All-optical helicity-dependent switching of ferromagnetic thin films using ultrafast laser pulses could be very interesting for data storage applications, but has been held back, because a large number of pulses are needed to write information. This study shows that full switching is obtained for a narrow window of laser fluence and pulse duration. It also shows that a few tens of pulses are enough to switch magnetization, and that even a $single$ $pulse$ can have a significant effect. These results will require a reassessment of the theoretical descriptions of optical spin control that have developed over the last decade.

Understanding the magnetic proximity effect in a van der Waals heterostructure made of monolayers of semiconductor and ferromagnet is of great interest, considering its potential applications in spin control. This article presents a general method to study the problem, combining $ab$ $initio$ and tight-binding calculations. The results reveal that a spin-polarized miniband with a localized state forms in the heterostructure, and can be tuned magnetically, mechanically, or electrically. The authors furthermore propose some programmable spintronic nanodevices with various functionalities, for applications in scalable quantum computation and high-density quantum circuits.

The maximum conversion efficiencies of photovoltaic devices and performance-limiting factors of practical solar cells remain ambiguous, and thus the strict determination of current technological limits is of significant importance. This study develops an analytical scheme that allows the evaluation of realistic maximum-power conversion efficiencies of important inorganic and hybrid perovskite solar cells in conventional thin-film form. The authors show that, although efficiencies greater than 30% can be achieved for absorber layers with sharp absorption edges, many record-efficiency polycrystalline solar cells are limited by open-circuit voltage and fill-factor losses.

The recent demonstration of neuromorphic computing with spin-torque nano-oscillators points to substantial energy-saving in data analysis. However, the limited intrinsic memory of these devices (much less than a microsecond) limits their utility for analyzing temporal sequences. Here the authors overcome the short memory of a spin-torque oscillator by using a feedback loop with a delay of 1 $\mu$s, reducing the error rate during temporal-pattern classification by 99%. In addition, they determine optimal operating point of the oscillator, in terms of the current and magnetic field, to take advantage of this memory for recognition tasks.

Potable water, while key to human survival, is relatively scarce. Seawater is plentiful, but its desalination by reverse osmosis requires membranes that pass water molecules, but reject ions and debris. The performance of current polymer membranes is limited by insufficiency in selective ion rejection, mechanical strength, thermal stability, resilience to cleaning agents, and electrical conductance. The authors’ atomistic computational design of a membrane with layers of graphite oxide, carbon nanotubes, and carbon-based fabric addresses all of those limitations and promises significant performance improvement at low cost, through its microscopic insight into the desalination process.

The study of elastic wave propagation is relevant to many engineering applications, such as crashworthiness, nondestructive testing, and energy harvesting. The authors discuss how elastic energy can be guided along the surface of a three-dimensional mechanical structure, without any penetration to its core. The mechanical design here is inspired by the recent discovery of Weyl semimetals. Using full numerical simulations, they demonstrate robust wave propagation on the surface of a fuselagelike hollow structure, with no backscattering at corners or surface defects.

Analysis and optimization of the electromagnetic response of material constituents, such as atoms, molecules, or meta-atoms, is challenging. This study demonstrates that the most general dipolar response of $any$ small scatterer can be conceptually decomposed into a set of basic, fundamental polarization phenomena. The results enable immediate, complete characterization of the electromagnetic properties of an arbitrary material or metamaterial (in the linear regime). With this approach, if you can imagine a material system, you can know its behavior—and optimize it.

The anomalous Hall effect related to scalar spin chirality—the $topological$ Hall effect (THE)—is studied intensely for its applicability in spintronics and Hall-effect devices. Knowing how to realize a large THE is essential, because the effect is often smaller than other contributions to transport. The authors show that thermal fluctuations enhance THE significantly, sometimes to 1000 times the zero-temperature value of the Hall conductivity. Numerical simulations and analytic calculations provide a detailed understanding of the temperature dependence typically seen in experiments.

Ubiquitous stacking faults in SiC are a major problem in power electronics, since they are predicted to cause the deterioration, and eventually failure, of devices. Understanding the thermodynamic stabilities of the different crystal polytypes in these faults is crucial to improving the quality of epitaxial SiC, and thus devices. This sophisticated density-functional study resolves the paradox of SiC polytype stability, predicting the correct free-energy hierarchy with varying temperature. The physical insight gained here should extend to other classes of semiconductors as well.

Controlling the random, seemingly chaotic motion of individual molecules, atoms, and electrons poses a major challenge for nanotechnology. For efficient, practical nanomachines we need to convert this disorderly motion to orderly, without any active regulation from the outside, ideally. The authors propose and analyze such an autonomous engine based on a single electron, combining ideas from electron transport and self-oscillating heat engines. Using parameters estimated from experimental data for available components, analysis shows that the engine can lift the mass of a small bacterium, is remarkably stable, and can be scaled up to increase power and further reduce fluctuations.

Mass sensors are actively used to detect biomolecules, viruses, and other nano-objects, and improved sensitivity is always sought. Owing to progress in nanofabrication technology, mechanically coupled optomechanical cavities featuring exceptional points (EPs) can be engineered. The authors propose an efficient optomechanical mass sensor operating at an EP, where any perturbation induces a frequency splitting that scales as the square root of the perturbation strength, resulting in giant sensitivity enhancement. These refined optomechanical sensors could see applications in many fields, including nanoparticle detection, precision measurement, and quantum metrology.

Speckle patterns have been utilized in various sensing applications, due to their excellent sensitivity to the states of light, yet the physical interpretation of that very feasibility has remained unclear. How much optical information could be encoded as intensity speckle, and retrieved from it? This work shows that speckle determines the “coherency matrix”, which contains information about the amplitude, phase, polarization, and coherence properties of incident light, as functions of position. The authors retrieve a 3520×3520 coherency matrix from a single intensity-speckle image, which would require about 24,773,760 individual measurements via conventional Stokes polarimetry.

Terahertz-frequency magnons (quantized spin waves) are notoriously short-lived, and therefore difficult to measure, but are currently of great interest for applications in spintronics and signal processing. This theoretical work shows that it is possible to detect magnons at 50–600 GHz, using reflection and attenuated total reflection of radiation from a heterostructure. Just 20 ultrathin layers (each 4–6 atoms thick) of iron are needed to measure considerable changes in reflectivity. These results indicate that strong interactions with electromagnetic radiation are possible, paving the way for magnetically tunable devices that operate in the far-infrared regime.

Electro-optical effects in nematic liquid crystals are extensively used in display technology. A major drawback that has hindered further application is the relatively slow time of the optical response to an applied field, on the order of milliseconds. The authors demonstrate a $microsecond$-scale electro-optical response, by using the electric field to change the orientational order of a liquid crystal with giant positive dielectric anisotropy. This phenomenon could be exploited in fast electro-optical shutters, phase modulators, beam steering, and more.

Phase-gradient metasurfaces (PGMs) provide remarkable control over the propagation of light. One expects that the number of unit cells $m$ in a supercell should not change the nature of a PGM’s diffraction characteristics, yet recent experiments have indicated $m$-dependent asymmetric absorptivity. An analytical description of the relationship between absorptivity and $m$ is needed, but lacking. This study offers a simple, intuitive semianalytical approach to explaining the observed asymmetric absorptivity. Here the phase gradient ensures additional, multiple total internal reflections inside the grating, leading to more dissipation and the asymmetric diffraction response.

Acoustic valley materials, usually constructed in a hexagonal lattice, provide an additional degree of freedom (so-called valleys in their band structures) for manipulating sound waves. This study realizes such a material in a $square$ lattice, in which the topological transport and negative refraction of acoustic kink waves are observed in experiment. At the intersection of channels, a counterintuitive transport phenomenon is seen: The kink waves tend to choose the path that contains a sharp turn. These findings may be useful for directional vocalization, noise reduction, and beam splitting.

Magnetic skyrmions are promising for information storage and processing, because of their nanoscale size, high stability, and large velocity, but understanding their dynamical excitations to capitalize on their characteristics remains a challenge. This study uses analytical results and micromagnetic simulations to investigate skyrmion breathing dynamics. A “breathing” skyrmion can be seen as analogous to a conventional amplitude modulator, with linear modulation functionality. This insight not only boosts basic skyrmion research, but also suggests magnonic devices with significant applicability in communication.

Recent advances in ultrafast laser 3D writing offer direct fabrication of in-chip silicon microsystems, but this technique remains extremely challenging, because of propagation nonlinearities that limit energy localization inside narrow-band-gap materials. This study examines the complex energy flux and subsequent unusual modification of silicon due to intense picosecond pulses from a thulium-doped fiber laser. The best conditions for reliable data inscription deep inside silicon are identified, which is a critical step toward engineering solutions to this problem. Also, the wavelength used here points to facing the next challenges associated with materials of even narrower band gap.

Optical and microwave reference signals with high phase coherence are needed for precision measurement and advanced technologies, especially as ultrastable signal generators move from the laboratory to the wider world. The authors explore laser-frequency stabilization with respect to a monolithic microresonator, housed without complex ambient-isolation systems. A comprehensive analysis of technical noise shows how to achieve laser coherence that is limited only by ambient thermal noise, and successful generation of a low-noise microwave signal reveals fundamental coherence between the optical and microwave frequency domains.

Wave manipulation for cloaking and holography is difficult in practice, because most techniques are limited in bandwidth and their ability to adapt their response to varying incoming wave fields. In this study, acoustic sources on closed surfaces have their responses calculated and fed back in real time, to suppress forward and backward scattering from objects without prior knowledge of the incident wave field. Furthermore, arbitrary virtual objects can be emulated by changing the boundary conditions applied to the surface. The authors demonstrate these active acoustic boundary conditions in a one-dimensional experiment, to perform broadband cloaking and holography in real time.

Metasurfaces (two-dimensional metamaterials) have attracted much attention in recent years. However, due to the symmetric design of its cascaded elements, a traditional transmission-type metasurface must be at least three layers thick to achieve high transmission and 360° phase control simultaneously. This study presents an ultrathin Huygens’ metasurface, composed of double-layer antisymmetric transmitarray elements, that can realize high transmission and 360° phase control at once. Based on this metasurface, a high-gain ultrathin (3.3% of the wavelength) transmitarray antenna is created, with an aperture efficiency of 61.04% at the working frequency.

Color centers in diamond are considered one of the most promising platforms for electrically driven single-photon sources operating under ambient conditions. Efficient electrical excitation of these emitters is challenging, due to the extremely high activation energy of donors in diamond, which limits the density of free electrons. It may possible to overcome this doping limit, though. The authors predict and numerically demonstrate a superinjection effect in homojunction diamond $p-i-n$ diodes that allows a 3000-fold increase in free-electron density, thereby enhancing the brightness of single-photon electroluminescence by three orders of magnitude.

Passive targeted drug delivery to solid tumors is driven by the permeation of drug carriers into a tumor’s interstitial matrix, which is slow and ineffective. This study suggests an $active$ delivery strategy to enhance the permeation of therapeutic molecules into the interstitium. The authors show that, by gradually changing the solutes of the interstitial fluid, liposomes can respond to the change in the chemistry of the surrounding fluid, leading to enhanced transport and controlled release.

Implanting gold ions in silicon at concentrations far higher than the equilibrium solubility limit promotes sub-band-gap optical absorption, and thus has been explored in the context of Si-based near-infrared photodetectors. This absorption enhancement decreases after annealing, though, according to the structural relaxation of the material. In this article, the evolution of composition and structure of Au-hyperdoped Si after thermal relaxation is investigated in detail. The results provide crucial information for fabricating devices using Au-hyperdoped Si, and may also pertain to other hyperdoped impurities.

Raman spectroscopy sees much use as a precise, noninvasive technique for quantifying the stages of intercalation in various graphene- or graphite-based batteries, but for most of these systems the staging process is still unclear. This study shows that the correlation between carrier concentration and Raman-peak position in aluminum-ion batteries (AlBs) can be determined by employing nonadiabatic electron-phonon coupling theory based on first principles. Using this methodology, the authors identify the intercalation staging mechanism in an AlB. Such microscopic insight could be crucial in developing the next generation of energy-storage devices.

Although spin-exchange relaxation-free (SERF) comagnetometers have been widely used for fundamental physics tests and rotation sensing, their applications encounter considerable obstacles, due to the uncontrolled compensation magnetic field in such a system. The authors demonstrate a real-time closed-loop control method to stabilize the compensation point of a K-Rb-${}^{21}$Ne comagnetometer, to improve systematic stability, enlarge the linear measuring range, and suppress cross-talk error.

Imaging techniques using nitrogen-vacancy centers in diamond are appealing for characterizing two-dimensional devices and materials, but their compatibility with gated devices is largely unexplored. This work uses wide-field N-$V$ techniques to examine graphene field-effect transistors fabricated on the diamond’s surface, and highlights some of the challenges in this approach. Current densities in the device are mapped at different doping conditions, and substantial modulation of the electric field at the diamond surface is seen, suggesting a complex electrostatic response of the multilayered structure. Pathways to mitigate the invasiveness of the imaging technique are discussed.

All-optical helicity-dependent switching of ferromagnetic thin films using ultrafast laser pulses could be very interesting for data storage applications, but has been held back, because a large number of pulses are needed to write information. This study shows that full switching is obtained for a narrow window of laser fluence and pulse duration. It also shows that a few tens of pulses are enough to switch magnetization, and that even a $single$ $pulse$ can have a significant effect. These results will require a reassessment of the theoretical descriptions of optical spin control that have developed over the last decade.

$Magnetostriction$ is a key property used in devices such as sensors, actuators, and energy harvesters, particularly in spintronics and straintronics. Here the authors perform an exhaustive magnetization-reversal study of galfenol films ranging from 5 to 60 nm in thickness, to tie the magnetic properties to film structure. The thinnest film possesses the best characteristics for potential applications: highest magnetostriction coefficient, lowest coercive and saturation fields, and pronounced anisotropy. These results should inform strain-engineering solutions for voltage control of magnetism in extrinsic multiferroics that combine piezoelectric and magnetostrictive materials.

Engineering and understanding microwave resonators is essential for the efficient electron-spin control and readout in electron-spin-resonance (ESR) spectroscopy. This work, focused on superconducting planar microresonators, presents finite-element simulations modeling the spin-photon coupling rate, in quantitative agreement with experimental data. Three resonator designs are optimized to address spectroscopy applications in $\mu$m-thick samples, with one exhibiting notably good magnetic field homogeneity at microwave frequencies.

Quantum yield (QY) is an essential parameter for the optical characterization of light-emitting materials, for applications in $e.g.$ optoelectronics. This study points out a systematic bias present in standard QY methodology for samples with lower absorption. The bias manifests itself as an absorption-dependent underestimation of QY, and might lead to misinterpretation of QY dependences on parameters that are linked to absorption, such as the excitation wavelength. This work addresses that bias and offers a correction procedure, and thus could have dramatic impact on a great number and variety of applications.

Photochemical upconversion is a means to turn light that a solar cell cannot use into light that it can use, and thus could boost efficiency beyond the Shockley-Queisser limit. So far, though, it has not been possible to quantitatively predict the increased photocurrent. The authors use ray optics and exciton kinetics to compute a figure of merit that directly reflects solar-cell performance. They furthermore illustrate the applications of these methods to device design, by investigating the performance penalty imposed by Boltzmann statistics.

Quantum gates induced by geometric phases are important for fault-tolerant quantum computing, due to their built-in resilience to noise, but their application is held back due to the complexity of physically implementing them. Based on conventional two-body interaction, this study uses single- and composite-loop techniques to tackle quantum computation with non-Abelian geometric phases. Furthermore, experiments verify that the composite-loop scheme can indeed improve the noise resilience of the implemented gates.

Spin selectivity in double-stranded DNA (dsDNA) molecules is unambiguously linked to their intrinsic chirality. Nevertheless, how the chiral-induced spin selectivity (CISS) effect in dsDNA influences spin-dependent thermoelectricity needs to be understood well, to comprehend the relationship between spin and heat transport. To this end, the authors explore the spin Seebeck effect of a chiral dsDNA-based magnetic junction, and elucidate the underlying physics relevant to the CISS effect by means of linear-response theory and nonequilibrium Green’s functions. Their results provide valuable guidelines for engineering innovative spin-caloritronic devices based on chiral organic molecules.

Unlike a conventional diffraction grating, a space-time-periodic (STP) grating produces spatial diffraction orders, each of which is formed by an infinite set of temporal diffraction orders. STP gratings offer enhanced functionalities and exotic characteristics, such as asymmetric diffraction patterns, nonreciprocal and asymmetric transmission and reflection, and an enhanced diffraction efficiency. Here a rigorous, general theory of diffraction by STP gratings is formulated, and a particular practical application in wireless communication is proposed: an inventive multiple-access communication system, featuring full duplex operation and high data-transmission rate.

An efficient way of controlling frequency relies on the strong phase modulation of propagating waves. The authors design a fiber-loop circuit with two phase modulators (PMs) to remarkably enhance phase modulation. Effective gauge potentials can also be introduced by changing the modulation phases of the PMs; varying these potentials shifts the incident frequency spectrum as much as 50 GHz, and yields threefold band expansion. This work is sure to boost applications in spectrum management for optical communication and signal processing.

In the manipulation of the propagation of light, nonlinear optical metasurfaces have enabled a plethora of applications. A key issue in this area is how to arbitrarily control the vectorial polarization profile of nonlinear optical waves. This study demonstrates an ultrathin nonlinear photonic metasurface that can generate a vectorial polarization profile of the second-harmonic-generation beam, and therefore encodes a high-resolution grayscale image. This rich polarization information has significant potential for nonlinear optical image encryption and other security applications, such as anticounterfeiting.

The surface exciton polariton (SEP), a photon-electron-hole quasiparticle at the surface of a crystal, has potential for sensing applications. SEP sensors have hardly been explored, as they would operate at cryogenic temperatures, but recently a room-temperature device in the so-called K-R configuration was successfully demonstrated. In this paper the performance of an SEP sensor in a modified K-R configuration is theoretically investigated; it is predicted to show higher sensitivity in both bulk and surface sensing, compared to a conventional Au-based surface-plasmon-polariton device.

“Slow” light is important for accessing various optical phenomena, due to the strong interaction with matter, but its realization in a free-form plasmonic waveguide requires consideration. The authors determine the optimized geometry for ultimate light trapping in a plasmonic cavity, with a quality factor near the theoretical limit, at an unusually short length. This saturation of quality factor enables the design of light-trapping devices of extremely small footprint. The ratio of quality factor to footprint is found to be comparable to that of state-of-the-art photonic resonators.

Understanding the magnetic proximity effect in a van der Waals heterostructure made of monolayers of semiconductor and ferromagnet is of great interest, considering its potential applications in spin control. This article presents a general method to study the problem, combining $ab$ $initio$ and tight-binding calculations. The results reveal that a spin-polarized miniband with a localized state forms in the heterostructure, and can be tuned magnetically, mechanically, or electrically. The authors furthermore propose some programmable spintronic nanodevices with various functionalities, for applications in scalable quantum computation and high-density quantum circuits.

Temporal fluctuations in solar irradiance are important in energy harvesting and the study of ecological systems and climate. As solar irradiance is not stationary, analysis is often made in the frequency domain, rather than the time domain, and the power-law spectrum is often attributed to atmospheric effects, such as dust and clouds. The authors show that the power-law spectrum of solar irradiance appears even for a clear sky, due to modulation of the diurnal oscillations caused by varying daylight duration. Thus the power law depends on geographical location, and one must account for the multiplicative effect of atmospheric factors on the clear-sky power-law spectrum.

Asymmetric acoustic manipulations (AAMs) attract increasing attention for applications in integrated and architectural acoustics. However, because a planar phased array system has the same phase distribution on both sides, designing a monolayer device with asymmetric phase modulation is a real challenge. This work experimentally realizes four types of phased array prisms with different AAM effects arising from asymmetric phase distributions, created by different exit interfaces on either side of a prism. With broad bandwidth, multifunctional AAMs, passive structure, and easy fabrication, these prisms offer a fresh approach to creating $e.g.$ multifunctional acoustic rectifiers and diodes.

Injection locking replicates the frequency and phase of a driven oscillator in a target oscillator, with applications in electronic, mechanical, and optical systems, and lately in mesoscopic circuits. The technique requires the driven oscillator to be almost resonant with the target oscillator. This study uses a superconducting circuit to demonstrate injection locking in a well-controlled parametric microwave oscillator. The authors furthermore present another technique, $parametric$ $locking$, that removes the constraint on detuning between the oscillators. This should impact any device that should not or cannot be close to resonant driving, as in quantum information processing.

The authors study the photoresponse of a broadband THz-frequency detector with stepped quantum wells in the absorption layer. Due to the asymmetric distribution of the wave function of the second excited subband $E_2$, the escape probability for photoexcited electrons is strongly dependent on bias polarity, which is responsible for the different shapes of photocurrent spectra under positive and negative bias voltages. These results show that not only band structure but also the nature of the wave function itself plays a key role in designing these high-performance quantum-well photodetectors.

Lattice deformations can be used to trigger functional processes in matter, such as manipulation of ferroic ordering, or tuning of band-gap energy. A prerequisite for related applications is the ability to control such deformations on short time and length scales. This study demonstrates full spatiotemporal control of deformation in solids via tailored laser excitations. In particular, control of accompanying thermal deformations—often considered an unwanted background to the coherent dynamics—is presented. These results make photoacoustic actuation of “straintronic” devices seem quite promising.

For a number of applications such as precision force-sensing experiments, characterization of an optically levitated test mass is critical to understanding both expected signals and possible backgrounds. Often it is difficult to probe or retrieve optically trapped particles $in$ $situ$. Using electrostatic colevitation and a specially designed mechanical probe, optically trapped microspheres are weighed against gravity, then $individually$ retrieved and imaged with electron microscopy to measure their size. One observation: Particles grown via the Stöber process are likely much more porous than typically assumed, and thus have reduced density and sensitivity to gravitylike forces.

The field of nanoscale thermoelectric devices continues to grow, due to their capability for highly efficient energy harvesting. Here achieving band-pass electronic transmission is the primary objective to maximize efficiency at a given power. Embedding a resonant-tunneling structure within a flexible electronic Fabry-Perot cavity, the authors reveal a specific design guideline for the cavity’s barriers to realize band-pass transmission. They further show that this class of devices can be superior thermoelectric generators compared to existing proposals, in terms of achieving excellent power-efficiency trade-off at a high figure of merit.

The maximum conversion efficiencies of photovoltaic devices and performance-limiting factors of practical solar cells remain ambiguous, and thus the strict determination of current technological limits is of significant importance. This study develops an analytical scheme that allows the evaluation of realistic maximum-power conversion efficiencies of important inorganic and hybrid perovskite solar cells in conventional thin-film form. The authors show that, although efficiencies greater than 30% can be achieved for absorber layers with sharp absorption edges, many record-efficiency polycrystalline solar cells are limited by open-circuit voltage and fill-factor losses.

Materials with negative stiffness are essential to the development of extreme composites with unbounded damping and stiffness. However, in traditional materials negative stiffness typically leads to localized deformation bands. This study uses simulation and theoretical analysis to demonstrate that a metamaterial with three negative material indices can satisfy the strong ellipticity condition, which prevents the formation of local bands. Here Poisson’s ratio and stiffness can approach -∞, with an instability-triggered mechanism. The proposed metamaterial concept will have an impact on developing “extreme” composites.

Free-space global quantum communication networks of the future are expected to be robust under background radiation, including sunlight and moonlight, so as to be available at any time of day, but this is difficult to achieve. This study proposes exploiting the inherent resistance of the coherent detection used in continuous-variable quantum key distribution (CVQKD) against background light. The components of background noise in CVQKD are found to be extremely small, and to have a very subtle influence on system performance. These results suggest that CVQKD is a viable option for real-world quantum communication networks.

Surface acoustic waves (SAWs) are ubiquitous in sensor technology, their main benefits being sensitivity, amenability to remote control, and no need for an embedded energy source. The authors discuss magnetic sensors based on the resonant interaction between a surface acoustic wave and the magnetization of a thin ferromagnetic layer. This interaction can be switched on and off by tiny angular variation of the magnetic field, which could be useful to remotely monitor periodic motion, such as the rotation of a shaft. Using a simple model of magnetization dynamics accounting for the properties of the layer, they describe the salient properties of the SAW versus in-plane magnetic field.

In “rainbow trapping”, waves of different wavelengths are spatially separated, which is interesting for $e.g.$ optical buffers, multiplexers, and energy harvesting. It is difficult to realize such trapping without reflections, though. Unlike previous designs, this study proposes a hybrid metamaterial–transmission-line structure with gradient resonant channels to trap waves with a long oscillation lifetime, enhancing wave-structure interaction. The trapped energy can be absorbed perfectly via the inherent losses of the materials. This phenomenon could be exploited in designs for mechanical waves as well.

The recently discovered ferroelectricity in thin films of doped hafnia has aroused much interest, because of the films’ stable ferroelectric polarization at small thicknesses and compatibility with Si-based semiconductor technology. Through density-functional calculations of a model Ni/HfO${}_2$/Ni (001) heterostructure, this study predicts stable, sizable ferroelectric polarization in the ultrathin HfO${}_2$ layer, and a significant induced magnetoelectric effect at the Ni/HfO${}_2$ interfaces. These findings reveal the promising prospects of ferroelectric/ferromagnetic composite multiferroics based on hafnia for applications in microelectronics.

So-called acoustic tweezers are a powerful tool for contactless selective manipulation of individual objects at the microscale. Assembling $multiple$ objects with these tweezers, though, has remained elusive, yet is critical for applications such as tissue engineering and microrobotics. The authors show that, under some conditions, this problem can be overcome by trapping target objects with two synchronized vortex beams. In this situation the destructive interference between neighboring vortices forms an attractive path to bring captured objects together.

In photonics, circulators are three-port nonreciprocal components that allow unidirectional signal transmission from one port to another, in a cyclic rotating fashion. Magnetless circulators based on spatiotemporally modulated networks have recently been proposed as improvements over traditional designs, but these devices have their own shortcomings. The authors show that a suitable arrangement of $N$ nonlinear, time-varying unit cells with a gradient phase shift between their modulation signals can actually yield an effectively linear time-invariant nonreciprocal response. Such a circulator could enable $e.g.$ full-duplex communication, radar, and quantum computing.

The exchange-bias (EB) effect is extremely important technologically, as a fundamental ingredient in the functionality of read-and-write heads in magnetic storage devices. Traditional EB systems, mainly based on multilayers of ferro- and antiferromagnetic films, suffer from relatively low EB fields and entraining effects that limit lifetime. This study shows how to create EB using chirality effects based on the Dzyaloshinskii-Moriya interaction in a Dy${}_{20}$Co${}_{80}$/Ni${}_{80}$Fe${}_{20}$ bilayer. Beyond straightforward, simple fabrication, this system affords isothermal switching between different EB directions, plus extreme stability, without any observable entraining.

In secure communication, quantum key distribution (QKD) based on coherent states is well known for its simple implementation, but it suffers from loss-dependent attacks, and generally underestimates the final secure key rate. To address this problem, the authors present a six-coherent-state phase-encoding QKD protocol based on non-phase-randomized coherent states, with which secret key rates and transmission distance can be significantly improved. The security of the protocol is determined using a recently developed security-proof technique based on semidefinite programming. These results point to this protocol as a promising candidate for high-speed, provably secure QKD.

The recent demonstration of neuromorphic computing with spin-torque nano-oscillators points to substantial energy-saving in data analysis. However, the limited intrinsic memory of these devices (much less than a microsecond) limits their utility for analyzing temporal sequences. Here the authors overcome the short memory of a spin-torque oscillator by using a feedback loop with a delay of 1 $\mu$s, reducing the error rate during temporal-pattern classification by 99%. In addition, they determine optimal operating point of the oscillator, in terms of the current and magnetic field, to take advantage of this memory for recognition tasks.

Although quantum key distribution (QKD) promises information-theoretic security, several studies have been carried out showing how its security can be compromised by targeting the detectors in the system. This work demonstrates a measure to mitigate a special class of attack, the $aftergate$ $attack$. By exploiting delayed detection events, which are usually considered detrimental for QKD, the authors show how an eavesdropper mounting such an attack can be unveiled.

Investigations of thermal relaxation in mesoscopic devices at low temperatures are important for understanding their electronic and thermal transport. For metal films at low temperatures, thermal decoupling of electrons from phonons is considered to provide high thermal resistance for electrons, and limits thermal relaxation. This study shows that, for copper on silicon, by increasing film thickness and temperature the thermal boundary resistance between phonons in film and substrate is comparable to the electron-phonon thermal resistance. These results are helpful for understanding heat flow in mesoscopic systems, and for improving detectors based on thermal effects at low temperatures.

Reservoir computing (a framework for machine learning) implemented in physical systems has attracted much attention for real-time computing in $e.g.$ time-series modeling and prediction, for which a single spin-torque oscillator (STO) has been proposed. The performance of a single STO will be limited, though. To solve this issue, the authors study reservoir computing on an $array$ of STOs, showing numerically that the system’s performance can improve with more STOs, and can become remarkably better than for a standard neural-network model. Interestingly, performance is best near a boundary between synchronized and disordered states, suggesting an enhancement at the edge of chaos.

The storage and processing of information on the same device, known as $memcomputing$, is a next-generation approach to computing. To this end, the authors present a ferroelectric-insulator-ferroelectric memcapacitor, based on different polarization configurations in the two ferroelectric layers. This setup allows digital and analog memory applications augmented by binary logic operations based on capacitive switching. Multiple stable capacitive states offer many possibilities: simple memory cells with nondestructive reading and improved fatigue; neuromorphic or chaotic circuits; low power consumption; simplified architectures; and reduced manufacturing cost.

Potable water, while key to human survival, is relatively scarce. Seawater is plentiful, but its desalination by reverse osmosis requires membranes that pass water molecules, but reject ions and debris. The performance of current polymer membranes is limited by insufficiency in selective ion rejection, mechanical strength, thermal stability, resilience to cleaning agents, and electrical conductance. The authors’ atomistic computational design of a membrane with layers of graphite oxide, carbon nanotubes, and carbon-based fabric addresses all of those limitations and promises significant performance improvement at low cost, through its microscopic insight into the desalination process.

Nuclear spins near nitrogen-vacancy (N-$V$) centers in diamond are an important resource for quantum computing and sensing, because of their extremely long coherence times, but it is difficult to initialize them perfectly, due to the negative effect of laser irradiation on the nuclear polarization. This study uses chopped laser pulses to reduce that effect and initialize a quantum register with up to 96.8% polarization, using double-resonance dynamical nuclear polarization. The authors also determine that laser-induced depolarization is probable due to the N-$V$ charge effect, at low magnetic fields.

Tunneling anisotropic magnetoresistance (TAMR) is a useful functional property that can be exploited in electronic devices. While the phenomenon has been explored in traditional magnetic tunnel junctions with $dielectric$ barrier layers, this work predicts that sizable TAMR effects can also be observed in multiferroic tunnel junctions with $ferroelectric$ barriers, utilizing the strong spin-orbit coupling of $e.g.$ orthorhombic HfO${}_2$. The switchable ferroelectric polarization of such materials allows control of TAMR by an applied electric field—a promising functionality for emerging technologies.

On-chip manipulation and concentration of propagating surface plasmons is of interest for ultracompact integrated optical circuits. Recently many plasmonic energy-storage devices based on patterned graphene have been proposed, but these nanostructures introduce large edge-electron-scattering losses. This study uses plasmonic Tamm states to enhance and trap propagating plasmons in graphene terminals, where just two unit cells of graphene are enough to squeeze the electromagnetic field into an extremely small mode volume, with remarkably intensity enhancement. This approach should facilitate deep-subwavelength integrated graphene-based trappers, resonators, and biosensors.

The study of elastic wave propagation is relevant to many engineering applications, such as crashworthiness, nondestructive testing, and energy harvesting. The authors discuss how elastic energy can be guided along the surface of a three-dimensional mechanical structure, without any penetration to its core. The mechanical design here is inspired by the recent discovery of Weyl semimetals. Using full numerical simulations, they demonstrate robust wave propagation on the surface of a fuselagelike hollow structure, with no backscattering at corners or surface defects.

Analysis and optimization of the electromagnetic response of material constituents, such as atoms, molecules, or meta-atoms, is challenging. This study demonstrates that the most general dipolar response of $any$ small scatterer can be conceptually decomposed into a set of basic, fundamental polarization phenomena. The results enable immediate, complete characterization of the electromagnetic properties of an arbitrary material or metamaterial (in the linear regime). With this approach, if you can imagine a material system, you can know its behavior—and optimize it.

In solar-cell technology, band-gap grading has been introduced to enhance the efficiency of charge collection, by inducing drift motion of charge carriers. Characterizing such systems accurately is tricky, though, due to technical difficulty in clearly separating carrier diffusion from drift. This study uses wavelength-dependent lateral photocurrent in a generic device structure to simultaneously characterize the electrical qualities of both the light absorber and the back surface in a graded-band-gap solar cell. The technique is readily applicable to many types of photoactive devices, because it works well without any knowledge of the actual band-gap grading.

For encrypted communication, the “sending or not sending” protocol of twin-field quantum key distribution has the advantages of proven unconditional security against any coherent attack, and fault tolerance to large misalignment error. The existing security proof is based on infinite key length, however. Here the authors consider the complete finite-key effects for this protocol under the universally composable framework. Numerical simulation shows that in practice this scheme can exceed a secure distance of 500 km for a typical, finite number of pulses, even with large misalignment error. The results of this work could be directly applied in experiment.

Secure quantum communication over long distances is hindered by photon loss—a simple problem, complicated by the fact that quantum signals cannot be amplified without adding noise. The authors present on-chip quantum storage of light at telecommunication wavelength (around 1539 nm) for up to 10 $\mu$s, as an enabling technology for quantum repeater networks, which use distributed entanglement to overcome attenuation. High-fidelity quantum storage of light is demonstrated in a nanophotonic resonator fabricated in yttrium orthosilicate doped with erbium-167. Looking ahead, an improved resonator would yield a benchmark device on the way to scalable quantum communication networks.

Materials exhibiting the quantum anomalous Hall effect (QAHE), and thus dissipationless chiral edge states, would be terrific for ultralow-power spintronic devices. Realizing a room-temperature ferromagnetic semiconductor and room-temperature QAHE have both been ongoing grand challenges. This density-functional study reveals that bulk PdBr${}_3$ and monolayer PtBr${}_3$ are promising candidates to address both challenges at once. The results offer guidance for developing a material platform for ultraefficient topological spintronic devices.

Vortex beams of light carrying orbital angular momentum (OAM) show promise for a wide range of microwave applications. However, classical OAM wave generators in the rf band are generally hindered by narrow operating bandwidth, complicated feed structures, or high loss. This study presents a method for OAM wave generation using the concept of spatial transformation at microwave frequencies. The all-dielectric device implementation enables broadband operation, and the technique efficiently generates vortex waves carrying OAM modes, thus illustrating the practicality of employing spatial transformations to realize innovative microwave devices.

A paramount example among spin-related optical phenomena is the quantum spin Hall effect of light, by which the direction of propagating guided modes can be controlled by the spin of the source. For unidirectional excitation of guided waves, the focus has been only on dipolar sources, leaving aside higher-order multipoles. Exploiting the angular-spectrum representation, the authors present a general analytical treatment of near-field directionality beyond the dipole approximation. This enables a considerable advance toward full control of spin-dependent directionality at the nanoscale, and should be useful for engineering light-matter coupling in nanophotonics and quantum optics.