Enhanced chiral light-matter interaction in metastructures could benefit applications in polarimetry and biosensing, but requires a high-performance chiral beam splitter that separates an incident beam into two circularly polarized ones of opposite chirality. This work unveils a photonic heterostructure that creates such chirality by stacking metasurfaces that are individually achiral, and hence provides an alternative approach. Analyses of symmetry, reciprocity, and microscopic dipolar interactions reveal the mechanism of interlayer coupling, extending the realm of artificial heterostructures into chiral photonics.

Ultrasensitive mechanical probes are central to many nanoscale sensing and transduction protocols. Membrane phononic resonators patterned in silicon nitride under tension allow for precise engineering of both the mechanical mode spectrum and mechanical dissipation. The authors present a paradigm for the geometric design and description of phononic crystals based on geometric mass contrast. Characterization of a variety of devices at ambient and cryogenic temperatures identify device designs with low-mass, low-dissipation mechanical modes. These design principles can be readily applied to optomechanical devices for tomorrow’s ultrasensitive force probes.

Domain walls (DWs) and skyrmions in ferromagnetic/heavy-metal thin-film multilayers are attractive for tomorrow’s magnetic devices, but such structures yield complex stray-field interactions that are known to limit their current-induced DW displacement. This work reveals that in these systems injection of spin current also leads to the phenomenon of Walker breakdown, which impedes DW motion even further, due to precession in the GHz regime, as confirmed by micromagnetic simulations and analytic theory. These findings identify limitations of conventional skyrmion-hosting materials, and identify a path for overcoming these limitations to enable high-speed racetrack memory.

Transparent conducting oxides (TCOs) are of great importance for numerous applications in optoelectronics. Though high-performance $n$-type TCOs are in commercial use, their $p$-type counterparts exhibit much lower performance, hindering critical technological development. This work sheds light on significant improvement in $p$-type TCOs through electron correlation due to on-site Coulomb interaction. This correlation not only promotes modification of the valence band but also shifts the screened plasma energy to below the visible region, and thus can be utilized as a key parameter in material design to optimize $p$-type TCOs.

In semiconductor physics, interaction of electrons with environmental disorder (phonons, impurity atoms, or other electrons) is important in shaping the band structure. These interactions result in “tailing” of the density of states near band edges, and band-gap narrowing. Using atomistic transport calculations with nonlocal scattering, this study assesses the impact of two important effects on band tailing and gap narrowing in $III-V$ compounds. Theoreticians modeling nanodevices will appreciate the benchmarked scattering models and formulas, while the predictions of band-gap narrowing and Urbach tails will appeal to experimentalists as they quantify their creations.

In the last decade it was demonstrated that the fastest way to write information employs ultrashort laser pulses. Naturally such experiments raise questions about the ultimate limit of repetition rate at which light can switch a medium between stable bit states. Here the authors demonstrate that with femtosecond pulses it is possible to write and rewrite magnetic bits in iron garnet with a frequency of up to 20 GHz, with the maximum repetition rate being defined by the frequency of ferromagnetic resonance in the field of photoinduced magnetic anisotropy. This finding reveals the principles to be employed in achieving magnetic recording at frequencies far beyond today’s state of the art.

Highly accurate and precise timekeeping supports many technologies for $e.g.$ navigation and telecommunication, as well as fundamental science. Nowadays optical clocks are far more stable than older microwave clocks, yet cannot be operated continuously. This study presents a hybrid microwave-optical time scale, in which the optical clock runs intermittently, and importantly explores improving this system aside from its optical clock. The hybrid system offers world-record accuracy, maintaining a fractional frequency uncertainty of 1.45×10${}^{-16}$ at 30 days and reaching the 10${}^{-17}$ decade at 50 days. This major improvement in accuracy could profoundly influence future architectures.

A major challenge in spintronics is reducing power consumption due to magnetization switching in ferromagnets. Here researchers demonstrate a scheme of orbital-controlled magnetization switching using a ferromagnetic oxide interface. Deterministic, magnetic-field-free 90° magnetization switching is achieved solely by applying an extremely small electric field, and the associated current density is reduced by 8 orders of magnitude, compared to current-driven switching. These results form a basis for a universal strategy for highly efficient magnetization control by band engineering.

While the magnetoelectric (ME) resonator is promising for magnetometers, very low-frequency and miniaturized antennas, and energy harvesters, devices based on the converse ME effect in amorphous Metglas alloy face frequency-response nonlinearity issues under high vibrational amplitude. This study aims to resolve this nonlinearity issue, by addressing the dependence of nonlinearity on bias field, driving voltage, mechanical quality factor, and frequency-sweep direction. With magnetic field as a control variable, the effects of nonlinear behavior on a ME antenna’s communication distance, bandwidth, and modulation are also presented.

The literature on slow dynamics in hysteretic elastic media has been limited to qualitative considerations, as comparison of results for different samples is only partly meaningful, due to the difficulty of achieving totally equivalent conditions. This study introduces measurable quantities to characterize these slow dynamics, and shows that different classes of materials ($e.g.$ intact versus damaged) exhibit significantly different behavior. This result is a necessary step to foster applications in nondestructive testing, for example, and separating and quantifying effects are key to understanding the physics and linking measurable macroscopic quantities to microscopic features.

Nitrogen-vacancy centers in diamond are promising as sensitive vector magnetometers, quantum repeaters, and qubits, but realizing this potential has been limited by the diamond material itself. Here a comprehensive study addresses creation of nitrogen-vacancy centers, balancing the impact of irradiation and annealing on the quantum coherence properties of commercial material. Photoluminescence decomposition analysis is used to quantitatively determine the neutral-to-negative charge-state ratio, and for insight into the impact of electron irradiation on material quantum coherence properties.

Optical topological insulators (OTIs) based on photonic crystals with $C_6$ symmetry are of interest for applications in integrated optics, offering special properties such as robustness, backscattering suppression, and defect immunity. These systems can be created with all-dielectric materials and no magnetic field, but progress is held back by technical difficulties. This study reveals the relationship between topological properties and structural parameters, and thus how to make OTIs with better adjustability by changing only the diameter of the cylinders in the basic structural unit of the photonic crystal.

Inertial sensors based on microelectromechanical systems (MEMS) have seen widespread success in automobiles, consumer electronics, and wearable healthcare devices, and there is significant interest in expansion to higher-accuracy applications in navigation, seismic recording, and gravimetry. This article presents a MEMS accelerometer based on recording variations in the degree of vibrational energy confinement in coupled microresonators in response to inertial forces. This approach enables enhancement of the sensor’s scale factor by a factor of over 100, and of resolution by 25, with tunable scale factor and sensor bandwidth and inherent filtering of external noise.

Spin waves are a promising medium for innovative approaches to information processing. A challenging problem for high-speed applications is the generation of spin waves that combine high frequency with low magnetic damping, in a weak external magnetic field. The authors use ultrafast magneto-optical Kerr effect to investigate standing spin waves (SSWs) and their damping in a thin film of magnetic insulator. They find that the damping of high-order SSW modes is several times lower than for the fundamental mode, over a broad range of magnetic field below the saturation level. This result is expected to have significant impact on ultrafast photomagnonic applications.

The current-driven motion of magnetic skyrmions is an essential functionality for next-generation memory and logic devices. This paper provides a complete, detailed experimental study of a model system composed of an ultrathin Pt/Co/MgO film, in which 100-nm skyrmions move at high speeds of 100 m/s and exhibit a velocity-dependent skyrmion Hall effect. Detailed characterization of the film properties using experiment, analytical modeling, and micromagnetic simulation reveals that this behavior can be fully accounted for by pinning effects, unlike some claims in the literature.

Extraction of particles or cells encapsulated in aqueous droplets into a continuous aqueous stream is important for single-cell studies. Controlled passive coalescence of droplets under coflow conditions in a microchannel is extremely challenging, requiring active chemical, magnetic, or electrical techniques. Such methods have disadvantages, though, in terms of system integration or biocompatibility. The authors’ sound-wave-mediated coalescence technique addresses the above limitations and promises rapid, on-demand extraction of a droplet’s contents in a microfluidic device.

Enhanced chiral light-matter interaction in metastructures could benefit applications in polarimetry and biosensing, but requires a high-performance chiral beam splitter that separates an incident beam into two circularly polarized ones of opposite chirality. This work unveils a photonic heterostructure that creates such chirality by stacking metasurfaces that are individually achiral, and hence provides an alternative approach. Analyses of symmetry, reciprocity, and microscopic dipolar interactions reveal the mechanism of interlayer coupling, extending the realm of artificial heterostructures into chiral photonics.

Magnetoelectric coupling plays a key role in multiferroic and spintronic devices, yet the mechanism of interfacial coupling between ferromagnetic Co and multiferroic BiFeO${}_3$ is still not clearly understood. The authors use x-ray dichroism to reveal the charge-transfer process at the Co/BiFeO${}_3$ interface, from which the interfacial exchange coupling arises with the formation of Fe${}^{2+}$. In BiFeO${}_3$ with periodic 71° or 109° domains, the combination of compressive strain and exchange coupling sensitively determines the enhancement of the coercive field of Co/BiFeO${}_3$. These results will facilitate the energy-efficient control of logic and data devices using electric fields.

Routing acoustic waves without backscattering losses is presently of great interest. Unlike both conventional approaches using defects in sonic crystals and emerging approaches based on topological edge states, this study proposes a radically different theoretical framework that utilizes extremely anisotropic media to engineer backscattering-immune waveguides. The exact condition for one-way wave propagation along zigzag paths is derived. The proposal is experimentally validated using spoof surface acoustic waves, and the results could have implications for on-chip wave manipulation, as well as noise control.

In high-energy physics, Penning traps are used to make very precise mass measurements, determine fundamental constants, and test symmetries. Magnetic field noise limits trap performance, though. A superconducting self-shielding coil can reduce magnetic field fluctuations, but technical difficulties make high shielding factors difficult to achieve in practice. This work describes a design for a coil system with adjustable shielding factor that overcomes many of these difficulties. In the BASE experiment at CERN, the authors use this design to achieve fourfold improvement in the precision of antiproton-to-proton charge-to-mass comparisons.

Superhigh-frequency (1−20 GHz) acoustic waves are an important tool for controlling electronic, optical, and magnetic excitations in semiconductor nanostructures. The authors demonstrate the piezoelectric generation of bulk longitudinal acoustic waves in GaAs up to 20 GHz. Studies of the wave-propagation properties over a wide temperature range show that scattering by thermal phonons is strongly suppressed below 80 K, thus enabling long propagation lengths and acoustic quality factors exceeding 104. These findings point to exciting opportunities for electrically driven optomechanics and acoustoelectric control in this frequency range.

Sensors and clocks based on ultracold atoms are now the technology of choice for demanding applications in navigation and geology, and are ideally suited to $e.g.$ the detection of dark matter and gravitational waves, and precision measurements of physical constants. Almost all of these devices operate in pulsed mode, which leads to aliasing of frequency noise (the Dick effect). Here researchers present a $continuous$ beam of ultracold Sr atoms with phase-space density reaching 10${}^{-4}$, 1000 times the conventional value. Such beams can be sources for uninterrupted atom interferometers, or can continuously replenish the gain medium in active optical superradiant clocks.

Although waveguiding is vital in mechanical signal processing and energy harvesting, conventional techniques are prone to significant backscattering from corners or junctions. Inspired by the quantum Valley Hall effect, the authors create and study topologically protected in-plane waveguides that are immune to backscattering. In addition, the waveguides are exploited to realize multi-interface junctions that act as logic ports with unconventional wave-manipulation capabilities, including asymmetric transport, energy trapping and rerouting, and mechanical signal delaying. These ports can ultimately serve as the building blocks for a practically endless array of elastodynamic circuits.

The $axion$ particle is one of the most promising candidates for the constitution of cold dark matter. As researchers of axion dark matter continue to scan for higher masses, the detectors become more sensitive to anomalous effects, which could confound the search. This analysis examines the effects of breaking translational invariance within cylindrical microwave cavities similar to those typically used in haloscope detectors. Breaking translational invariance causes avoided mode crossings, a discovery that enables experiments to mitigate these effects in the detectors used in the axion quest.

At the microscale, noncontact control of objects plays a key role in physics, chemistry, biology, and information science, but is susceptible to external disturbance. This study demonstrates a method based on an innovative trapping mechanism to manipulate a micro-object controllably, using a toroidal vortex at ambient conditions. By taking advantage of the nonlocal topological property of the vortex flow field, the object is intrinsically protected from external disturbance. This manipulation technique will be useful in scenarios such as scanning-image short-range weak-interaction sensing, as well as a broad range of applications involving rotational degrees of freedom.

The construction of quantum wells (QWs) in graphene nanoribbons is interesting for applications in optoelectronics and quantum information processing, but is hindered by insufficient understanding of quantum mechanical effects in these systems. This study uses theory to reveal that, unexpectedly, electronic states confined by higher confinement energy exhibit $weaker$ quantum confinement, due to the wave-function symmetries of the electronic states from the constituent graphene nanoribbon segments. This mechanism leads to a design method for such QWs via modulation of geometric parameters and displacement between constituent nanoribbon segments.

Recently it was shown that ferrimagnetic nanodisks support a magnetic vortex state that can oscillate at frequencies up to 30 GHz, suggesting that a ferrimagnetic free layer in a spin-torque-driven nanopillar oscillator would have a significantly higher frequency than the ferromagnetic oscillator. Here theory shows that a nanopillar spin-torque oscillator with a current perpendicular to the ferrimagnetic free-layer will indeed drive high-frequency vortex oscillations. This work indicates that it is feasible to construct ferrimagnetic spin-torque oscillators with frequencies increased tenfold, or more. Moreover, the Oersted field from the current provides enhanced frequency control.

Efficient, reliable characterization of quantum states is a basic and important step in most tasks in quantum information processing, but standard approaches, including quantum tomography, are notoriously inefficient. This research offers solutions to the problem of verifying nonstabilizer states by proposing efficient, practical protocols for verifying arbitrary $n$-qubit Dicke states. These protocols require only two distinct settings based on Pauli measurements, and thus are readily applicable with current experimental techniques, and are able to verify the robust Dicke states of hundreds of qubits.

Despite the commercial success of organic emitters based on thermally activated delayed fluorescence (TADF), a general understanding of emitter-matrix interactions is still lacking. How does the dielectric constant around an emitter impact its delayed fluorescence? The authors study the photophysical properties of three emitters in solution, showing that a higher excited triplet state at low dielectric constants can boost the efficiency of blue TADF emitters. Experiments are supported by state-of-the-art quantum simulations. The findings may be generalized to solid-state systems, pointing to tuning of host-guest architectures to improve the efficiency of TADF-based blue OLEDs.

The recent introduction of two-dimensional materials into magnetic tunnel junctions (2D MTJs) offers very promising properties for spintronics, such as atomically defined interfaces, spin filtering, perpendicular anisotropy, and modulation of spin-orbit torque. Nevertheless, the difficulty of integrating exfoliated 2D materials into spintronic devices has limited exploration. Here the authors find a fabrication process leading to superior performance in MTJs based on transition-metal dichalcogenides, and further suggest a path to alleviate basic issues of technology and physics for 2D MTJs.

Metamaterials have great potential to control near-field electromagnetic response at will. Lately metamaterial-based devices have been used to manipulate the rf magnetic field in magnetic resonance imaging (MRI) via Fabry-Perot resonances or subdiffraction lenses. This work shows that a negative-permeability metamaterial incorporated into an MRI setup supports magnetic surface plasmons, allowing local boosting of the magnetic field and an improved signal-to-noise ratio. These results could pave the way for high-performance MRI systems.

The SiC/SiO${}_2$ interface is a central component of many SiC electronic devices. Defects intrinsic to this interface can have a profound effect on device operation and reliability; it is therefore crucial to both understand the nature of these defects, and develop characterization methods to enable optimized SiC-based devices. The authors show confocal microscopy to be a noncontact, nondestructive, and spatially resolved means of addressing individual defects and assessing SiC/SiO${}_2$ interface quality. This may provide a more rapid method to optimize the SiC surface for future devices.

Electromagnetically induced transparency (EIT) and its acoustic analogue (AIT), featuring a transparency window in the transmission spectrum, share similarities with Autler-Townes splitting (ATS), though the intrinsic mechanisms differ. They are usually distinguished only after a device has been built, via a numerical method that does not consider their mechanisms. The authors theoretically, numerically, and experimentally distinguish AIT from ATS with a pair of Helmholtz resonators, finding that one must consider both the usual numerical criterion and the physical mechanisms. This work helps to guarantee AIT or ATS by design, for applications in $e.g.$ sensing, imaging, and filtering.

As lithographic nanostructures shrink to critical dimensions less than 10 nm, the semiconductor industry searches for nondestructive metrology techniques to extract three-dimensional structural information with subnanometer precision. This study develops a modeling algorithm that extracts a unique, complex line profile from x-ray scattering measurements collected in a nondestructive grazing-incidence configuration. This algorithm represents an important step toward demonstrating the utility of grazing-incidence x-ray scattering as a viable tool for measuring the dimensions critical to semiconductor processing, and more.

Ultrasensitive mechanical probes are central to many nanoscale sensing and transduction protocols. Membrane phononic resonators patterned in silicon nitride under tension allow for precise engineering of both the mechanical mode spectrum and mechanical dissipation. The authors present a paradigm for the geometric design and description of phononic crystals based on geometric mass contrast. Characterization of a variety of devices at ambient and cryogenic temperatures identify device designs with low-mass, low-dissipation mechanical modes. These design principles can be readily applied to optomechanical devices for tomorrow’s ultrasensitive force probes.

Surface-acoustic-wave (SAW) devices are often made in acoustically soft polymers due to ease of fabrication, but at the cost of high acoustic attenuation. To improve the design of SAW devices, the authors present a comprehensive three-dimensional numerical model that includes SAW transducers, a chip containing a water-filled microchannel, and the acoustic streaming and radiation forces acting on particles suspended in the channel fluid. The model is validated by experiment, and explains how the use of glass leads to SAW devices with increased energy efficiency and improved acoustophoretic focusing ability, compared to polymer-based devices.

Designing hardware specifically for neural-network algorithms is crucial for low power consumption in “big data” analysis tasks. The challenge is to realize reliability, endurance, and scalability in the same synaptic device. Here a spintronic memristor, based on an isotropically coercive free layer in a magnetic tunnel junction, is modeled by the Landau-Lifshitz-Gilbert equation with an additional dissipative term reminiscent of dry friction. Simulations indicate subnanosecond data writing, and synaptic behavior in an in-plane magnetic field. Meeting all three of the criteria above, this device would be a good building block for neuromorphic circuitry.

Inspired by the quantum spin Hall effect, the authors propose a tunable nonlinear topological insulator for acoustic waves, which could be an ideal framework for lossless information transport. The band structure of the hexagonal unit cell is obtained analytically, using Bloch’s theorem and zone-folding techniques, revealing doubly degenerate Dirac cones. Breaking of inversion symmetry creates energy-dependent band gaps with topologically protected edge states that are robust against backscattering at arbitrary interfaces. The proposed topological insulator could be a stepping-stone platform toward building tunable acoustic devices, interconnects, and electroacoustic integrated circuits.

Domain walls (DWs) and skyrmions in ferromagnetic/heavy-metal thin-film multilayers are attractive for tomorrow’s magnetic devices, but such structures yield complex stray-field interactions that are known to limit their current-induced DW displacement. This work reveals that in these systems injection of spin current also leads to the phenomenon of Walker breakdown, which impedes DW motion even further, due to precession in the GHz regime, as confirmed by micromagnetic simulations and analytic theory. These findings identify limitations of conventional skyrmion-hosting materials, and identify a path for overcoming these limitations to enable high-speed racetrack memory.

In spintronics, ferrimagnets continue to draw intense attention for their ultrafast dynamics, but their reduced magnetization near the compensation point plus small spin-mixing conductance do not help the spin-orbit torque in ferrimagnet/heavy-metal multilayers. This study uses two heavy metals with opposite spin Hall angles flanking a typical ferrimagnetic alloy to achieve effective enhancement of the spin Hall angle, as well as stable, controllable multilevel current-induced switching based on the proposed heterostructure. This work, combining efficient and multibit magnetization switching, provides an alternate route to leveraging ferrimagnets in spintronic devices.

The magnetically coupled dielectric elastomer actuator (MCDEA) is an emerging two-degree-of-freedom system that demonstrates rich dynamical behavior, which is important for applications in robotics, energy harvesting, and smart structures. However, the complex nonlinear dynamical behavior that can arise in such a system is challenging to predict. The authors develop a numerical model that can accurately characterize these dynamics, and thus suggests control strategies to manage $e.g.$ the appearance and amplitude of a specific resonance. These insights will impact engineering solutions for active vibrational control, energy harvesting, and programmable soft motors.

While the use of mixed-matrix membranes to improve the efficiency of gas separation by filtration is an important technological breakthrough, full exploitation of this technique is limited by the rich complexity of the transport and adsorption phenomena involved. This study uses kinetic Monte Carlo simulations to gain deeper insight into the separation performance of various membranes with complex structures, predicting optimal shapes for filler particles in defect-free membranes. This filtration technique is particularly interesting for removing CO${}_2$ from the atmosphere, to ameliorate global climate change.

Transparent conducting oxides (TCOs) are of great importance for numerous applications in optoelectronics. Though high-performance $n$-type TCOs are in commercial use, their $p$-type counterparts exhibit much lower performance, hindering critical technological development. This work sheds light on significant improvement in $p$-type TCOs through electron correlation due to on-site Coulomb interaction. This correlation not only promotes modification of the valence band but also shifts the screened plasma energy to below the visible region, and thus can be utilized as a key parameter in material design to optimize $p$-type TCOs.

Antiferromagnetic (AFM) materials attract increasing interest in spintronics research, yet a lack of efficient electrical control and detection of the AFM state is a major issue for device applications. This first-principles study proposes a tunnel junction with large tunnel magnetoresistance (on the order of hundreds of percent), by exploiting the magnetic phase transition of AFM Mn${}_3$Pt, which can be controlled by an electric field. These results offer valuable guidelines for engineering innovative AFM memory based on the magnetic transition in Mn${}_3$Pt and similar materials.

Decoherence is one of the limiting factors in quantum technology. To use plasmonic components here, knowing whether the quantum properties of individual surface plasmons can be protected over long distances is crucial, yet has not been fully addressed experimentally. The authors investigate the quantum decoherence of single surface plasmons in the high-confinement regime, where the excitation’s significant matter component is expected to cause prompt decoherence. Surprisingly, the coherence properties of plasmons are well preserved even in this regime, highlighting that, despite intrinsic losses, plasmonic devices can have a remarkable range of utility for quantum applications.

This work illustrates that near-field thermophotovoltaics based on graphene-BN-InSb heterostructures can be a comparable or even superior alternative to high-performance thermoelectric energy converters. Resonant coupling between layers, mediated by surface plasmons and surface phonon polaritons, allows this system to efficiently convert waste heat at 400–800 K (the common temperature range for industrial waste heat) to useful electricity.

High spatial resolution and low perturbation are required for mapping the electromagnetic field at the nanoscale, to provide crucial information about local light-matter interaction. Using optical superresolution microscopy for detection via a nitrogen-vacancy center in diamond, this work demonstrates far-field noninvasive sensing of the local electromagnetic field with high accuracy and a spatial resolution of approximately 50 nm. As the nitrogen-vacancy center is sensitive to various physical quantities, a multifunctional far-field quantum nanoscope is anticipated.

Superconducting nanowire single-photon detectors (SNSPDs) with high system detection efficiency (SDE) would enable remarkable experiments in quantum information processing. However, realizing a NbN SNSPD with saturated intrinsic detection efficiency (IDE) that retains high SDE at near-infrared wavelengths is challenging, due to the high critical temperature of NbN. This study uses defect engineering by helium-ion irradiation to enhance the IDE of NbN SNSPDs to saturation without sacrificing SDE. This technique also allows direct comparison of irradiation-induced changes in detector performance, making it a useful tool for studying the physics of superconducting devices.

Nanoparticles in a laser-produced plasma can act as a sensitive control knob for inducing highly localized electron injection. The benefits of nanoparticles for laser-plasma electron accelerators have not been examined, though, due to complex effects on the plasma fluid and difficulties in getting nanoparticles into the plasma. Here the authors demonstrate nanoparticle-assisted laser wakefield acceleration by succeeding at incorporating nanoparticles into the plasma medium, obtaining prominent enhancement of beam quality by controlling the electron injection process. This work could promote applications of nanoscience to laser-plasma accelerators, and open a path to $attoscience$.

Studies of geometric phases arising in metasurfaces have focused on the Pancharatnam-Berry (PB) phase, locking spin states to a symmetric profile. This work shows how to realize instead the Aharonov-Anandan (AA) phase, by mimicking a single ballistic Aharonov-Bohm ring with metastructures. Spin-symmetry restriction is broken by combining the AA and PB phases, and the merged phase can impose a desired phase profile on an arbitrary polarization, enable manipulation of the photonic spin Hall effect, superpose spin-½ charges, and break the conjugate constraints of output orbital-angular-momentum modes. These results have significant potential for polarization control and spin-enabled optics.

This article reports a type of magnetic tunnel junction (MTJ) with an expanded middle layer, for spin-transfer-torque magnetic random-access memory (STT-MRAM). This data-storage layer of the form Fe-Co-B/W/Co/W/Fe-Co-B sandwiches a ferromagnet with high exchange stiffness between two tungsten films, and thus is much less sensitive to temperature than that in a conventional MTJ. Such a storage layer is promising for spintronic memory that must operate across a wide range of temperatures, as in automobile applications.

Femtosecond laser pulses are now a powerful tool for controlling spin waves, which is promising for magnonic data processing. Unveiling the full potential of ultrafast photomagnonics requires extending the range of technologically relevant mechanisms and materials. Thus the authors investigate propagating magnons in a thin film of the ferromagnetic alloy galfenol, which supports spin-wave excitation via ultrafast laser-induced changes in its pronounced in-plane magnetic anisotropy. Magnetostatic surface waves are clearly detectable over 10 $\mu$m from the excitation spot, while abrupt localized changes in anisotropy provide a path to ultrafast optically reconfigurable magnonic elements.

In semiconductor physics, interaction of electrons with environmental disorder (phonons, impurity atoms, or other electrons) is important in shaping the band structure. These interactions result in “tailing” of the density of states near band edges, and band-gap narrowing. Using atomistic transport calculations with nonlocal scattering, this study assesses the impact of two important effects on band tailing and gap narrowing in $III-V$ compounds. Theoreticians modeling nanodevices will appreciate the benchmarked scattering models and formulas, while the predictions of band-gap narrowing and Urbach tails will appeal to experimentalists as they quantify their creations.

Plane waves are required for underwater acoustic locating, navigation, and survey, but generating them efficiently with traditional approaches is impractical, particularly at low frequencies. This study proposes an easy means to transform cylindrical waves into plane waves, through phase rectification with graded aluminum honeycomb structures. The honeycomb metasurface allows mimicking of the acoustic behavior of fluids, and its effective acoustic index and impedance can be independently tailored as desired. Experiment verifies efficient wave-front modulation of the metasurface with high transmission over a broad frequency range.

Quantum-enhanced sensing with spin-bearing defects in solids is important for measuring magnetic fields at the atomic scale, but progress is impeded by the difficulty of accessing ensembles of strongly coupled spins that can be entangled deterministically. This study exploits an optically dark electronic-nuclear spin defect near a single nitrogen-vacancy center in diamond to create an entangled state of two electronic spins, and measure the amplitude of time-varying magnetic fields with superior performance. With their stability under optical illumination, dark electronic spins can be repeatedly measured through the N-$V$ center to further improve the signal-to-noise ratio.

Thermal metamaterials are typically designed to work at room temperature, where thermal conduction (mediated by phonons) is the dominant mode of heat transfer. Unfortunately, at higher temperatures thermal radiation (mediated by photons) dominates, and those metamaterials no longer work. Thus the authors propose an effective-medium theory for manipulating thermal radiation described by the Rosseland diffusion approximation. They proceed to design radiative metamaterials that are well-behaved in both steady and transient states, for three different functions. These results may inspire innovations in heat management, including radiative camouflage and thermal diodes.

Electric field control of discrete states of resistance and spin using ferroelectricity is promising for next-generation multistate memory devices. In contrast to most ferroelectric devices, which employ merely two states, this article presents nonvolatile, multilevel resistance states in multiferroic tunnel junctions (MFTJs) with a PbZr${}_{0.2}$Ti${}_{0.8}$O${}_3$ tunneling barrier. The phenomenon is due to the “mixed” state of two ferroelectric domains, gradually interconverted by tunneling electroresistance and magnetoresistance in the external electric field. This work shows a fresh route to ferroelectric spintronic devices for potential neuromorphic computing applications.

Levity to grasp gravity: The superconducting gravimeter (SG) is the most sensitive and stable instrument for temporal gravity observations, but amplitude attenuation and phase delay at high frequencies limit its application in seismometry. This study uses a superconducting oscillator plus SQUID-based displacement detection to broaden the SG’s frequency band to cover ambient seismic noise. In addition, this negative-stiffness design could also have many other uses, such as for a superconducting gravity gradiometer. This approach provides a means to adjust the stiffness of the magnetic levitation force (as in the Meissner effect).

With potentially quantum limited performance and wide frequency bandwidth, traveling-wave Josephson parametric amplifiers (TWJPAs) based on superconducting circuits are in urgent demand for quantum information processing. This study designs a TWJPA in which the interacting pump and signal/idler microwaves propagate with similar phase velocities through two different transmission lines, thereby enabling parametric gain. Such operation is possible due to a chain of SQUIDs that form the signal transmission line, which is magnetically coupled to a separate pump $LC$ line. The proposed circuit may greatly simplify the measurement setup and solve the problem of pump depletion.

The ion-cavity quantum interface is an important building block for tomorrow’s quantum networks, but miniaturizing such devices remains an outstanding challenge. This study introduces and characterizes an on-chip design that uses microelectromechanical-systems (MEMS) technology to integrate a fiber-based optical resonator with an ion trap. Simulations show that the device’s performance is expected to be similar to that of much larger existing systems, and that it is compatible with strong ion-cavity coupling. A MEMS approach thus offers a promising route to scalable quantum networks based on miniaturized, fast, high-fidelity interfaces.

Parametric resonators have wide utility as bits, sensors, and frequency references. Controlling the phase states of a parametric resonator enables signal generation and computation, but so far the states’ rich dynamics in the presence of nonlinearities have received little attention. The authors delineate the regimes of nonlinearities for which parametric phase control is possible, and use this to experimentally operate on now-accessible solutions of the parametric response. In the context of micro- and nanoelectromechanical resonators, this work can foster numerous applications for parametric pumping in $e.g.$ scanning probe microscopes, signal amplifiers, and inertial sensors.

A quantum computer comprises both qubits and their classical electronic interface. While much research is currently devoted solely to qubits, an efficient electronic controller is also urgently needed for a scalable quantum computer. This study uses analytical techniques to expose the effect of nonideal circuit blocks in a classical controller on qubit fidelity, for all required operations, and how fidelity is affected by the limited performance of the general-purpose, room-temperature equipment typically used with the few qubits types available today. Tailor-made controllers can achieve significantly lower cost, power consumption, and size, as required for scaling up.

Solar cells with parallel optical transitions promise a 10% boost in conversion efficiency over the Shockley-Queisser single-junction limit. This study classifies various concepts according to their internal physical processes, and shows that voltage matching through “ratchet steps” of carrier relaxation may be crucial to realizing efficiency benefits. The ideal exothermic ratchet-step size is shown to depend logarithmically on the etendue ratio for emission and absorption, in addition to the luminescence extraction efficiency. While for some cells infinite Auger interaction rates lead to endothermic optimal ratchet configurations, more realistic rates lead to an exothermic arrangement.

Photoelectron emission from nanotips via strong-field lasers is a powerful technique for coherent control of ultrafast electron dynamics, and so is important in many areas, including time-resolved electron microscopy and free-electron lasers. The effects of dc bias are typically ignored, though. Exact solution of the time-dependent Schrödinger equation reveals that, under large dc bias, strong modulation persists in the emission current, the magnitude of which increases significantly. In the authors’ model, the dynamics of the multiphoton excited states depend strongly on the applied dc field. This work suggests a practical approach to strong control of high-current photoemission.

In the last decade it was demonstrated that the fastest way to write information employs ultrashort laser pulses. Naturally such experiments raise questions about the ultimate limit of repetition rate at which light can switch a medium between stable bit states. Here the authors demonstrate that with femtosecond pulses it is possible to write and rewrite magnetic bits in iron garnet with a frequency of up to 20 GHz, with the maximum repetition rate being defined by the frequency of ferromagnetic resonance in the field of photoinduced magnetic anisotropy. This finding reveals the principles to be employed in achieving magnetic recording at frequencies far beyond today’s state of the art.

$Accelerating$ $beams$, which as they travel in free space bend themselves without the help of any optical device, are not only fascinating but can also be useful, as they can circumvent obstacles in their paths. So far applications have been limited to beams with convex trajectories (such as a parabola), but here the authors propose image transmission based on accelerating beams with nonconvex trajectories (a helix, for example). It is found that, because of the special mapping relationship between Fourier space and real space for nonconvex accelerating beams, images encoded in these beams are less affected by obstructions than images in convex beams ($e.g.$ Airy beams).

Light emission from group-III nitride semiconductors is of crucial interest, as it underpins efficient LED lighting. Perhaps surprisingly, the underlying physics here remains poorly understood. This study demonstrates experimentally that the spontaneous emission of III-nitrides has an unexpected behavior. An advanced numerical model reveals that this is caused by the interplay between two phenomena: The strong disorder in III-nitride compounds leads to carrier localization, and the pronounced Coulomb interaction affects the electron-hole coupling. This insight enables a deeper understanding of III-nitride luminescence, to promote optimization of future LEDs.

Graphite is a common electrode material in metal-ion batteries, and understanding the diffusion of atoms and small molecules in it is instrumental for improving graphite-based batteries. Here first-principles molecular dynamics simulations reveal an intriguing cooperative effect in AlCl${}_4$-intercalated graphite, whereby the motions of the graphene layers and the intercalant AlCl${}_4$ molecules considerably enhance each other. This cooperative behavior is a key factor behind the ultrahigh-rate capability of a recently reported aluminum-ion battery. Furthermore, it turns out that an AlF${}_4$-based battery could be even better.

NMR measurements are generally designed to be performed in a stable magnetic field, yet temporal field fluctuations occur in many practical implementations. To analyze this complication, the authors decompose the magnetization into eigenmodes of the instantaneous effective Hamiltonian, which allows them to quantify the adiabatic and nonadiabatic regimes. This work has direct implications for the interpretation of measurements in fluctuating fields, the design of NMR hardware, and the development of new pulse sequences with improved robustness against field fluctuations.

Nanoscale superconducting quantum interference devices (SQUIDs) on a tip are currently of high interest for ultrasensitive scanning-probe magnetic and thermal imaging. Their applicability, however, is curbed by volatility to ambient conditions and a limited range of operating fields and temperatures. In this work, the authors develop a fabrication method applicable to a wide range of previously inaccessible superconducting materials, and demonstrate operation of a SQUID-on-tip at a high magnetic field of 5 T, paving the way to nanoscale imaging of magnetic and spintronic phenomena, and of dissipation mechanisms in quantum states of matter.

The circular photon drag effect (CPDE) is important for helicity-dependent optoelectronic emitters and detectors, but in graphene the relatively weak light-matter interaction gets drowned out by other nonlinear optical effects. Vertically grown graphene (VGG), though, with its particular symmetry point group, can exclude the other effects and enhance the light-graphene interaction. This study of the CPDE from VGG shows that the emitted THz states can be tuned to have linear or left- or right-handed elliptical polarization, by changing the helicity of the pump laser. This observation points to graphene-based polarization-sensitive THz sources for chiral analysis.

van der Waals heterojunctions based on two-dimensional materials are attractive for tunnel field-effect transistors, but their atomically clean and electronically sharp junction interfaces fail to realize satisfactory tunneling behavior. This work reveals the phenomenon of band offset degradation in these heterojunctions, induced by interlayer charge transfer, which can impede the band-alignment configuration that is key to turning on such devices. These discoveries help to explain the calculated deviations in heterojunction band alignment, and should be highlighted in tunnel-device exploration.

Realizing single-photon sources plays an essential role in quantum information science. The key step for generating a single photon is to attain strong photon blockade, based on either strong energy-spectrum anharmonicity or quantum interference to eliminate two-photon excitation. However, the strong coupling in a high-finesse cavity that is needed for these mechanisms is still a challenge. In this work, strong photon antibunching with a large cavity photon number is predicted, by combining the optical Stark shift with anharmonicity and quantum interference beyond the strong-coupling regime. This proposal suggests exciting opportunities for applications in $e.g.$ quantum networks.

When operated in a dilution cryostat, a MHz-frequency nanomechanical device relies on active cooling to reach its quantum ground state. However, a proper understanding of its environmental interactions and mechanical quantum decoherence would require in-equilibrium cooling, with the thermal bath reaching sub-mK temperatures. This is very demanding and requires cooling via adiabatic nuclear demagnetization. The authors successfully demonstrate the technique, with a scheme to measure the temperature on-chip to verify equilibrium with the cryostat. An unexpected instability of unknown origin is observed at very low temperatures, presenting a strong limitation to progress in this field.

One of the most promising ways to transfer quantum information between superconducting qubits is with microwave photons. Enhancing direct quantum channels by time-bin encoding techniques, which map qubit states to single-photon states emitted at different times, allows the detection of photon-loss errors, and thus heralded quantum communication is possible. The authors realize and experimentally benchmark an error-detection scheme that allows them to select only experimental runs in which the photon state was transmitted successfully, yielding significantly improved fidelity of the transfer in this post-selected setting.

High-power broadband terahertz dual-comb sources are of great importance for fast high-resolution spectroscopy, but are rare because of the lack of high-performance terahertz radiation sources. Here the authors demonstrate a broadband terahertz dual-comb source on a chip, based on electrically pumped quantum cascade lasers and microwave double injection. By injection locking the two lasers at slightly different round-trip frequencies, even weak microwave power can significantly broaden the dual-comb bandwidth. Furthermore, the double-injection technique allows direct evaluation of the carrier offset noise of the terahertz laser combs.

Highly accurate and precise timekeeping supports many technologies for $e.g.$ navigation and telecommunication, as well as fundamental science. Nowadays optical clocks are far more stable than older microwave clocks, yet cannot be operated continuously. This study presents a hybrid microwave-optical time scale, in which the optical clock runs intermittently, and importantly explores improving this system aside from its optical clock. The hybrid system offers world-record accuracy, maintaining a fractional frequency uncertainty of 1.45×10${}^{-16}$ at 30 days and reaching the 10${}^{-17}$ decade at 50 days. This major improvement in accuracy could profoundly influence future architectures.

Powerful terahertz masers are required for various important applications, such as heating and current drive in controlled nuclear fusion. Traditional relativistic electron masers are based on emission of a fixed transverse mode of the operating cavity; at terahertz frequencies, oversized cavities are obviously needed, and it is difficult to provide single-mode generation. This study proposes a fresh strategy for solving this problem, based on excitation of a supermode formed by a fixed set of partial transverse cavity modes. The approach is interesting as a means to realize high-power sources of coherent terahertz radiation.

Software-defined radio (SDR) requires multi-use or reconfigurable radio-frequency front ends, with filters capable of dynamically shifting their operating frequencies. Current approaches to tunable filters typically require prohibitively large electric fields to achieve large tuning. This study utilizes a multiferroic approach, with an electric field straining magnetoelastic elements to tune the system’s magnetic resonance. Furthermore, dipole coupling of the elements yields mode splitting, for additional resonance peaks that depend on element spacing and applied strain. This could lead to innovative types of rf filters for SDR, where communication frequencies are rapidly changing.

Subwavelength plasmonic nanoparticles offer interesting possibilities as meta-atoms in metamaterials, due to their sharp resonance and strong field enhancement. While the surface-plasmon resonance is usually in the ultraviolet or visible regime, a spherical nanoparticle with a lossless dielectric core and thin plasmonic shell exhibits a strongly redshifted resonance. This study numerically shows that that scheme can be applied to nonspherical plasmonic meta-atoms to shift the resonance all the way to the near-infrared regime, without spectral broadening or weakened field enhancement. This insight could impact photothermal biomedical imaging, and ultracompact photonic components.

Reducing noise to the ultimate limit is a challenge in developing lasers for ultrasensitive applications in precision sensing and fundamental science. This study utilizes hollow-core optical fiber to implement a passive noise suppression scheme known as “collinear balanced detection” to suppress classical noise to the shot-noise limit, while simultaneously preserving the spectrum and temporal profile of a laser pulse. The work expands on this scheme to show how the technique can be adapted to reduce noise across an arbitrary band of frequencies. These demonstrations enable passive, broadband, all-guided fiber-laser technology operating at the shot-noise limit.

Optimizing the effective charge-to-spin conversion, or effective spin Hall angle, in a ferromagnetic/nonmagnetic (FM/NM) system is crucial for next-generation magnetic storage and spin logic devices. The authors present a fresh approach for enhancing the effective spin Hall angle, by introducing strain to a FM/NM system. High, tunable efficiency in charge-to-spin conversion is achieved in Ta/Fe/Pt films grown on mica, by applying compressive strain in the flexible substrate, with over 50% enhancement of the effective spin Hall angle. These findings hopefully will spur further work on flexible spintronics.

Efficient, fast, high-density magnetoelectric random-access memory (MeRAM) requires both large perpendicular magnetic anisotropy and high-efficiency voltage-controlled magnetic anisotropy. Thus the authors propose a double-barrier ferromagnetic heterostructure with an atomically thin layer of a late transition metal $X$ (Rh, Ir, or Pt). First-principles calculations reveal its anticipated performance to be due to the synergy effects of the large strain-induced magnetism in $X$, the large spin-orbit coupling of $X$, and the giant modulation of magnetocrystalline anisotropy at the $X$/MgO interface. This work provides rules for designing the next generation of spintronic memory devices.

In recent years, remote traps based on the $radiation$ $force$ of sound or light beams have been used to manipulate atoms and even living cells, but have been limited to tiny objects. Here the authors’ work in acoustics confirms the accuracy of a general theoretical model of radiation force on solid objects for complex beams. The force is quantified experimentally and theoretically for a range of spherical objects of various sizes and compositions. Spheres up to 6 mm in diameter are moved along a controlled path by a single beam. These results lay the groundwork for noncontact manipulation of large objects for various physical, biological, and medical applications.