Schottky-barrier contacts to amorphous semiconducting oxides are essential building blocks for transparent, flexible, low-cost electronics, but rectification is typically insufficient, unless an oxygen-plasma surface treatment or a reactive deposition of the contact metal is employed. This study of inertly and reactively sputtered Schottky contacts on zinc tin oxide, aimed at identifying the mechanisms governing contact formation, reveals that migration of oxygen alters carrier density in the vicinity of the interface, and with it the diode’s rectification. A general, step-by-step recipe for high-performance Schottky-barrier diodes on transparent semiconducting oxides is included.

Entanglement-based quantum networks are strongly pursued worldwide, because of their potential impact on secure communication, distributed quantum computing, and timekeeping, for example. Among quantum emitters, the N-$V$ center in diamond is a leading candidate for implementing such networks, but high photon loss at the N-$V$ emission wavelength hinders long-distance entanglement (beyond ~1 km). The authors surmount this hurdle by down-converting single N-$V$ photons to a telecom wavelength, via nonlinear optics plus efficient filtering and excellent control of a lone emitter. This technological achievement is a critical step toward large-scale quantum networks.

To address a chief environmental challenge of society worldwide, brackish water from wells or the ocean can be desalinated by a class of porous electrodes in which ions are stored within the crystal structure of a redox-active material. The authors present a comprehensive theoretical framework for transport of ions and charge in an entire electrochemical device, treating desalination as a dynamical process. Interestingly, though both electrodes in the proposed device adsorb only cations, the water (which also bears anions) can still be desalinated very effectively, due to the inclusion of a single anion-exchange membrane.

Combating decoherence—the randomization of qubit values in a physical system—is a key challenge in quantum computing. Control theory provides a powerful set of tools to stabilize classical systems, but much work remains to bring its full weight to the quantum domain. Aiming to track and predict qubit-state evolution under various forms of decoherence, the authors employ machine-learning algorithms to optimize qubit-state forecasting, and adapt classical algorithms to work directly with discrete, single-shot qubit measurements. These results suggest that exciting opportunities exist for implementing real-time feedback using purely classical, hardware-agnostic techniques.

Many engineered objects endure stress throughout their lifetimes, typically leading to material fatigue and failure, which can turn to tragedy, as in the Versailles rail accident of 1842. Thus quantitatively assessing the level of cumulative fatigue damage is of great interest in many contexts, and should promote our insight into the underlying physics at the several stages of fatigue. The authors find that the frequency of a particular surface-acoustic-wave elastic Lamb mode is a reliable indicator of the fatigue level in a thin metal film. This technique should also work for other systems in which these Lamb modes are observable, including alloys, composites, glasses, and polymers.

Spatially resolved studies of surface acoustic waves (SAWs) are important for advancing acoustoelectronic applications in telecommunication, quantum materials, and phononic crystals, but it is difficult to achieve mesoscopic resolution of such surface-displacement fields. This study uses transmission-mode microwave impedance microscopy to directly image the piezoelectric potential carried by the elastic surface wave. Interference of counterpropagating SAWs and diffraction due to a small domain of opposite polarization are also observed. This technique is sure to facilitate the engineering of SAW devices and complex acoustic metamaterials.

Schottky-barrier contacts to amorphous semiconducting oxides are essential building blocks for transparent, flexible, low-cost electronics, but rectification is typically insufficient, unless an oxygen-plasma surface treatment or a reactive deposition of the contact metal is employed. This study of inertly and reactively sputtered Schottky contacts on zinc tin oxide, aimed at identifying the mechanisms governing contact formation, reveals that migration of oxygen alters carrier density in the vicinity of the interface, and with it the diode’s rectification. A general, step-by-step recipe for high-performance Schottky-barrier diodes on transparent semiconducting oxides is included.

The next generation of alkali-vapor-cell atomic clocks are under development in labs around the world, with prototypes already demonstrating short-term fractional frequency stabilities one or two orders of magnitude better than for commercial clocks. However, stability is often spoiled at medium time scales (>100 s) by light-power-induced frequency-shift effects. The authors tackle this problem by applying autobalanced Ramsey interrogation, recently demonstrated on a Yb${}^{+}$ clock, to coherent-population-trapping systems—an approach that could really boost the development of vapor-cell clocks with white-noise-limited frequency stability over greatly extended time scales.

Mesoscopic magnetic resonance at lengths of 0.1—10 $\mu$m is vital in many areas of research, including structural biology and medical diagnostics. Magnetic resonance based on detecting spin fluctuations with nitrogen-vacancy centers in diamond is a powerful tool at the nanoscale, but this approach sputters in the mesoscopic regime, as signals fall off cubically with distance. This study surmounts the problem by instead detecting spin $polarization$, extending the sensor-to-sample distance to tens of $\mu$m, rather than the tens of nm in previous work. This remote-sensing approach will have an impact on bioimaging applications at the subcellular level.

Widely tunable nano- and microscale light emitters based on a single material do not suffer from alloying-induced defects, and thus are very desirable for photonic devices. Suitable materials are scarce, though. This study of Bragg-reflector microcavities in Ga${}_2$O${}_3$:Cr microwires, with an intense red-infrared luminescence band, reveals strongly modulated, tunable spectra. Experimental data, analytical calculations, and simulations show good agreement, in a comprehensive analysis of propagated and confined modes and reflectivity along the emission band. This microcavity approach to tunable light sources is promising for applications in optoelectronics and photonics.

With strong spin-orbit coupling, the largest spin Hall effect among transition metals, tunable interfacial Dzyaloshinskii-Moriya interaction (IDMI), robust perpendicular magnetic anisotropy (PMA), and excellent process compatibility with CMOS devices, tungsten is a key material for spintronic devices. Brillouin light-scattering spectroscopy shows that the thickness-dependent phase transformation of W in thin-film multilayers leads to phase-driven oscillations of PMA and IDMI, due mainly to the variation of distance between heavy-metal and ferromagnetic atoms. Simultaneously tuning PMA and IDMI through this effect would be a powerful tool for atomic-scale device engineering.

The superradiant quantum phase transition (QPT) is of fundamental interest, and has potential applications in modern quantum technology. The existence of an equilibrium superradiant QPT in cavity and circuit QED systems is still debated, though, considering the no-go theorem. Combining cavity QED and optomechanics, the authors predict at thermal equilibrium a single-photon-triggered superradiant QPT that is immune to the no-go theorem. Being able to manipulate a QPT in this manner is expected to inspire both the creation of innovative photonic quantum devices and the exploration of single-photon quantum-criticality physics.

In the drive for terahertz-frequency technology, a spatial phase shift between a THz near field and a periodically modulated two-dimensional electron gas dramatically enhances the detection responsivity of a multigate “ratchet” structure, by breaking its symmetry. In high-mobility structures the response exhibits a plasmonic peak; finite ratchet structures also present such peaks, at lower plasmonic frequencies, corresponding to the wavelengths determined by overall structure length. These results are important for the modeling, design, and optimization of very sensitive sub-THz and THz plasmonic detectors, modulators, mixers, phase shifters, and sources.

The photoluminescence of a semiconductor is a very sensitive means to determine its absorption coefficient $\alpha$, using Planck’s generalized law. The standard method (suitable only for self-supported thick samples like wafers) is here extended to multilayer thin films, by using the transfer-matrix method to include the effects of the substrate and optional front layers. This technique allows one to circumvent parasitic absorption from the substrate, and thus to determine $\alpha$ accurately down to very low values, for investigation of deep band tails. Accounting for the substrate effect is important for a clear understanding of how a solar cell is functioning, and how to improve it.

The generation of “structured” light beams bearing nonvanishing orbital angular momentum is important for a wide range of microwave and optical applications. Unfortunately, current methods allow only narrow bandwidth, require an optically thick device, or are inefficient. This study leverages geometric phases due to spin-to-orbital interaction and tailored subwavelength transmit-arrays to enable highly efficient creation of broadband multiple vortex beams, free from normal-mode background interference. The design methodology and proposed structure can be further extended, for two-dimensional (de)multiplexing of vortex beams in momentum space.

Detecting faint light from deep space requires an excellent signal-to-noise ratio, as does quantum information processing. For microwave and terahertz frequencies, a simple directional amplifier offering minimal added loss and easy on-chip integration would be a major step forward. This study envisions such an amplifier as two Josephson-junction oscillators plus an on-chip passive circuit, promising 20 dB of gain while adding only ~1 photon s${}^{-1}$ Hz${}^{-1}$ of noise. This work addresses scale-up in circuit QED and detector research by integrating directional signal routing and amplification, allowing more qubits or pixels per unit area.

Precise manipulation of microparticles, cells, and even biomolecules has become increasingly important in medical science and technology. The authors study acoustic streaming induced by Lamb-wave resonators (LWRs) in the context of microfluidics, and design a microelectromechanical device consisting of four LWRs. The LWR array efficiently drives multiple horizontal cylindrical vortices in a 1-$\mu$L droplet and traps suspended particles at a desired location, with no need for microfluidic channels. This capability for highly efficient, specific flow stimulation and particle manipulation in a small droplet is expected to facilitate biomolecule detection and other biomedical applications.

This study provides a rigorous analytical solution for electromagnetic wave propagation within and scattered from a slab of engineered material possessing general space-time modulation. In contrast to the weak photonic transitions in conventional space-time permittivity-modulated media, an equilibrated space-time-varying medium provides energy and momentum for strong, unidirectional photonic transitions from the excited mode to its four adjacent modes. Equilibrium in the electric and magnetic properties of such a medium yields various interesting phenomena, pointing the way to optimal insulators, nonreciprocal integrated systems, and subharmonic frequency generators.

Mechanical metamaterials that harness the buckling instability to yield advanced functionalities suggest numerous potential applications, including soft robotics, flexible electronics, and medical implants. Most approaches so far have used geometrical design to adjust the onset of instability in such materials, leading to a relatively narrow range of tunability. The authors investigate an additional dimension, the spatial distribution of material properties, to widen the range of instability thresholds. Adding this further design dimension could extend the instability threshold (critical strain ) by a factor of three, compared to the purely geometrical approach.

The design of modern circulators and isolators relies on nonreciprocal (discriminating forward and backward wave vectors, breaking time-reversal symmetry) electromagnetic waves in magnetic materials. Creating such devices that are small enough for portable electronics (think mobile phones) remains challenging. The authors show theoretically the nonreciprocal propagation of surface acoustic waves in multilayers with simultaneous magnetoelastic and Dzyaloshinskii-Moriya interactions. Despite both interactions being relatively weak, their interplay creates up to 45 dB of isolation for counterpropagating waves, making these multilayers promising for miniaturized rf isolators.

From quantum optics, we are familiar with electromagnetically induced transparency (EIT), particularly in association with “slow light”. The plasmonic analogue, PIT, now also receives attention due to its potential applications in sensing, communication, and more. However, the conventional dipole-quadrupole coupling yields only a modest quality factor $Q<100$. Here controlling the $Q$ factor of the Fano lineshape is not trivial. The authors demonstrate quadrupole-quadrupole interaction in engineered meta-atoms to control $Q$ and attain $Q>100$, with the potential to reach 1000, bringing high-$Q$ applications based on PIT one step closer.

Controlling the generation of spin-orbit torque (SOT) with an electric field is important for spintronic applications in digital memory and magnetic sensors, but for the heavy metals typically used as SOT sources, insensitivity of bulk carrier density to an electric field makes generating torque difficult. This study demonstrates that creating SOT in an ultrathin film of gold can be managed effectively by ionic-liquid gating, which enables tuning of SOT-generation efficiency by a factor of two with a gate voltage of just ±1 V. The results offer an avenue to exploiting SOT efficiently in spintronic devices.

Noise mitigation is critical in a myriad of scientific disciplines and applications, including spectroscopy, telecommunication, and sensing. The task is particularly difficult when working in the Fourier (frequency) domain. This study presents a methodology to redistribute the energy of a spectrally periodic wave along its frequency spectrum, in noiseless fashion, to obtain a new set of waveforms with higher spectral peak power over the input noise floor. The mechanism of passive amplification is based on an elegant, powerful generalization of the spectral Talbot effect, and it enables interesting capabilities for noise reduction in important applications.

Stochastic computing (SC), a radical rethinking of computation, defines operations on streams of random bits; it trades precision for large advantages in speed. Implementation has been thwarted, though, by the lack of an efficient means to properly decorrelate bitstreams at each logic gate in an SC circuit. This study harnesses recent advances in manipulating magnetic skyrmions to propose a technique for telegraph-signal reshuffling that is tailor-made for SC applications. Leveraging the two-dimensional diffusive character of skyrmion motion shows how useful these exotic magnetic textures can be in tackling problems that require compact, scalable, energy-efficient device architectures.

Applications of solid-state quantum emitters in photonic quantum information science generally require nanofabrication, to enable appreciable interactions with confined optical fields. Open questions include the extent to which nanofabrication can induce detrimental effects in a quantum emitter’s behavior, and under what circumstances such effects might be expected. The authors combine nanoscale optical location techniques, precise nanofabrication, and high-resolution optical spectroscopy to quantitatively investigate the influences of nanofabrication on quantum-dot single-photon sources, and the potential for eliminating fabrication-induced problems via surface passivation.

Designing metamaterials with near-zero electric permittivity (ENZ) is of great importance for practical applications at optical frequencies, but designs based on traditional approaches are restricted to a narrow range of operating frequencies. The authors invoke the rigorous spectral representation of the effective permittivity for a composite system, treating the design challenge as an inverse problem of effective-medium theory. Their approach, based on analytical solution of the inverse problem plus finite-element simulations, may be the way forward to broadband ENZ metamaterials.

The electromagnetic response of a superconducting nanowire exhibits relaxation oscillations, due to strong electrothermal nonlinearity. Resultant technology has been limited, though, by a lack of understanding of how the nonlinear behavior can be externally modulated. This study of electrothermal oscillations in superconducting nanowires uses microwave modulation to show that the oscillations mix with an external microwave drive, and lock to its frequency. This interaction is reminiscent of the ac Josephson effect, despite its thermal origin. Nanowire devices based on these phenomena could see use in microwave applications like frequency mixing and parametric amplification.

Quantum computing hardware is much more susceptible to errors than classical hardware. While quantum error correction can combat these errors, the noise affecting the quantum hardware must be understood to apply the proper error-correcting code. Presenting an approach for determining in real time whether qubit errors are correlated, the authors simultaneously monitor two fluxonium qubits and measure the correlations between their relaxation times. This analysis method and architecture can be generalized to multiqubit systems, where applying the right error correction is crucial for reliable computation.

Recent research on nanomechanical resonators has shown that their mechanical motion can be controlled via the interactions between different flexural modes, in diverse structures. Strong coupling in flexural modes has been attributed to strain mediation, but further study is needed to harness the dynamics for applications. The authors use a doubly clamped resonator to demonstrate coupling between flexural modes of different parity, and then use theory to show that the thermomechanical motion of a nanomechanical resonator can be amplified with high gain. This technique could be useful for amplifying an infinitesimal signal in sensing applications.

To advance high-energy detectors, we need a deeper understanding of the exciton-activator interaction in systems such as x-ray storage phosphors and scintillators. This article revisits the general molecular interpretation of self-trapped excitons known from binary alkali halides, and justifies extending this interpretation to the ternary quasi-one-dimensional halides CsMg$X_3$. Direct evidence of interaction between polaronic self-trapped excitons and local luminescent Eu${}^{2+}$ activators is found. These results highlight the family of inorganic halide perovskites as interesting scintillators for tomorrow’s detectors, in addition to solar-cell applications.

Pore-scale flow behavior is important in many areas of science and engineering that deal with porous media, including geophysics, hydrogeology, biophysics, and filtration. This numerical study shows that variable structures dependent on flow rate exist in dynamic porous media, which contradicts the Darcy permeability model. In spite of $microscopic$ flow variations (tortuosity), the $macroscopic$ flow behavior (permeability) remains constant. The possibility of these variable structures (shown to be induced also by local pressure fluctuations) raises questions about the universality of constant-tortuosity models for Darcy flow, challenging our view of transport in porous media.

Through the anomalous Hall effect (AHE), a ferromagnet can generate spin current in the direction transverse to an applied charge current, and hence exert a torque on another ferromagnet. The authors observe, however, that when in-plane current is passed through a spin-valve structure, an out-of-plane effective magnetic field is created, which exerts torque on the free layer. This effective field, originating from the broken mirror symmetry of the spin valve, completely dominates the dampinglike torque that one expects from the AHE. Such a strong out-of-plane field would be useful for switching perpendicular magnetic bits in spintronic high-density nonvolatile memory applications.

As the march toward a spintronic future continues apace, the authors find that surface tension plays a critical role in the dynamics of curved magnetic domain walls (DWs). Direct observation with a Kerr microscope reveals that the switching-field distribution shifts to $lower$ values of magnetic field as the size of Co-Fe-B/MgO nanodots decreases—quite the opposite of previous results. In the framework of an elastic interface, this can be explained as Laplace pressure applied to DWs nucleated at the edges of the dots. These findings suggest a path toward scalable spintronic devices with lower switching energies, based on controlling the nucleation and pinning potential of DWs.

Ferroelectric materials with high mechanical quality factors $Q_m$ are used in high-power electromechanical devices, but the physical mechanism of $Q_m$ enhancement is not well understood. This study uses Rayleigh analysis to quantitatively investigate the pinning effect in acceptor-doped relaxor ferroelectric crystals. Enhancement of the longitudinal factor $Q_{33}$ is attributed to restricted polarization rotation, while that of the shear factor $Q_{15}$ is ascribed to both polarization rotation and clamped domain-wall motion. This insight into the underlying physics will guide the material optimization of device performance in $e.g.$ transducers and sensors.

Superconducting circuits are an excellent platform for quantum simulation, with various protocols enacted using highly coherent transmon qubits in a digital (gate-based) approach. Analog simulations with transmons, though, are typically limited by restrictive intercircuit coupling. The authors propose an alternative strategy based on periodic modulation of an effective magnetic field, enabling a Floquet description of the system. This allows one to modify the existing two-qubit interaction and the effective spin Hamiltonian, without modifying the setup. Detailed analysis shows this Floquet-type simulator to be quite advantageous, outperforming state-of-the-art digital approaches.

Transition edge sensors (TESs) are remarkable detectors that resolve the number of photons in an ultraweak optical pulse, allowing the determination of the full photon statistics, which is not possible using standard commercial detectors. In demonstrating the potential of these TESs for nanophotonics, the authors uncover subtle differences in the optical properties of two bimodal quantum-dot micropillar lasers with nominally similar characteristics, yet different photon-number distributions, whether in standard single-mode lasing or polarization-mode switching.

Entanglement-based quantum networks are strongly pursued worldwide, because of their potential impact on secure communication, distributed quantum computing, and timekeeping, for example. Among quantum emitters, the N-$V$ center in diamond is a leading candidate for implementing such networks, but high photon loss at the N-$V$ emission wavelength hinders long-distance entanglement (beyond ~1 km). The authors surmount this hurdle by down-converting single N-$V$ photons to a telecom wavelength, via nonlinear optics plus efficient filtering and excellent control of a lone emitter. This technological achievement is a critical step toward large-scale quantum networks.

Current-induced magnetization switching is an emerging technology for low-power spintronic applications, such as magnetic nonvolatile memory. The authors investigate this phenomenon in ferrimagnetic Co${}_x$Tb${}_{1-x}$ alloys grown atop a W layer, observing current-induced switching with a strong thermal contribution, which can lower the external in-plane magnetic field needed for the switching. Furthermore, they identify a characteristic temperature associated with the switching, above the magnetic and angular-momentum compensation temperatures yet below the Curie temperature. This insight should impact both the physical understanding and application of ferrimagnetic alloys.

Generating spin currents with controllable spin direction is important to spin-torque technology, which includes spin-based computing and magnetic random-access memory, but it is difficult to produce such currents through conventional means, like the spin Hall effect. This study shows that the anomalous Hall effect can be used to yield spin currents with controllable spin polarization. The magnetization of the generating ferromagnetic layer, and hence the spin polarization, is governed by exchange bias and external field. This result offers a great deal of flexibility in creating spin torque, which should enable innovative device designs.

For the surface electronic states of a metal, the Tamm plasmonic (TP) mode can be excited without an additional phase-matching component, but in a typical device the TP resonance wavelength is fixed, which is quite limiting. This work develops an approach to tuning TP resonance by incorporating a thin layer of liquid crystal (LC) into the design. Here phase retardation by the LC layer is an ensemble effect, due to the entire layer, not just the molecules at the metal’s surface, as in devices based on surface-plasmon polaritons (SPPs). Also unlike SPP systems, the proposed structure is highly amenable to mature thin-film processing, for scalability to lengths of several meters.

Complete control of photon emission at the nanoscale is crucial for scalable quantum technology. Control over the polarization of light is traditionally achieved $a$ $posteriori$ using birefringent optics, but this work takes a different approach by intrinsically altering the polarization $emitted$ by a source. The authors use a nanofiber’s evanescent field to induce radiation of arbitrary polarization by a linear dipole emitter in a waveguide, potentially spanning the entire Poincaré sphere. This remarkable manifestation of the mapping of an emitter’s purely geometrical degrees of freedom to the polarization states of its emission opens the door to applications in nanophotonics.

To address a chief environmental challenge of society worldwide, brackish water from wells or the ocean can be desalinated by a class of porous electrodes in which ions are stored within the crystal structure of a redox-active material. The authors present a comprehensive theoretical framework for transport of ions and charge in an entire electrochemical device, treating desalination as a dynamical process. Interestingly, though both electrodes in the proposed device adsorb only cations, the water (which also bears anions) can still be desalinated very effectively, due to the inclusion of a single anion-exchange membrane.

The success of superconducting single-photon detectors has recently led to theoretical analysis of the impact of photons on a current-carrying superconductor, accompanied by the emergence of resistance due to vortices. Based on this analysis, a device unlike the standard long, narrow variety is developed, enabling operation close to the intrinsic critical pair-breaking current of the superconductor. These devices, which can be as wide as a few $\mu$m, clearly show single-photon detection for optical wavelengths from 408 to 1550 nm, making them compatible with optical fibers and promising for next-generation large-area detectors for quantum optics applications.

While three-dimensional arrays of metallic nanoparticles in a dielectric matrix are promising candidates for plasmonic optical components and metamaterials, their fabrication remains challenging, and the types of structures that can be produced by conventional methods are limited. The authors overcome these limitations by using ion-beam irradiation to produce a periodic arrangement of prolate nanoparticles in a symmetric matrix, controlling both particle aspect ratio and orientation. Applications aside, such a structure is an ideal physical model for evaluating and comparing analytical theories and numerical calculations of the plasmonic resonance in metallic nanoparticles.

Spinel ferrites are auspicious for the sources of “pure” spin current being developed for spintronic applications, but until recently have exhibited extremely high damping, making these materials unsuitable for coherent spin-current generation. This work demonstrates efficient spin pumping across an epitaxial ferrite/Pt interface, revealing the coexistence of proximity-induced magnetism and spin pumping at these interfaces, and that pumping dominates the microwave-induced voltage. These insights highlight this class of materials as the means to devices based on pure spin currents.

Current-induced switching via Néel spin-orbit torque in thin films of the antiferromagnet Mn${}_2$Au offers the prospect of a magnetic multilevel nonvolatile memory cell. The authors investigate this switching with experiments and modeling, explaining the strong dependence of the switching amplitude on current density and temperature. They also point out the importance of thermal assistance of the electrical current to the switching. The energy barrier to switching in Mn${}_2$Au is high enough to preserve a magnetic state for many years at room temperature, making this system suitable for real-world spintronic devices.

Although it is well known that structural disorder has a strong impact on a resonant optical device’s performance, the exact relationship still needs clarification. This article presents the effective refractive-index approximation, an elegant method for simple, quantitative, and comprehensive insight into the link between a resonator’s structural disorder and the consequent deterioration of its optical performance. The proposed method is validated both theoretically, against transfer-matrix simulations, and experimentally, by comparison to measured properties of highly disordered Bragg reflectors.

Combating decoherence—the randomization of qubit values in a physical system—is a key challenge in quantum computing. Control theory provides a powerful set of tools to stabilize classical systems, but much work remains to bring its full weight to the quantum domain. Aiming to track and predict qubit-state evolution under various forms of decoherence, the authors employ machine-learning algorithms to optimize qubit-state forecasting, and adapt classical algorithms to work directly with discrete, single-shot qubit measurements. These results suggest that exciting opportunities exist for implementing real-time feedback using purely classical, hardware-agnostic techniques.