In the treatment of infertility, many methods for assisted oocyte activation have been proposed, yet each has drawbacks that could limit the efficiency of or introduce uncertainty to the activation. This study adopts a physical approach, using nanosecond pulsed electric fields (nsPEFs) to trigger repetitive calcium oscillations in mouse oocytes, which boosts the activation rate and improves embryo development. The underlying activation mechanism is unveiled via simulations based on a kinetic calcium-channel model. This effective, controllable technique may provide a better solution to male infertility.

As a complement to conventional high-field nuclear magnetic resonance (NMR), zero- and ultralow-field (ZULF) NMR has important applications in chemical analysis and fundamental physics. Breakthroughs in this field are being thwarted in part by intrinsic susceptibility to magnetic field noise. This study uses a two-channel atomic magnetometer to eliminate common-mode magnetic field noise. This approach could promote the engineering of a portable ZULF-NMR system, and provide a sensitive method to probe the frontiers of fundamental physics, such as exotic spin-dependent interactions and molecular chirality.

Mask-based pattern generation is a crucial step in microchip production, but there are many technical challenges in scaling current techniques down to nanometer resolution. Lithography using metastable atoms has been suggested as a cost-effective, less complex alternative to conventional techniques, including extreme ultraviolet (EUV) photolithography. This study presents a method based on binary holography that can be used to shape atom beams into arbitrary patterns. Simulations demonstrate the potential for state-of-the-art helium sources to produce patterns with nanometer resolution.

Waves with nonzero angular momentum have shown capability in boosting communication efficiency, particle manipulation, and creating source illusions, among many other possibilities. Generating such waves has led inevitably to scattering, which limits their performance, and nearly perfect generation of high-order angular momentum remains challenging. This study presents experimental realization of theoretically perfect creation of wavefronts with angular momentum via metasurfaces. This design strategy shows the way to practical, highly efficient metasurfaces for various functionalities, and to complete control of fields radiated by compact sources.

The $reciprocity$ $relations$, theorems describing the fundamental operation of solar-cell devices, are powerful tools for extracting a device’s internal properties that cannot be easily evaluated by direct measurements. This study generalizes the theorems to devices with arbitrary depletion regions, and carefully investigates the conditions under which the standard relations are no longer valid. This generalization extends our understanding of device behavior and enables accurate interpretation of the optoelectronic properties of unconventional devices, such as $p-i-n$-junction solar cells.

There are standard computational protocols for studying defects in $crystalline$ solids, with repeating lattice structures, but the study of extrinsic impurities in technologically relevant $amorphous$ oxides is drastically complicated by the structural disorder. This study presents a general methodology for addressing such systems, combining $ab$ $initio$ molecular dynamics, hybrid-functional calculations, and an electron-counting rule based on maximally localized Wannier functions. This approach yields, for example, the interesting finding that carbon and nitrogen impurities in amorphous alumina occur only in neutral and +1 charge states respectively.

Ferroelectric materials offer an important platform for realizing nonvolatile digital memory. Inspired by the recently discovered room-temperature ferroelectricity in $IV-VI$ semiconductor thin films, the authors study electron tunneling in such thin films and propose a type of random-access memory that they call the “in-plane ferroelectric tunnel junction”. Apart from nonvolatility and lower power consumption and faster writing than with traditional dynamic memories, their proposed system has the advantage of a faster and nondestructive reading process, thus overcoming the write-after-read problem of present-day ferroelectric memories.

Magnetic skyrmions are promising as highly scalable, stable, and individually addressable elementary units for next-generation memory and computing devices. However, their stability in the absence of external fields and evolution upon confinement are important open questions. The authors present the zero-field stabilization of room-temperature (RT) skyrmions in multilayered Ir/Fe($x$)/Co($y$)/Pt nanodots. They further show that skyrmion size can be modulated by a factor of four—down to 50 nm—by systematically varying dot size and magnetic interactions. This insight into creating and tailoring RT skyrmions in multilayer nanostructures is of immediate value to material and device design.

In the treatment of infertility, many methods for assisted oocyte activation have been proposed, yet each has drawbacks that could limit the efficiency of or introduce uncertainty to the activation. This study adopts a physical approach, using nanosecond pulsed electric fields (nsPEFs) to trigger repetitive calcium oscillations in mouse oocytes, which boosts the activation rate and improves embryo development. The underlying activation mechanism is unveiled via simulations based on a kinetic calcium-channel model. This effective, controllable technique may provide a better solution to male infertility.

The authors discuss the peculiar behavior of the critical current $I_c$ of a temperature-biased Josephson tunnel junction between different superconductors. Their theoretical exploration of coherent thermal transport in such a junction reveals the abrupt variation of $I_c$ as the electrodes forming the device reside at specific temperatures. This phenomenon could provide a basis for applications such as heat rectification or radiation sensing. Specifically, the authors suggest a single-photon detector operating in the dissipationless dynamical regime.

Strongly driven superconducting circuits are a versatile platform to implement a multitude of Hamiltonians for quantum computation, simulation, and sensing. Unfortunately, they can display complex dynamics with instabilities, so predicting and preventing these instabilities is crucial for applications. This paper proposes an inductively shunted transmon qubit as the elementary circuit optimized for strong parametric drives. Developing a numerical approach that avoids the limitations of perturbative analysis, the authors demonstrate that adding the inductive shunt significantly extends the range of pump powers in which the circuit is stable.

$Radiative$ $thermal$ $transistors$ are promising devices for controlling heat exchange at the nanoscale, providing a functional mechanism to manage thermal radiation in structures without contact. The authors propose a transistor exploiting the properties of a phase-change material such as VO${}_2$, and implementation under dynamical conditions. Studying the response of the device analytically, they show that the heat radiated by a source can be dynamically modulated and amplified. These results could have significant impact on managing radiative heat exchange in out-of-equilibrium systems.

As a complement to conventional high-field nuclear magnetic resonance (NMR), zero- and ultralow-field (ZULF) NMR has important applications in chemical analysis and fundamental physics. Breakthroughs in this field are being thwarted in part by intrinsic susceptibility to magnetic field noise. This study uses a two-channel atomic magnetometer to eliminate common-mode magnetic field noise. This approach could promote the engineering of a portable ZULF-NMR system, and provide a sensitive method to probe the frontiers of fundamental physics, such as exotic spin-dependent interactions and molecular chirality.

Multiferroics are important for controlling magnetization via electric field (rather than electric current), which is a main theme of spintronics research. However, single-phase multiferroic materials are not practical, due to their small magnetoelectric couplings. Here first-principles calculations predict giant magnetoelectric coupling at the interface between a single-phase multiferroic and a ferromagnet. The study provides microscopic physical insights into the control of magnetization by the electric field at a multiferroic interface, which should open a route to enhancing the efficiency of spintronic devices through low-power magnetic bit manipulation.

Non-Hermitian Hamiltonians give rise to exceptional points (EPs), which are invariably realized in identical resonantly coupled oscillators. The authors develop an electromechanical system featuring two vibrational modes that differ massively in spatial profile, frequency, and even the algebraic sign of the eigenvalue related to dissipation. An optomechanical-like interaction can parametrically couple these disparate modes, and with appropriate parameter selection a virtual EP emerges. These results show that non-Hermitian singularities can be accessed in a realistic physical system, thus accelerating the practical exploitation of their many unusual features.

Optically pumped atomic magnetometers achieve impressive sensitivity in measuring dc or rf magnetic fields, competing with state-of-the-art SQUID detectors. Noise cancellation in rf atomic magnetometers is important, and is often used for applications in unshielded environments. Common noise-rejection techniques rely on differences in the spatial or temporal properties of signal and noise. This study demonstrates noise suppression of linearly polarized noise based on the magnetometer’s polarization sensitivity. Directional phase sensitivity of the magnetometer is also demonstrated.

Mask-based pattern generation is a crucial step in microchip production, but there are many technical challenges in scaling current techniques down to nanometer resolution. Lithography using metastable atoms has been suggested as a cost-effective, less complex alternative to conventional techniques, including extreme ultraviolet (EUV) photolithography. This study presents a method based on binary holography that can be used to shape atom beams into arbitrary patterns. Simulations demonstrate the potential for state-of-the-art helium sources to produce patterns with nanometer resolution.

Quantum computers based on trapped ions have approached the point of executing interesting near-term algorithms, but scaling beyond tens of ion qubits will require either increasingly large experiments or integration of control technology. This article presents the design and implementation of a cryogenically compatible dc voltage source incorporated into the substrate beneath the electrodes of an ion trap. An analog switch enables the integrated circuit to reach noise levels similar to those in much larger commercial electronics. These results herald the application of similar designs in future experiments focused on creating scalable or deployable quantum systems.

Fabricating integrated waveguides using light is key to realizing miniaturized optical circuits and networks in a full three-dimensional volume, an achievement that presently is hampered by diffraction in the writing beams. The authors demonstrate the use of nondiffracting Bessel beams to write waveguides that support localized modes, so that no diffraction occurs during fabrication. Creations include single, double, and multiwaveguide splitters and couplers, along with electro-optic modulators, a family of components that can pave the way to densely packed passive optical devices for applications ranging from communication to classical and quantum optical computing.

Strain engineering in monolayer GaSe has been predicted to significantly tune its optical and electronic properties, which is promising for device applications, but assessing the applied strain is difficult. Here first-principles calculations unveil the influence of strain in the monolayer’s structure. The authors suggest the use of infrared and Raman vibrational spectroscopies to monitor the applied strain, and to estimate important mechanical and thermodynamic parameters. They find that monolayer GaSe is softer than other two-dimensional materials, yet able to sustain the strain values typically applied in flexible electronics.

The silicon vacancy in SiC has long-lived electronic spin states that can be used for quantum sensing, communication, and computation, but knowledge of the optical transitions used to measure and control these spin states has been lacking. This study uses high-resolution laser spectroscopy of individual silicon vacancies in 4$H$-SiC to isolate the spin-dependent optical transitions. Each defect has two narrow, nearly lifetime-limited optical transitions that correspond to different spin states, and result in quite different spin-polarization dynamics when driven. These results are promising for interfacing spins and photons, including efficient spin initialization and readout.

Predicting the “when” of material failure has a wide range of applications, but how far in advance that prediction can be made is not clear. This study follows the advancement of cracks under creep (constant load) conditions, showing that although intermittent crack propagation results in acoustic emission (AE), analysis of the AE-event data cannot be used for reliable predictions. An optical-speckle technique is then employed to measure the length scale of the fracture process, which $can$ be used to predict when a sample fails. These results should be of interest for $e.g.$ monitoring the stability of structures or machinery.

In quantum-dot solar cells and photodetectors, the Coulomb interaction between charge carriers plays a key role in the dissociation of photogenerated excitons, yet is rarely investigated quantitatively at the level of single charges. By means of high-resolution photocurrent spectroscopy, this work reports a Coulomb-induced giant photocurrent enhancement of positively charged trions in a single self-assembled quantum dot, and explores the hole-hole Coulomb interaction quantitatively. The results yield important understanding of energy conversion in solar cells, to improve internal quantum efficiency for energy harvesting.

Waves with nonzero angular momentum have shown capability in boosting communication efficiency, particle manipulation, and creating source illusions, among many other possibilities. Generating such waves has led inevitably to scattering, which limits their performance, and nearly perfect generation of high-order angular momentum remains challenging. This study presents experimental realization of theoretically perfect creation of wavefronts with angular momentum via metasurfaces. This design strategy shows the way to practical, highly efficient metasurfaces for various functionalities, and to complete control of fields radiated by compact sources.

While highly conductive, intermetallic compounds are typically poor thermoelectrics, due to a small Seebeck coefficient. $B20$-type CoSi, however, exhibits an anomalously large, negative Seebeck coefficient. First-principles calculations reveal the underlying reason to be the strong energy dependence of carrier lifetimes due to phonon scattering, which leads to effective energy filtering of holes and low-energy electrons. The phenomenon originates from a band structure with both heavy and massless fermions near the Fermi level, and is thus intrinsic to the material. This insight opens an avenue for discovering and designing high-performance thermoelectrics for energy recovery.

Acoustic focusing has been highlighted as a promising tool to handle biological cells in aqueous suspensions. Manipulation of submicrometer-sized objects such as pathogens and extracellular vesicles is challenging, though, due to the streaming induced by the acoustic field. The authors experimentally characterize the suppression of acoustic streaming in two miscible aqueous solutions of different density and speed of sound, confirming the correlation between evolution of acoustic streaming and the solute concentration field. The results support recent theoretical findings and indicate that acoustic manipulation of such small particles in inhomogeneous fluids is feasible.

Antiferromagnetic spintronics are hoped to enable ultrafast, high-density memory arrays working at picosecond time scales, but current methods to switch the antiferromagnetic order cost a lot of energy. This work proposes an ultrafast yet low-energy method for switching the Néel vector of an antiferromagnetic layer, by reconfiguration of the anisotropy energy when an electric field is applied. Theoretical analysis shows the possibility of switching the Néel vector of a 50-nm circular antiferromagnetic element with an energy dissipation of 250 aJ in less than 30 ps, without any current-induced torque.

Quantum simulation of the associative memory in Hopfield neural networks is an interesting crossover between quantum information and machine learning. The $quantum$ $stochastic$ $walk$ has been proposed for such simulations, yet not realized experimentally. The authors successfully map this scheme to a three-dimensional photonic chip, and achieve quantum stochastic walk evolution. A good match rate between the experimental quantum scheme and the expected result for a Hopfield neural net is attained. This proof of principle, combined with the scalability of low-loss integrated chips and straightforward Hamiltonian engineering, is a primary step toward photonic artificial intelligence.

Current-driven domain-wall motion has attracted intense attention, due to its rich physics and promising application in “racetrack” nonvolatile memory. However, the interplay between antiferromagnetic/ferromagnetic (AFM/FM) phase boundaries and current has remained elusive. To investigate, the authors use pulses of current to induce the formation of anisotropic FM domains in an AFM matrix. The phase boundaries can be reversibly oriented by the applied (writing) current, giving rise to the orthogonal alignment of phase boundaries and current, which could pave the way for multistage digital memory based on AFM/FM boundaries.

Although conventional acoustic absorbers work well for noise suppression, they also block fluid (air) flow and visible light, which is inconvenient if $e.g.$ one wants a quiet house near a busy road. This study uses sparsely arranged acoustic resonators that permit fluid flow yet effectively prevent sound propagation. Furthermore, when such a sparse absorber is combined with optical cloaking technology, it can be transparent to the eye. This approach enables innovative applications of multifunctional acoustic metamaterials.

Domain-wall memory devices are expected to replace today’s hard disk drives, but controlling the position and speed of a domain wall within a magnetic nanowire remains a challenge. The authors study domain-wall pinning and dynamics in constricted nanodevices, and find that pinning is very sensitive to the dimensions of the constriction. Stability of a domain wall for $more$ $than$ $a$ $year$ could be achieved in materials with in-plane anisotropy, such as Ni-Fe (permalloy). Archiving data requires even longer stability times, but this is a clear step in the right direction.

Ultrasonic waves are now recognized as a key element not only for signal processing in wireless communication (as in your mobile phone), but also for information transfer between different physical systems. Here one of the most promising platforms is the phononic crystal (PnC). However, the dispersion relation in conventional PnCs is based on passive, not active (adaptable), structures. To overcome this drawback, the authors propose a graphene-based PnC that enables acoustic transmission to be electrostatically tuned from transparency to opacity, in a noninvasive manner. This architecture can extend the utility of PnCs and acoustic phonons for applications.

Plasmonic lasers, active and nonlinear metasurfaces, and room-temperature plasmon-exciton-polariton strong coupling all rely on strongly scattering nanoparticle arrays coupled to dense active media. Quantitatively assessing the strength of feedback arising from nanoparticle scattering is crucial to the design rules for such systems. This study quantifies metrics in $k$-space and real space for the strength of feedback in plasmonic array lasers. Surprisingly, coupled-wave theory—the gold standard for diffractive systems with distributed feedback and gain—$fails$ quantitatively for these systems.

The heterojunction (HJ) band structures of graphene nanoribbons (GNRs), generated by modulating ribbon width, are currently considered for use in tunnel field-effect transistors and optoelectronic applications, but interfacial states often exist and remove the band offsets. Here first-principles calculations show that these states are induced by C—C bonds at the interfacial edge, and can be eliminated by “spare” coupling of wide and narrow GNR segments, which is predicted to be stable at room temperature. Knowing how best to construct GNR HJs naturally will promote the technologies based on them.

In the emerging field of $magnonics$, the aim is to use magnons (quantized spin waves) as information carriers in computing technology. For magnonic operation, it is important to control the direction of spin-wave propagation at will. Employing the concept of a phased array antenna, this study shows that it is possible to control the direction of spin waves in thin films without any waveguide, magnonic crystal, or other specifically designed geometry. Furthermore, the phased-array concept enables spin-wave focusing, leading to local confinement with enhanced amplitude. These results pave the way for future work in magnon-based spintronic applications.

Understanding the critical behavior of the alloy FePt is important for the development of heat-assisted magnetic recording (HAMR), since recording takes place in the critical region, but there are numerous conflicting reports of the critical exponents for this material. This study uses finite-size scaling in multiple critical variables to explore the cause of these conflicting results, and resolve which exponents should actually be used in the development of HAMR. The authors identify phase-transition crossover effects induced by two-ion anisotropy as the cause, and find that the Heisenberg critical exponents should be used in all cases involving a magnetic field.

The $reciprocity$ $relations$, theorems describing the fundamental operation of solar-cell devices, are powerful tools for extracting a device’s internal properties that cannot be easily evaluated by direct measurements. This study generalizes the theorems to devices with arbitrary depletion regions, and carefully investigates the conditions under which the standard relations are no longer valid. This generalization extends our understanding of device behavior and enables accurate interpretation of the optoelectronic properties of unconventional devices, such as $p-i-n$-junction solar cells.

The authors consider the guiding of surface acoustic waves by the local resonance band gap of a ZnO pillar-based phononic crystal, by introducing a hollow-cylindrical linear defect. Such a defect can be structurally engineered to yield narrow-band guiding modes near the middle of the gap. Moreover, the authors point out the frequency tunability of this waveguiding behavior, thanks to the acoustoelectric response of ZnO to ultraviolet illumination. This approach is attractive for designing reconfigurable, high-$Q$ surface-acoustic-wave devices, such as filters and duplexers for wireless communication.

$1T$-TaS${}_2$ is a layered material that has attracted much attention for device applications, due to its distinct resistivity states, but the related resistivity-switching effects are often weak and cannot be controlled by a gate voltage. The authors show that the usual switching in TaS${}_2$ due to phase transitions is accompanied by a surprisingly strong modulation of a heterojunction’s Schottky-barrier height, for further control via the gate voltage. These achievements—tenfold enhancement of resistivity switching plus the additional control by gating—will boost applications involving $e.g.$ ultrabroadband photodetection, negative differential conductance, and neuromorphic circuitry.

In considering the dielectric properties of materials at the nanoscale, measurements of negative permittivity at low frequencies often have been dismissed as experimental error. However, this study demonstrates a transition from positive to negative permittivity in a ferrofluid, due to the ordering of magnetic nanoparticles induced by a dc electric field, and the formation of conduction paths. This effect may find applications in soft-matter research on particle assembly, as well as in sensors based on such a change in permittivity.

Introducing defects into photonic or phononic crystals leads to the emergence of defect modes within formerly forbidden bands, and has found application in functional devices. The authors experimentally observe dipolelike antisymmetric localization of acoustic waves in a defect-embedded periodic duct. This phenomenon is due to non-Bragg resonances, unlike symmetric localization by Bragg resonances. From the viewpoint of a multimode-interaction mechanism, the localization induced by high-order modes exhibits unusual antisymmetry. This insight will have an impact on the engineering of devices, such as noncontact tweezers, spatial wave modulators, and detectors.

Seeing atoms and electrons in motion via ultrafast electron diffraction or microscopy requires electron pulses a few femtoseconds in duration, or shorter. The authors demonstrate terahertz-based electron compression with a simple planar mirror, yielding pulse lengths of less than 30 fs. Furthermore, this approach could potentially deliver isolated $attosecond$ pulses of electrons.

The interplay between the structural, magnetic, and charge properties of multiferroic materials allows functionalities that could enable the next generation of fast, low-power data storage and logic devices. Unfortunately, multiferroics are typically comprised of many domains, which diminishes their useful properties; achieving control over domains spanning all three aspects is key. Using state-of-the-art techniques, the authors are able to engineer the domain structure of a BiFeO${}_3$ film to macroscopically bias the growth of a chosen magnetic, ferroelectric, and structural domain—a critical step toward deterministic, robust device operation.

The simultaneous readout of photons emitted due to Cherenkov and scintillation processes is a promising feature of SiO${}_2$:Ce optical fibers, suggesting applications in innovative radiation sensors for calorimetry in high-energy physics, as well as radiation monitoring in medicine, security, and industrial controls. This study presents a detailed characterization of the scintillation properties of doped silica fibers exposed to high-energy electrons, and demonstrates the feasibility of a simultaneous dual-readout approach.

A strain gradient $\nabla S$ deeply influences the properties of a disordered ferroelectric, but the exact mechanism whereby $\nabla S$ controls the disorder dynamics, especially in bulk material, is still largely unknown. The authors create a macroscopic strain gradient in KTa${}_{1-x}$Nb${}_x$O${}_3$ single crystals by coupling a composition gradient and 180° domains, and field-dependent electro-optic effects reveal the $\nabla S$-controlled disorder dynamics and local dipole behaviors. This work provides a basis for understanding and harnessing the potential of disordered ferroelectrics, and the $\nabla S$ design method is valuable for developing functional materials and devices.

Semitransparent ceramics increasingly attract attention for various applications, due to unique combinations of mechanical and optical properties, chemical durability, and biocompatibility. The authors observe accurate delamination of high-aspect-ratio layers from surfaces of yttria-stabilized zirconia, upon exposure to a femtosecond laser. The phenomenon is explained by an intricate interplay between surface ablation, surface-layer breakdown, and self-focusing of the beam fraction penetrating toward the bulk of the material. This effect presents opportunities for slicing hard, brittle, semitransparent materials into arbitrary shapes for applications.

The spin Hall effect (SHE) may be used to control the magnetization state of a nanomagnet by exerting spin torque. In a practical system, both spin Hall and interfacial contributions to the torque coexist, making quantitative analysis of SHE difficult. This study uses the dependence of SHE on the thickness of the heavy-metal layer in heterostructures with different combinations of W, Pt, and Co-Fe-B to distinguish the various torque contributions. Both significant interfacial effects and accumulation of spin current in a sandwiched ferromagnet are observed, improving prospects for applications in spintronics.

There are standard computational protocols for studying defects in $crystalline$ solids, with repeating lattice structures, but the study of extrinsic impurities in technologically relevant $amorphous$ oxides is drastically complicated by the structural disorder. This study presents a general methodology for addressing such systems, combining $ab$ $initio$ molecular dynamics, hybrid-functional calculations, and an electron-counting rule based on maximally localized Wannier functions. This approach yields, for example, the interesting finding that carbon and nitrogen impurities in amorphous alumina occur only in neutral and +1 charge states respectively.

Modeling supercurrent density and magnetic field penetration in a superconducting wire is important in the design of quantum computing circuits and kinetic-inductance detectors. Standard tools are based on London’s local electrodynamics, and do not account for the nonlocality arising from Cooper-pair quantum coherence. The authors develop an exact numerical method for computing the electrodynamic properties of superconducting wires that takes full account of nonlocality. They find that wires in the nonlocal regime ($i.e.$ with large mean free path) have supercurrent density flowing away from the surface, impacting device parameters and their sensitivity to flux noise.

Thermal transport at the nanoscale is much slower than plain diffusive transport in the bulk, and has only lately become experimentally accessible. This limits the informed design of nanosystems for $e.g.$ heat management in nanoelectronics, or killing cancer cells using laser-heated nanoparticles. Using ultrafast pulses of short-wavelength light, the authors map the continuous transition from isolated heat sources that cool slowly to closely spaced heat sources that can cool quickly. This improvement in heat transfer simply by tuning geometry carries important implications for the design of functional nanostructures.

$Thermal$ $diodes$ are attractive for applications in harvesting waste heat and phonon manipulation, but a sufficient rectification ratio has yet to be achieved. Here nonequilibrium simulations of “molecular bridges” covalently bonding a carbon nanotube to gold reveal asymmetric heat flux across these nanostructures, resulting in significant thermal rectification. This is due the mismatch of vibrational modes between neighboring sulfur and carbon atoms. The results show a path toward $e.g.$ high-efficiency chip cooling, and molecular phononics.

Cellulose nanofibrils are attractive as an excellent, sustainable material for thermal insulation, when assembled in an aerogel (“solid air”, a gel in which gas has replaced the liquid). To advance beyond trial and error in developing these aerogels, it is important to understand the several contributions to their thermal conductivity $\kappa$, as well as $\kappa$ for an individual nanofibril. Here an open-cell model is used to obtain this information by fitting the measured $\kappa$ of such an aerogel, under atmospheric and vacuum conditions. The results can be used to predict the optimum solid volume fraction and performance of modified fibril networks.

Surface defects in colloidal semiconductor quantum dots are responsible for many undesirable effects. Plasmonic effects in nearby metallic nanostructures can offer some partial remedy, by enhancing radiative decay rates. This work reveals that when metallic nanostructures are placed in the vicinity of an Au/Si Schottky junction and a Si/aluminum oxide charge barrier, unusual plasmonic effects offer not only enhanced spontaneous emission, but also suppressed impact of surface defects. This material platform for superplasmonic processes can render these quantum dots truly bright emitters that are ultrafast and immune to environmental noise, for applications in photonics.

The authors show that a photonic structure involving a phase-change material incorporated into an asymmetric resonant cavity can display unidirectional transmittance, for a broad range of input light intensity. This effect can be used for high-power, magnet-free optical isolation. In the example here, VO${}_2$ undergoes a dielectric-to-metal transition when heated by light, becoming highly reflective above its transition point. Crucially, the critical intensities for forward and backward incidence differ by orders of magnitude. Thus phase-change materials, as opposed to nonlinear optical materials, allow better isolation over a much broader intensity range.

Type-II superlattices (T2SLs) of narrow-band-gap semiconductors hold great promise for mid- and long-wavelength infrared (IR) detectors. To improve photodiodes based on these superlattices, understanding of minority-carrier transport along the growth direction is required, yet still lacking. Here researchers use electron-beam-induced current and time-resolved microwave reflection to investigate the key transport properties in a midinfrared T2SL photodetector, presenting a comprehensive study of carrier dynamics and the effect of surface recombination in the structure. This work will help to optimize the design and growth of T2SL structures, for better IR photodetectors.

Ferroelectric materials offer an important platform for realizing nonvolatile digital memory. Inspired by the recently discovered room-temperature ferroelectricity in $IV-VI$ semiconductor thin films, the authors study electron tunneling in such thin films and propose a type of random-access memory that they call the “in-plane ferroelectric tunnel junction”. Apart from nonvolatility and lower power consumption and faster writing than with traditional dynamic memories, their proposed system has the advantage of a faster and nondestructive reading process, thus overcoming the write-after-read problem of present-day ferroelectric memories.

The low-frequency signal from ultralow-field magnetic resonance imaging (ULF-MRI) setups leads to the use of high-sensitivity noninductive detectors. The simplicity and robustness of optical atomic magnetometers (OAMs) make them excellent candidates to this end, but they suffer from magnetic field inhomogeneities. Appropriate magnetic “dressing” of precessing atomic spins can overcome this limitation: Applying a dressing field with opportune inhomogeneity compensates the atomic line broadening induced by the MRI gradient, restoring OAM sensitivity and enabling $in$ $situ$ detection of ULF-MRI signals by optical magnetometry.

The demonstration of enhanced nonlinear optical effects at terahertz frequencies is important for the development of reconfigurable planar THz devices. However, high input intensities are usually required to efficiently excite nonlinear optical effects in ultrathin structures—an acute problem here, as high-power THz sources are not available. This work presents a class of ultrathin, nonlinear, hybrid planar THz devices based on graphene-covered plasmonic gratings. These devices exhibit very large nonlinear responses to low input power, and are expected to be useful for THz nonlinear spectroscopy, noninvasive subwavelength imaging, and communication applications.

Magnetic skyrmions are topologically protected spin textures that are promising as carriers of information for computing and storage beyond CMOS-based systems. An electrostatic approach to creating a skyrmion can be more energy efficient than those based on a driving current. In this work, formation and dissolution of Néel skyrmions are examined theoretically via electrical modulation of magnetic anisotropy, in both ferromagnetic and antiferromagnetic structures. Simulations clearly illustrate the feasibility of the mechanism, enabling local control for versatility in applications. Also noteworthy is that the dynamical processes involved are much faster in antiferromagnetic materials.

Lamb waves and surface acoustic waves (SAWs) are of great interest for frontier technologies, including structural health monitoring, energy harvesting, sensors, and acoustic tweezers. This work presents elastic phased diffraction gratings, passive structures for controlling guided ultrasonic waves in solids. Such a grating adopts an array of interchangeable superstrates to modulate wave dispersion, and thus the diffraction wave field. Experiments demonstrate the ability to modulate antisymmetric and symmetric Lamb waves at 100 kHz, and Rayleigh SAWs at 100 MHz. These gratings are comparatively easy to fabricate, interchangeable, and theoretically feasible for all Rayleigh-Lamb modes.

A substantial fraction of our energy consumption is due simply to maintaining a desired constant temperature. Based on an existing approach for energy-free temperature maintenance, the authors develop a temperature-trapping theory for coupled thermoelectric fields and propose a “negative energy thermostat”, which can convert ambient thermal energy into electricity without loss of its thermostatic ability. They also design a thermoelectric thermostatic cloak, which could be interesting for designing energy-saving buildings, vehicles, and spacecraft.

Understanding the effects of surface-related strain is important in developing technologies based on nanoscale forms of silicon. This investigation of physical phenomena related to surface oxidation of free-standing silicon nanocrystals (Si-NCs) reveals strongly surface-dependent lattice strain. The time evolution of natural oxidation in air shows a clear correlation between oxide formation and the appearance of compressive strain in a Si-NC’s core. Strain is negligible in H-terminated nanocrystals, but increases continuously as oxidation progresses. These results clarify contradicting results in the literature.

In terahertz science and technology, developing radiation sources is extremely important. Progress here is held back because traditional electronic or photonic technologies are not efficient enough in the THz frequency regime. This study presents an electronic-spintronic THz emitter (E-STE), which combines the principles of spintronics and vacuum electronics and emits continuous, monochromatic THz radiation from a compact structure. This E-STE can cover the entire THz frequency range at relatively low voltage and current, providing relatively high radiation power density and working efficiency. Such devices may have far-reaching impact on the development of chip-integrated THz sources.

A nearby organic molecule such as benzene induces colossal conductivity modulation in phosphorene, an analogue of graphene with great promise for applications in nanoelectronics and spintronics. Here first-principles calculations are employed to analyze the electronic behavior of a phosphorene nanoribbon under various conditions, and in the presence of a gate voltage. The observed conductivity modulation is highly robust against external influences and scales with size, and thus should be useful in energy-efficient field-effect transistors and other nanodevices for switching or sensing.

While “spin Hall” heavy metals like Pt have been widely studied for generating spin-orbit torque (SOT) in spintronics, alternatives are also sought. Rare-earth metals are thought to have potential here, but their efficiency remains uncertain. Thus the authors use an innovative technique to measure the effective fields and SOT switching efficiency generated in rare-earth-bearing magnetic multilayers. They find significant enhancement of both dampinglike and fieldlike torques, for enhanced switching efficiency with increasing thickness of a Tb layer, plus a strong angular dependence of both torques. This enhancement indicates that rare-earth metals are promising for spin-orbitronic devices.

$Retention$ is of critical concern when a memory device stores data. While ferroelectric diodes continue to emerge as a promising memory technology, their retention behavior and underlying physics have not been well investigated. This paper proposes a theoretical model for retention loss in ferroelectric diodes, combining polarization relaxation kinetics and polarization-controlled Schottky emission. The model describes well the behavior of metal/BiFeO${}_3$/La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$ diodes, and explains the dependences of retention-loss rate on BiFeO${}_3$ thickness and electrode screening. These findings should help to guide the development of reliable ferroelectric memory devices.

Ferrites could see extensive use in spintronics, microwave communication technology, and biomedical applications, because of their excellent magnetic properties and high resistivity. At the nanoscale, though, surface disorder significantly decreases the magnetization of these beach-ball structures. In this article, the magnetization of Fe${}_3$O${}_4$ hollow nanospheres is increased substantially by Zn doping, up to a maximum. The same doping also increases the resistivity of the hollow nanospheres, further enabling them for applications.

The energy loss $E_{loss}$ seen in organic photovoltaics sets a fundamental limit to their open-circuit voltage, and hence power conversion efficiency. This study compares molecular structures of fullerene and nonfullerene acceptors and quantifies the relationship between $E_{loss}$, exciton binding energy, and intra- and intermolecular electron-phonon couplings. Molecular design strategies derived from this analysis provide elementary approaches to reduce $E_{loss}$ yet also achieve efficient exciton dissociation. While addressing the source of $E_{loss}$ is particularly relevant for solar cells, it has wide-ranging implications for any system with an organic heterojunction.

The field effect, as in a field-effect transistor (FET), allows control of the switching current in a metallic superconductor, without affecting the critical temperature or normal-state resistance. Here magnetotransport experiments on Ti-based superconducting FETs reveal several physical insights: The phenomenon occurs at the sample surface, the field effect causes a transition from ballistic to tunnel-like behavior, and a mixed superconducting–normal-metal state is possible at high gate voltages. Such a device could be the cornerstone of easily fabricated monolithic architectures for classical or quantum computing, and a host of other applications in (opto)electronics.

Tunable phononic metamaterials, which change their response to elastic wave propagation upon actuation, have important applications in cloaking, wave guiding, active noise reduction, superlensing, and acoustic mirrors. Most systems either require substantial morphological changes for actuation, or exhibit only high-frequency band gaps. The authors present a framework for developing tunable phononic metamaterials with low-frequency band gaps, using negative-stiffness inclusions in an elastic matrix, and demonstrate two implementations of this concept. The designs are robust and achieve substantial band-gap manipulation, without elastic instability or significant structural changes.

Recent advances in nanotechnology allow very accurate manipulation of small collections of quantum dots (QDs), an important step in semiconductor-based quantum computation. As the number of QDs in a device grows, it becomes increasingly useful to control the system with as few voltage changes as possible. The authors offer a protocol, “CODA”, both to efficiently identify sparse sets of voltage changes for controlling a quantum system, and to introduce a metric for comparing device designs. As a method for efficiently controlling $and$ designing a device, CODA will impact engineering solutions to the extension of semiconductor-based quantum computation.

Magnetic skyrmions are promising as highly scalable, stable, and individually addressable elementary units for next-generation memory and computing devices. However, their stability in the absence of external fields and evolution upon confinement are important open questions. The authors present the zero-field stabilization of room-temperature (RT) skyrmions in multilayered Ir/Fe($x$)/Co($y$)/Pt nanodots. They further show that skyrmion size can be modulated by a factor of four—down to 50 nm—by systematically varying dot size and magnetic interactions. This insight into creating and tailoring RT skyrmions in multilayer nanostructures is of immediate value to material and device design.

“Magic wavelength” traps are used in state-of-the-art optical atomic lattice clocks, to hold atoms without perturbing their delicate internal states. Such traps have only been possible for specific atomic species, or have required trapping light that is difficult to produce. This work reveals that, by augmenting a standard optical trap with a weak additional light field of a different color, it is possible to achieve the “magic” configuration through the interaction of the two light sources. This opens the field of optical control and precision measurement to a much wider range of atoms, and thus could extend cold-atom sources to a much wider range of applications.

A broadband metamaterial lens that does not suffer from chromatic aberration is of particular interest for optical applications at midinfrared frequencies. Due to the intrinsic dispersion of the building blocks, though, such a broadband achromatic metalens has remained elusive. The authors use the Pancharatnam-Berry phase and propagation phase to control the wave front of light, from which the chromatic aberration can be eliminated effectively. This result points the way to practical midinfrared devices, $e.g.$ for communication technology.

Three-dimensional (3D) waveguide cavities with high coherence are used extensively in quantum information technologies. In particular, they provide a well-engineered electromagnetic environment for the readout of superconducting qubits, and thereby improve coherence times. Application of 3D microwave cavities to optomechanical experiments is now of interest. This study describes design guidelines for and demonstrates high cooperativity in an optomechanical system based on a 3D cavity. The high dynamic range and plug-and-play nature of this platform could enable experiments in unexplored regimes of optomechanical interaction.

Recent improvements in dynamic scanning-probe techniques allow enhanced spatial resolution, as better tips retain their integrity in the face of repulsive interactions, but the ultimate $temporal$ resolution at high spatial resolution remains an open question. The authors’ computational analysis of temporal resolution in atomic force microscopy using atomically engineered tips reveals that nonmetal-terminated tips ($e.g.$ O-terminated Cu) work well for enhanced temporal resolution up to video-rate imaging speeds, at pm-range spatial resolution. On the contrary, the maximum spatial resolution with low-stiffness tips ($e.g.$ CO-terminated) deteriorates with increasing imaging speed.

Fullerenes and their derivatives are well-known electron-transport materials for organic (opto)electronic devices, but the extent to which they can transport holes is strongly disputed. To resolve the matter, this study quantifies bulk hole transport in a fullerene derivative. Remarkably, hole transport here is trap-free, with a high mobility equal to that for electrons. This demonstration that fullerenes can exhibit balanced, bipolar charge transport, which is unusual for organic semiconductors, is interesting for application in organic photovoltaics and ambipolar transistors.

In quantum secure communication, using high-dimensional (HD) resources is an important approach to increasing the secure key rate, especially in quantum key distribution (QKD). However, high-dimensional quantum key distribution (HDQKD) has remained far from practical applications, due to the technical difficulties in HD state preparation, transmission, and measurement. This study solves the first two issues by realizing a state-mapping approach between different degrees of freedom of a photon: orbital angular momentum and spin. This noninterferometric state-mapping approach offers immediate impact on practical HDQKD systems.

Integration of strongly coupled quantum-dot–cavity systems into silicon photonics is important for large-scale quantum photonic integrated circuits, to implement photon-photon interactions in optical quantum information processing, but technical difficulties have thwarted progress here. This study uses a transfer-printing method, which is applicable regardless of the materials used, to solve the problem. This approach is expected to have real impact on the production of quantum photonic integrated circuits.

In industrial semiconductor fabrication, it is important to determine the quality of an entire wafer, not just a small area. Energy-resolved x-ray diffraction topography is important for visualizing the tilting distribution of a crystal lattice over a large area, yet in spite of technical progress it has been difficult to analyze a whole wafer qualitatively. By analyzing diffracted-angle positions of a two-dimensional detector under one-dimensional polychromatic (white) x-ray illumination, the authors visualize the periodic lattice-tilting structure over a large area on a substrate. This approach will have an impact on improving manufacturing processes for single-crystal wafers.

The Si-$V^{-}$ center in diamond has emerged as an excellent single-photon source with outstanding properties for photonics and quantum information processing. Here surface-treatment techniques enable researchers to obtain single Si-$V^{-}$ centers with bulklike spectral properties in nanodiamonds. The authors resolve the fine structure of individual Si-$V^{-}$ centers in low-strain nanodiamonds, formulate an analytical strain model for this center, and experimentally find the strain coefficients. They also explore the potential for bottom-up assembly of complex quantum systems, using cantilever nanomanipulation to achieve efficient positioning, rotation, and declustering of nanodiamonds.

Metamaterials continue to change the way we think about manipulating light. For example, although conventional diffraction gratings are well-studied and widely used, only a few configurations (as in blazed gratings) allow efficient, accurate control of diffraction. The authors show that to construct arbitrary diffraction patterns, each propagating diffraction order requires $exactly$ two degrees of freedom, represented by structured “wires” (in a metamaterial, say). This important result solves an old problem of power management by a diffraction grating. From the physical point of view, it is achieved by accurately adjusting the optical near field scattered from a grating.

As usual, simpler is better: This study uses optoelectronics, rather than optomechanics, to directly couple optical and microwave cavities, addressing the issue of low-temperature operation of the traditional tripartite system. The approach here allows one to generate and control the quantum entanglement of output cavity modes at $room$ $temperature$, which is important for practicality in applications such as quantum sensing (think quantum radar).