Current thermometry techniques lack the spatial resolution needed to map temperature inside modern transistors, leaving it difficult to understand and mitigate the performance-limiting effects of self-heating. As a step toward solving this problem, the authors determine the temperature dependence of the bulk plasmon energy in silicon nanoparticles. With this knowledge, individual silicon nanoparticles can now be used as miniature thermometers, and a silicon transistor can be seen as a collection of them. If they can be queried, we will be able to map temperature inside the transistor with nanoscale spatial resolution.

The unusual transient negative differential capacitance (NC) of a resistor–ferroelectric (FE) capacitor circuit is of keen interest for use in next-generation transistors, but a clear physical picture and theoretical framework are needed to interpret experiments. The authors show both numerically and analytically that transient NC comes from the mismatch of free charge and polarization in the capacitor during polarization switching, and a procedure to experimentally determine the viscosity coefficient in Landau theory is provided. These results should have real impact on metrology and device physics involving the NC effect.

Focusing is an ubiquitous mode of transforming waves—even within the human body, as it turns out. Recently, high-frame-rate ultrasound has enabled the observation of shear shock waves in soft solids, such as the brain. The present study uses that technique to further show that shear waves emitted by a cylindrical source into tissue-mimicking gelatin can be focused, and that they form a shock at the focus. This could explain why traumatic brain injuries, such as diffuse axonal injury, occur deep inside the organ, rather than near the skull.

The development of high-density voltage-torque magnetoresistive random-access memory (MRAM) looks to voltage-induced magnetization switching (bit writing) without a biasing magnetic field—but how? In mainstream technology based on magnetic tunnel junctions (MTJs) with perpendicular magnetization, voltage-induced switching is not possible at zero bias. However, the authors show that switching at zero bias should be possible, in an MTJ with elliptical cross section and a conically magnetized free layer. Their results provide a practical guide to designing bias-field-free voltage-controlled MRAM.

Microwave readout of charge states and spin states is important for quantum information science, but is difficult to scale to a large number of qubits, due to cost and the size of the components required to faithfully transmit the signal from room temperature to mK qubit temperatures. In this study, a voltage-biased semiconductor double quantum dot is used to generate microwave photons, yielding a cryogenic on-chip source for charge-state readout. Surprisingly, the emission properties of the double dot are affected by other qubits placed in the same microwave cavity. These results should facilitate the development of a large quantum processor to realize true quantum supremacy.

Anisotropic thermal conductivity would be useful for thermal management of electronic devices, but few naturally occurring materials show much anisotropy between the in-plane directions of a thin film. In this study, simulations reveal that phonon scattering from features engineered by modulating thickness leads to significant anisotropy in heat flow. This approach can tune the anisotropy ratio across an order of magnitude without sacrificing the mechanical robustness of the continuous film, and the proposed structures are well within the capabilities of available silicon-based nanofabrication techniques, with no need for exotic or expensive materials.

Developing cache memory cells is of great interest for next-generation integrated circuits, where both high speed and low energy consumption will be essential. This study utilizes the giant spin Hall effect to demonstrate that, by successfully modifying the interfaces in a magnetic tunnel junction to improve its magnetic properties, a significant reduction of switching current can be achieved. The experiments also demonstrate nanosecond-scale pulse switching with high reliability, due to beneficial assistance from fieldlike torque. With clear potential for further optimization, these developments show great promise for high-performance memory technology.

Controlling the flow of mechanical energy in elastic structures has been a long-standing challenge in many areas of engineering, particularly those dealing with vibrations and structure-borne noise. In light of recent studies on the quantum valley Hall effect in electronic systems, the authors show that $acoustic$ topological edge states can be achieved, at a boundary or interface between two elastic waveguides having broken space-inversion symmetry. The topological properties of such waveguides can be tuned by applying a controlled strain field, while the resulting edge states show a relatively high insensitivity to defects.

Quicker is not always better. Standard microfluidic techniques efficiently produce microdroplets at high rates in applications, from materials science to bioengineering, but are not well suited to the $low$ rates that may be needed $e.g.$ when biological surfactants are used to stabilize the droplets. This study presents a quasistatic method to create aqueous microdroplets in oil, based on the periodic withdrawal of a glass capillary full of aqueous phase across an air/oil interface. These results enable an easily implemented and operated, robust, versatile, and inexpensive technique to produce microdroplets when slower is better.

The flux and spectrum of antineutrinos emitted by a nuclear reactor can provide timely estimation of the quantity of fissile material in the core, which is valuable for campaigns to irradiate surplus plutonium, particularly as mixed-oxide (MOX) fuel, to render it undesirable for weaponization. This study shows how to track fissile inventories in MOX cores precisely and robustly, even with plain antineutrino detectors that lack spectral sensitivity. Also, this rate-based tracking method can see past changes to total reactor power, which might be used deliberately to mask diversion of Pu. These results provide a means of verification, to satisfy cutoff treaties and agreements.

Surface acoustic waves have become a key phononic technology to probe and control a wide range of excitations in condensed matter. Here wide-passband transducers establish stable phase locking between a pulsed source and electrically generated “nanoquakes” on a chip. An inherently stable, yet tunable, phase locking between a free-running pulsed laser and the sound wave is used for time-domain optomechanical spectroscopy of a dynamically strained quantum dot. This technique can be directly adapted to any pulsed source of electromagnetic radiation, or even particles, to probe the acoustically driven dynamics of structural, electronic, optical, or magnetic excitations.

Current thermometry techniques lack the spatial resolution needed to map temperature inside modern transistors, leaving it difficult to understand and mitigate the performance-limiting effects of self-heating. As a step toward solving this problem, the authors determine the temperature dependence of the bulk plasmon energy in silicon nanoparticles. With this knowledge, individual silicon nanoparticles can now be used as miniature thermometers, and a silicon transistor can be seen as a collection of them. If they can be queried, we will be able to map temperature inside the transistor with nanoscale spatial resolution.

The ability to manipulate magnetism via electric fields, to achieve an emergent multiferroic response at the interface of a ferromagnetic material with a ferroelectric one, carries enormous potential for nanoscale devices. Here strong electric-field control of magnetism is reported for a heterostructure of cobalt on lead magnesium niobate–lead titanate. A strong, direct, magnon-driven magnetoelectric effect at the interface is resolved and quantified. These results are promising for technology based on engineered artificial multiferroics.

Photons are massless, yet can exert force on small particles. This $radiation$ $pressure$, though discussed by Kepler, still needs investigation for modern systems. This study reveals that the optical force exerted on a parity-time-symmetric bilayer with balanced gain and loss can be $asymmetric$, depending on the direction of impinging light. The authors explain the direct physical link of the optical pulling/pushing force to the optical characteristics embedded in the bilayer, which has a non-Hermitian Hamiltonian. This finding suggests taking advantage of the optically generated asymmetric force to tailor flexural vibrations, for contactless probing of mechanical deformations.

The interfacial Dzyaloshinskii-Moriya interaction (iDMI) plays a key role in stabilizing magnetic skyrmions, tiny topological hedgehogs that could provide a basis for spintronics. Investigating the iDMI as a function of layer thicknesses in technologically important heterostructures, the authors observe that the iDMI constant $D$ varies linearly with the inverse of Co-Fe-B thickness, demonstrating its purely interfacial origin, whereas its relationship to Ta thickness is complex, and nearly independent above a threshold value. Boron segregation at the Ta/Co-Fe-B interface is crucial in controlling $D$.

Guided sound waves from arrays of acoustic transducers are key to myriad techniques in medicine, engineering, and the physical sciences, but digital methods of wave-field control are restrictive. This study investigates physical reconfiguration of array elements according to the folding sequences of origami, which allows portability. Reconfiguring a tessellated array is more effective than digital methods of acoustic-wave guidance, using fewer array elements, while refinement of array topology tends to the ideal case of geometrical acoustics. These findings may promote adaptive, deployable waveguides for $e.g.$ medical ultrasound or the study of underwater ecosystems.

The unusual transient negative differential capacitance (NC) of a resistor–ferroelectric (FE) capacitor circuit is of keen interest for use in next-generation transistors, but a clear physical picture and theoretical framework are needed to interpret experiments. The authors show both numerically and analytically that transient NC comes from the mismatch of free charge and polarization in the capacitor during polarization switching, and a procedure to experimentally determine the viscosity coefficient in Landau theory is provided. These results should have real impact on metrology and device physics involving the NC effect.

Focusing is an ubiquitous mode of transforming waves—even within the human body, as it turns out. Recently, high-frame-rate ultrasound has enabled the observation of shear shock waves in soft solids, such as the brain. The present study uses that technique to further show that shear waves emitted by a cylindrical source into tissue-mimicking gelatin can be focused, and that they form a shock at the focus. This could explain why traumatic brain injuries, such as diffuse axonal injury, occur deep inside the organ, rather than near the skull.

Spintronic devices require channels to efficiently conduct spin current. Carbon nanofibers based on polyacrylonitrile (PAN) are already of keen interest in nanotechnology and energy science; could they be useful in spintronics as well? Modeling PAN nanofibers as N-terminated zigzag graphene nanoribbons, the authors study the electronic structures and quantum transport properties of these quasi-one-dimensional systems as a function of width. They demonstrate that narrow ribbons bear finite magnetic moments, with spin-polarized electronic states exhibiting similar spin configurations on both edges, resulting in spin-dependent transport channels.

Phononic crystals (PnCs) have been studied extensively for two decades, as an enticing platform for ultrahigh-frequency signal processing. However, struggles with geometric scaling and efficient coupling of acoustic energy at the microscale have hindered the development of basic building blocks for devices. Through a systematic design approach, this study demonstrates waveguiding in pillar-based PnC slabs operating in the GHz regime, laying a foundation for more sophisticated signal-processing systems built of PnCs.

Research on metamaterials has evolved to include engineering adjustable, rather than fixed, properties. Shunted piezoelectric materials have previously been used to to tune elastic waves propagating in beams and plates, and in this work the authors incorporate them into tunable resonators in a lattice structure. This cellular lattice exhibits highly directional yet symmetric pathways for energy propagation at specific frequencies, but the real draw here is tuning subsets of resonators to alter the symmetry. This can override any inherent lattice pathway, and thus allows the tailoring of material properties for electromechanical control of vibrations.

High-index dielectric nanostructures are of particular interest for nonlinear nanophotonics, as they offer inherent magnetic-resonance-enhanced frequency conversion, and special types of optical nonlinearity. This study proposes a nanoantenna consisting of a chain of Si nanoparticles excited by a quantum emitter, with radiation properties that can be tuned by photoexcitation of the electron-hole plasma. This system is very sensitive to the refractive indices of the nanoparticles—a fact that can be exploited for efficient all-optical modulation. Laser pumping of this nanoantenna allows unidirectional launching of surface plasmon-polaritons, for applications in plasmonics and photonics.

Oceanic rogue waves can exceed 30 m in height, and thus pose a threat even to large ships and offshore stations. Being able to model such waves faithfully would be very helpful in unraveling their dynamics. In this context (and others as well), “breathers”—deterministic, localized, coherent pulsating structures—could be important to our understanding. Using fully nonlinear hydrodynamic simulations, this study aims to shed light on the applicability of breathers in modeling rogue waves under realistic sea conditions. The results provide a quantitative criterion for validating the relevance of breather dynamics for tackling diverse problems in oceanic engineering.

Spin-based nano-oscillators are tiny devices that convert direct electrical current to microwave signals. One particular variety offers an amazing propensity for robust long-range synchronization, which is very interesting for next-generation rf applications and neuromorphic computing. State-of-the-art micromagnetic simulations reveal that, for these oscillators, the spatial extent of magnetization dynamics can be dramatically altered with an applied magnetic field, with spatial symmetry breaking appearing only in strong oblique fields. These findings allow sensible engineering of the couplings in complex networks of such oscillators, to promote brain-inspired spintronic computing.

Amorphous indium gallium zinc oxide ($a$-IGZO) has drawn considerable interest as a channel material in flexible thin-film transistors (TFTs), to replace amorphous-Si-based TFTs, but $a$-IGZO-based TFTs show environment-dependent instability in threshold voltage. Investigating the variation of defect structures and subgap states with post-annealing temperature, this study reveals that the excess oxygen atoms surrounding cations or cation-vacancy-related defect clusters give rise to the instability. These results open a route to understanding the physical origin of device instability, and offer a feasible approach to tailoring such defects for practical applications.

Optical frequency standards surpass their microwave counterparts in both stability and accuracy, yet they are often bulky, power-hungry, and unable to operate outside of a well-controlled laboratory environment. Leveraging a two-photon transition in ${}^{87}$Rb vapor and recent advances in fiber frequency combs, the authors build an optical clock to beat the current portable standards, with an architecture that can be made compact and low-power. These results point the way to a real-world optical frequency standard of even higher stability, for applications such as satellite navigation.

Magnetic resonance imaging (MRI) is one of the most important noninvasive imaging techniques for human diagnostic medicine. Over the decades MRI quality has improved substantially, through the design of very sensitive multichannel receivers, but designs based on conventional materials are nearly topped out in terms of performance. Thus artificial materials are sought to further boost MRI sensitivity. The authors demonstrate a type of metasurface that can dramatically enhance MRI image quality, allowing the resolution of smaller features of tumors, for example, much more quickly and efficiently.

The authors discuss a fast memory application based on thermal hysteresis in a magnetically driven superconducting quantum interference device (SQUID). Their theoretical exploration of the coherent thermal transport in an inductive temperature-biased SQUID reveals appreciable bistability of electrode temperature in the device. This property could be used to encode “0” and “1” logic states (bits) in a thermal memory, which would be well placed in the context of both superconducting memory elements and thermal devices. Write times of ~0.2 ns seem possible—better than prior propositions for thermal memory, by a factor of 10${}^7$.

Spin-orbit torque (SOT) is a promising means to control magnetization dynamics in spintronic applications. Much effort has been devoted to addressing the physical origin of SOT, and improving its efficiency. The authors observe an abrupt enhancement of SOT for Cr thinner than 6 nm in a Cr/Ni heterostructure, and more remarkably a change in the sign of the fieldlike torque, due to a very strong Rashba interaction. This purely interfacial effect could provide a basis for extended functionality in next-generation memory and logic devices.

Although we use it in countless applications, glass remains a bit mysterious. For example, amorphous solids lack long-range order, and thus challenge the classical dislocation-mediated view of plasticity in crystals. It is widely accepted that shear transformations underlie amorphous plasticity, but how they work remains unclear. This study uses a colloidal glass to directly observe shear-transformation events in real space, providing a clear picture of the interplay of these transformations with free-volume dynamics. This is a crucial step toward developing a general theory of amorphous plasticity, and improved engineering to avoid failure of amorphous solids under stress.

Originally introduced in the context of string theory and quantum field theory, the ideas of supersymmetry have lately been extended to photonics, as a tool to design unique optical structures with degenerate spectra. Here the authors study several aspects and applications of one-dimensional supersymmetric transformations in optical fibers. As an example, they discuss the possibility of building a broadband all-fiber true mode (de)multiplexer, requiring no mode conversion between optical waveguides, which addresses an outstanding real-world problem in the field.

The exotic transport physics of single-crystalline transition-metal pentatellurides has been debated for years. The authors discover that in fact polycrystalline samples show the same unusual properties, and that viewing these compounds as semiconductors with bipolar conduction explains their behavior. This insight allows for the prediction and optimization of their thermoelectric properties. Considering the dearth of work on materials for waste-heat recovery at lower temperatures, and on generators employing a single compound for both legs, this study could change the way we think about what is possible in thermoelectric applications.

The development of high-density voltage-torque magnetoresistive random-access memory (MRAM) looks to voltage-induced magnetization switching (bit writing) without a biasing magnetic field—but how? In mainstream technology based on magnetic tunnel junctions (MTJs) with perpendicular magnetization, voltage-induced switching is not possible at zero bias. However, the authors show that switching at zero bias should be possible, in an MTJ with elliptical cross section and a conically magnetized free layer. Their results provide a practical guide to designing bias-field-free voltage-controlled MRAM.

Acoustic manipulation of fluids and particles garners increasing interest for use in medicine and biotechnology. One approach is based on actuation of confined, acoustically “leaky” systems, but finding accurate physical models for them has been stymied by a lack of relevant empirical data. To advance the field, the authors provide experimental benchmark results for particle trajectories in three dimensions, and describe them with a minimal numerical model. They reveal that a pseudo-standing wave drives acoustic streaming and the acoustic radiation force on suspended particles—important insight for developing clinical applications.

If dark matter cannot be detected using photons, then how can we confirm or rule out its existence? Well, if it’s made of axion particles, we should use an instrument known as a haloscope. Mounting evidence suggests a need to search for higher axion masses, but progress is being held back by numerous technical difficulties. This study employs resonant designs based on dielectric structures and “supermode” tuning to aid in the push toward higher masses. The authors also discuss implementation of tunable Bragg resonators in transverse magnetic modes, which are applicable to broader problems in microwave engineering.

Magnonic crystals (metamaterials to control magnetism) are attractive for manipulating spin waves in digital logic and storage applications, but in the end one needs to be able to actually produce them—an interesting challenge in nanotechnology. This work shows how interlayer coupling and magnetization strength can be engineered in Co/Mo/Co ultrathin-film structures by ion irradiation. Modifications of magnetic properties are correlated to and explained by structural evolution, as revealed by numerical simulations. The authors propose numerous types of magnonic crystals that can be fabricated according to these insights.

Microwave readout of charge states and spin states is important for quantum information science, but is difficult to scale to a large number of qubits, due to cost and the size of the components required to faithfully transmit the signal from room temperature to mK qubit temperatures. In this study, a voltage-biased semiconductor double quantum dot is used to generate microwave photons, yielding a cryogenic on-chip source for charge-state readout. Surprisingly, the emission properties of the double dot are affected by other qubits placed in the same microwave cavity. These results should facilitate the development of a large quantum processor to realize true quantum supremacy.

Volumetric methods of nanoparticle synthesis (such as arc discharges, flames, and laser ablation) yield a plethora of particle types, but in general we lack a deep understanding of the physical processes behind such synthesis. This is due in part to a lack of diagnostic tools. The authors demonstrate a four-wave-mixing laser technique, termed coherent Rayleigh-Brillouin scattering, for $in$ $situ$ detection of ~5 nm particles produced in an arc discharge, with a spatial resolution of ~150 μm. By enabling detailed $in$ $situ$ monitoring of nanoparticle nucleation and growth during large-scale synthesis, this approach will advance our physical understanding of the growth mechanisms.

Free control of electromagnetic wave fronts remains a key issue in photonics. Traditional wave-shaping elements, used for centuries, suffer from limited functionalities and excessive size. Now metasurfaces offer exotic abilities to reshape the wave front of light, but are mainly used to manipulate propagating waves. Extending the idea of metasurfaces for propagating waves to surface waves (SWs), the authors create a gradient metawall that yields focusing effects and high-efficiency reflection per a generalized Snell’s Law for SWs. This development may inspire applications such as SW holograms, superresolution imaging, and enhanced nonlinear optical effects.

Though confinement of spin waves (SWs) through nanochannels (NCs) is essential for the development of magnonic logic hardware, voltage-controlled reconfigurable SW nanochannels and logic devices have remained elusive. Here numerical micromagnetic simulations show that NCs of arbitrary shape and size, down to a few tens of nanometers, can be easily configured to transmit SWs with variable wave vectors as needed, even in the absence of a magnetic field. Operation of reconfigurable logic devices using the NCs is also demonstrated. This study is expected to have a large impact on the development of all-voltage-controlled low-power nanoscale magnonic devices.

The topologically protected magnetic textures called skyrmions may provide a suitable basis for $reservoir$ $computing$, one approach to brain-inspired cognitive computing. Reservoirs of self-organized skyrmions offer potential advantages in size, efficiency, and complexity, compared to systems of memristive devices, quantum-dot lasers, and atomic switches. The basic element here is an isolated skyrmion in a ferromagnetic ribbon; thus the authors examine current flow through magnetic skyrmions based on anisotropic magnetoresistance, analyzing the nonlinear current-voltage characteristics. The scheme they provide offers a path to spintronic neuromorphic computing.

Generating microjets of highly viscous fluids is important to prevent blurring in key modern applications such as inkjet printing, but such technology is being held back because most printers handle only low-viscosity liquids, such as water-based ink. This study proposes a simple structure that uses an impulsive force to generate high-viscosity microjets of fluids similar even to honey, and with non-Newtonian properties. These results seem very promising for advancing state-of-the-art devices, including bioprinters and needle-free injection systems.