ultracompact signal processing in the midinfrared and terahertz spectral regimes.

The Au${}_x$Ge${}_{1-x}$ alloy in the thin-film limit is an interesting material for nanoscale thermoelectric applications, particularly close to its metal-insulator transition, which occurs for $x$ around18–20%. This study presents a detailed characterization of this system, revealing good properties for thermal sensing. This alloy’s unexpectedly low Seebeck coefficient combined with its resistive behavior as a function of temperature makes it particularly apt for use as a local heater and thermometer. Moreover, the work demonstrates the ability to integrate this thin film as an electrode in large-area junctions based on ultrathin molecular layers.

Thorough scanning of parametric design space can yield devices with superior performance. This study follows such an approach in the case of core-shell nanowires under visible light, and in the presence of realistic losses. Upper limits on absorption and scattering by nanocylinders are reported, and the responsible resonances identified. These results determine the best members from a large class of objects, and accordingly provide additional degrees of freedom for metasurface modeling, and the design of optoelectronic or photonic devices.

In the quest for rare-earth-lean permanent magnets for $e.g.$ electric motors, the authors investigate so-called “composite crystal” permanent magnets of transiton-metal-doped CeCo${}_5$. The optimized material achieves high anisotropy and coercivity, while containing no critical rare-earth elements. Moreover, Cu- and Fe-doped CeCo${}_5$ systems cover the “gap” energy product of 10–20 MGOe. The particular coercivity mechanism here does not require complicated microstructure development, opening the potential for advanced manufacturing. This supply-independent, versatile, process-effective magnet can be used in a wide range of applications in the automotive and sensor industries.

Quantum coherence can be manifested in the time domain as quantum interference and diffraction, in analogy to optical interference and diffraction in real space, but time-domain diffraction has not received much attention. This work harnesses a superconducting qutrit (three-level system) under periodically modulated microwave fields to demonstrate diffraction in the time domain, revealing that the widths of the diffraction fringes are independent of the control-field power. These findings may see application in quantum gate operations in the strong-driving regime of superconducting quantum information hardware.

Strain engineering is a powerful technology that exploits spatial stress distribution to control various properties of condensed matter at the nanoscale. This study brings strain engineering to the subpicosecond time scale, demonstrating enhancement and suppression of an optically driven insulator-to-metal phase transition in VO${}_2$ nanohillocks by a picosecond strain pulse. The transient strain applied during subpicosecond optical excitation controls the fraction of material undergoing the phase transition, while not affecting its dynamics on a longer time scale. These results show the way to strain engineering of the phases of matter at nanometer and picosecond scales.

In noncontact particle manipulation by acoustic or optical beams, which is important for applications in the biological sciences and materials engineering, a $tractor$ $beam$ can pull particles toward the beam’s source, but long-range pulling is impossible without simultaneous trapping in the transverse direction. This study uses theory to gain insight into the parameters and conditions for simultaneous trapping and pulling of a spherical particle by an acoustic Bessel beam. Trapping at the pressure maximum of an axisymmetric Bessel beam in the Rayleigh regime is more flexible than the trapping by focused beams currently used for acoustic tweezers.

Thanks to advances in photolithography and nonfouling, photoactivatable surface chemistry, it has become possible to apply the holographic principle to sensitive molecular detection. Such molecular holograms, termed $molograms$, can be used for analysis of biomolecules in complex biological samples. In $molography$, biomolecules on a chip become apparent through diffraction, their coherent signal standing out from the speckled background generated by randomly arranged scatterers. The signal is hardly affected by changes in refractive index due to temperature gradients, buffer changes, or nonspecific bindings, which all are largely incoherent with respect to the molographic pattern.

The weakest link: Flexible electronics are an exciting prospect, but reliability in their connections is crucial to real-world deployment. Using a dispersion of conductive particles in an insulating fluid, researchers discuss the physics and engineering behind self-healing interconnects in which repair is automatically triggered upon the development of an open-circuit fault. Healed spots possessing metallic conductivity and nearly plastic stretchability are demonstrated. This work promises self-healing stretchable interconnects to improve circuit reliability.

In harvesting waste heat for electricity via thermoelectric materials, the figure of merit $zT$ has stagnated at values below 3, as it is difficult to simultaneously achieve low lattice thermal conductivity yet high electronic conductivity. Using state-of-the-art first-principles techniques, the authors predict a tremendous $n$-type $zT$ of 5 at 800 K in the full-Heusler compound Ba${}_2$BiAu, due to a “pocket” in a highly dispersive conduction band. A parallel study of Fe${}_2$TiSi allows the generalization that highly dispersive bands at off-symmetry points ought to be pursued for next-generation thermoelectrics.

Electric current can induce an ultrafast phase transition in vanadium dioxide that completely changes the material’s conductivity, offering potential device applications such as memristors for information technology. This transition is thought to be induced by strong correlation among electrons, not by Joule heating, though the situation remains murky. The authors use phase-field modeling to investigate the possible mechanism of a current-induced nonthermal transition, and construct the temperature-current phase diagram for VO${}_2$. They also show that domain walls can be moved by the current. These findings should help to guide the design of VO${}_2$-based electronics.

Eddy-current detection is of great interest for applications in biomedical science, materials science, and security, as it enables nondestructive evaluation of materials with different electrical conductivities. This study demonstrates eddy-current imaging using N-$V$ centers in diamond. Compared to existing technologies, the diamond-based sensor offers superior bandwidth, sensitivity, and spatial resolution, which could advance the field significantly. Using state-of-the-art N-$V$ sensors, a device would be able to distinguish between different kinds of biological tissues, including cancerous versus healthy tissues, which is of great interest for $e.g.$ navigation in neurosurgery.

Ferromagnetic Josephson junctions (FJJs), comprised of two superconductors separated by a ferromagnet, have been developed extensively because of their unconventional physical properties and potential applications in superconducting spintronics. Nevertheless, the dynamical properties of FJJs have not been sufficiently investigated. The authors’ experiments show that for thick FJJs, the magnetization dynamics of the middle layer are significantly modified by Meissner screening and magnetic flux pinning in the thick outer layers. Proper selection of the superconducting material provides dynamic access to the spin-polarized supercurrents in FJJs for high-frequency device applications.

The main challenge in scanning SQUID microscopy (the most sensitive technique for magnetometry) is the inverse proportionality between magnetic field sensitivity and spatial resolution. This work shows that a scanning-probe sensor made of a single planar superconductor/ferromagnet/superconductor Josephson junction can help to resolve this difficulty. Having a similar field sensitivity, planar junctions may outperform SQUIDs of the same size in terms of spatial resolution. This is important for developing advanced scanning-probe sensors with both high sensitivity and resolution.

The recent demonstration of ultrafast photonic computing devices exhibiting optical switching of phase-change materials has piqued interest in their viability for neuromorphic computing, but scaling these standalone devices to parallel computing platforms poses a major challenge. The authors leverage the parallelism offered by wavelength-division multiplexing (WDM) to propose a photonic “in-memory” platform, which can be used to emulate a spiking neural network. This solution could potentially bridge the gap between isolated computing devices and large-scale implementations of neuromorphic systems.

Quantum noise spectroscopy is an important tool for understanding the ubiquitous noise sources in quantum systems. Traditional spectroscopic methods are usually inefficient in measuring noise spectra that are broadband and decay nonmonotonically (exhibiting resonances, for example). Here a fresh approach uses a continuous driving method called gDYSCO. Performance of the continuous and traditional pulsed sequences is quantified analytically and numerically, and compared, revealing gDYSCO’s advantages in reconstructing nontrivial spectral features. Experiments confirm the findings, pointing to improved understanding and handling of quantum noise sources through this superior technique.

Naturally inspired: For propulsion and mixing under conditions of low Reynolds number, one could replicate the locomotion of microorganisms that have evolved to swim in highly viscous environments. Thus the authors simulate the motion of the head segment of the $C.$ $elegans$ roundworm with a reciprocating rocking blade. The biomimetic blade motion chaotically mixes the surrounding fluid via circulatory flow, and when confined between parallel walls creates steady Poiseuille flow, with a pumping efficiency comparable to the worm’s swimming efficiency. These pumping and mixing functions have potential applications in microfluidic devices, such as microprocessors cooled by polymeric flows.

$Radiation$ $balancing$ is a viable technique for heat mitigation in lasers and amplifiers, relying on anti-Stokes fluorescence cooling to offset heat generation in the gain medium. To achieve radiation balancing, the gain medium should be amenable to solid-state optical refrigeration. As Yb-doped silica is the gain medium of choice for most fiber lasers and amplifiers, it is important to determine whether this material can be optically cooled. The authors use optical spectroscopy to explore the possibility of solid-state optical refrigeration of Yb-doped silica, and show that cooling is in fact feasible.