Mott insulators exhibit an electric-field-induced insulator-metal transition, which may be used to implement artificial neurons. This work focuses on the poorly understood relaxation of the (filamentary) metallic state back to the insulating state. Extending their previous model of the “leaky-integrate-and-fire Mott neuron” to include electroelastic effects, the authors show that strong electrical pulsing may increase the relaxation time by thickening the metallic filaments. Numerical simulations agree qualitatively with recent experiments. This work sheds light on the dynamics of firing and relaxation in these systems, which is important for developing future neuromorphic circuitry.

A quantum network consisting of more than one physical system can combine the advantages and avoid the inherent drawbacks of those different systems. However, a compatible quantum interface is needed to connect them and form a larger quantum network. The authors use nondegenerate narrow-band polarization-entangled photon pairs to entangle different nodes, creating a universal photonic quantum interface that will significantly aid in the development of more complex networks, for quantum communication or distributed quantum computing.

The advanced computational method presented in this study is important for a variety of effects studied in spintronics that involve interplay between spin-transfer torque, spin pumping, and the damping of magnetization dynamics. Other approaches either use a purely time-dependent classical scheme, such as micromagnetics, or combine it with a steady-state quantum description that cannot take into account the impact of time-dependent fields due to evolving magnetic moments on electrons. This numerically exact, and thus nonperturbative, framework will impact the computational design of spintronic nanodevices utilizing magnetic domain walls or skyrmions for digital and bioinspired computing.

As device sizes in micro- and optoelectronics continue to shrink, accurate control over defect distributions in semiconductor thin films becomes ever more important—particularly for extended defects that spoil the crystal quality of important material systems such as Ge/Si heterostructures. The authors combine state-of-the-art characterization techniques with advanced modeling to understand the impact of misfit dislocations on the lattice of Si-Ge/Si layers. Strong agreement between the predicted and measured distribution of tilt angles is obtained, shedding further light on plastic relaxation in semiconductor heterostructures.

Guiding waves at scales shorter than the wavelength is crucial for many applications, including compact signal-processing systems and concentration of wave energy. High sensitivity to geometrical imperfections and disorder-induced backscattering, however, pose major problems. This study proposes using a chiral metamaterial, in which the waves guided at the subwavelength scale are strongly protected by the chirality against various types of disorder. Through rigorous statistical studies, the authors demonstrate that this scheme is more robust than other waveguiding solutions, including recently proposed topological designs.

Microfluidic sensing is of interest for biomedical applications, and for monitoring trace contaminants in the environment. However, a sensor offering physical robustness, simple integration into microfluidic systems, high sensitivity, and good analyte specificity is lacking. The authors present theory and experiments on a lasing sensor that can address many of these requirements, with sensitivity of over 1000 nm of wavelength shift per refractive-index unit in robust, untreated capillaries, and the potential for surface-based specificity. This work could lead to a fresh direction in research on capillary-based microfluidic sensors, and an alternative to whispering-gallery devices.

Mott insulators exhibit an electric-field-induced insulator-metal transition, which may be used to implement artificial neurons. This work focuses on the poorly understood relaxation of the (filamentary) metallic state back to the insulating state. Extending their previous model of the “leaky-integrate-and-fire Mott neuron” to include electroelastic effects, the authors show that strong electrical pulsing may increase the relaxation time by thickening the metallic filaments. Numerical simulations agree qualitatively with recent experiments. This work sheds light on the dynamics of firing and relaxation in these systems, which is important for developing future neuromorphic circuitry.

Measuring and understanding the properties and interactions of spin currents at interfaces are important for the design of future low-dissipation devices for logic and data storage. Traditional detection schemes are indirect, using the inverse spin Hall effect to convert the spin current into a voltage. Taking a different approach, the authors present a contact-free probe of spin current, based on paramagnetic adsorbates, that combines electron-spin-resonance detection with ferromagnetic-resonance excitation to monitor spin-pumping effects at an interface $directly$, with atomic precision.

Colloidal nanostructures that are not iridescent could find use in ink-jet printing, reflective displays, or other optical technologies, but producing noniridescent structures involves complicated chemical methods, and control of the thickness of the colloidal structure is limited. Here a simple, robust, $physical$ method yields a uniformly thick photonic film, by suppressing “coffee ring” patterns with the Marangoni effect from fluid dynamics. Noniridescence is achieved by depletion attraction and the friction effect of polymer brushes, leading to short-range-ordered packing of colloidal nanoparticles. This method could be used in many printing and light-filtering applications.

In computing, clear evidence of the advantage of quantum optimization techniques over optimization approaches on classical hardware remains to be demonstrated. A fully connected qubit architecture is needed, to reduce the embedding overhead and to generate hard problems for testing quantum effects in such devices, but fabricating such systems is extremely difficult. Using techniques from the study of disordered magnetic systems, the authors show that a good design compromise is given by a modified small-world graph. These achievable small-world architectures should give a boost to the quest for proof of quantum speedup.

Laser-induced cavitation and subsequent fragmentation of free-falling tin microdroplets finds application in producing extreme ultraviolet (EUV) light for state-of-the-art EUV nanolithography. Studies of such laser-droplet systems are scarce, though. This work combines analysis of high-resolution stroboscopic shadowgraphy with intuitive fluid-dynamics models to further our understanding of the late-time dynamics of a tin microdroplet after the impact of a picosecond laser pulse. The insight obtained offers a means to engineer optimal laser targets for EUV light generation from laser-driven plasma.

Electronic defects in semiconductor devices play important roles in device performance and reliability. These defects are created during fabrication using plasma-based processing technology. The authors investigate the generation and annihilation of defects in amorphous silicon by measuring the photocurrent during processing with hydrogen and argon plasmas. Unfavorable species for defect generation and favorable species for defect recovery are distinguished. These results are beneficial for improving semiconductor plasma processing, which is used to make state-of-the-art devices such as FinFETs, computer memory, and solar cells.

The development of organic light-emitting diodes (OLEDs) requires simulation methods that can predictively model all electro-optical processes. Here three-dimensional (3D) kinetic Monte Carlo simulation is the “gold standard”, but the method is computationally inefficient for OLEDs at low voltages. To address this regime, the authors present a 3D master-equation model that accurately includes carrier recombination, making simulation at low voltages feasible.

Recent technological advances have boosted research related to $exceptional$ $points$ (EPs), singularities arising in non-Hermitian quantum mechanics that once seemed purely mathematical. Device-level implementation of EPs has been primarily in gain-loss-balanced toroidal optical microcavities, but too much gain can cause such a system to become unstable. This study proposes an all-lossy dual-mode planar waveguide structure, in which the topological properties of an EP are achieved by patterning the longitudinal loss profile only. This scheme needs no active pumping, is accessible to many conventional optical elements, and offers a platform for topological control of light signals.

Quantum information processing requires faithful on-chip transfer of quantum states, but the couplings in solid-state systems are usually preset and not tunable, and thus in general cannot meet the special configurations needed for perfect quantum state transfer. The authors demonstrate such a perfect transfer in a chain of four superconducting qubits with nearest-neighbor coupling through $in$ $situ$ parametric modulation, in a single step, with high fidelity (99.2%) in a short time (84 ns), thus overcoming the fixed-coupling problem. Their scheme for flexible tunability of multiqubit coupling can be easily extended to larger systems.

In the realm of solid-state qubits, the strength of the coupling of a point defect’s spin to the local strain of its host crystal is important for developing a nanoscale quantum sensor. The authors use density functional theory to calculate the key parameters for a divacancy in SiC, and predict the stress sensitivity that could be achieved, which is competitive with that of an N-$V$ center in diamond. This result highlights the potential for defect qubits in SiC, which has advantages in crystal growth and microfabrication techniques at wafer scale that point to integrated, all-silicon-based chip sensors.

Encoding information in the temporal-spectral mode of single photons attracts growing attention in the community of photonic quantum technology. The temporal mode, with ultralong coherence time, of single photons from atomic ensembles is easy to control, but the conventional photon-counting technique provides only the amplitude of the temporal-mode function. This study develops a cavity-free homodyne detection scheme to characterize the $complete$ temporal state of narrow-band single photons, paving the way to exploit the temporal-spectral degree of freedom in photonic quantum information processing.

$Single-inclusion$ $polarizability$ quantifies the ability of subwavelength scatterers to interact with light, and allows the implementation of such inclusions in metamaterials or metasurfaces. Studying the physical mechanisms involved can offer practical insight for the control or harvesting of radiant energy. Here the polarizability of radially inhomogeneous spheres is determined, for three analytically solvable cases. These exact solutions reveal particular scattering peculiarities that can be used as the “smoking gun” for recognizing particular types of inhomogeneities, and present avenues for controlling or harvesting light via single inclusions or surface/bulk composite materials.

Magnetic random-access memory (MRAM) has emerged as a promising candidate to meet the requirements of high-density, low-power data storage. The authors use micromagnetic simulations to study the magnetic configurations of ring-shaped magnetic tunnel junctions (MTJs) with an in-plane magnetic storage layer, and experimentally study the spin-transfer-torque switching process and dynamic-resistance state diagram. In addressing both nanotechnology and physical mechanism, the results illustrate the advantages of nanoring MTJs, compared to in-plane elliptical MTJs or MTJs with perpendicular magnetic anisotropy, as memory units for nonvolatile MRAM.

Spin-torque efficiency and asymmetry are key parameters of the magnetic materials used in advanced devices such as spin-torque nano-oscillators (STNOs) and spin-transfer-torque magnetic random-access memory (STT-MRAM). Few studies have reported actual values, though, and even those come with large uncertainty. By experimentally mapping the magnetic-droplet current-field phase diagram in orthogonal spin-valve STNOs, the authors determine both parameters as functions of stoichiometry, for alloys of Co, Ni, and Fe. This work offers a method to determine, improve, and tailor spin-torque efficiency and asymmetry in STNOs and other spintronic applications.

Fe${}_{50}$Rh${}_{50}$ is an unusual alloy with a first-order metamagnetic phase transition above room temperature. Thin-film structures of FeRh with engineered interfacial exchange interactions are multifunctional, responding to temperature, magnetic field, and strain. The authors use experiments and simulations to study reduced switching fields in both macroscale Fe-Pt/FeRh thin films and mesoscale islands, allowing the exchange constants between layers to be determined. This will promote the development of nanoscale, multifunctional magnetic materials for applications including heat-assisted magnetic recording, spintronics, antiferromagnetic memory, and medical hyperthermia treatments.

Despite the rapid progress in lead-halide perovskite solar cells, our understanding of the basic optoelectronic properties of these materials is still incomplete. Here the authors show a clear, universal correlation between the photoexcitation-flux dependences of photoconductivity and photoluminescence, which helps to uncover the unique photophysics of these materials. While the photoconductivity exhibits a crossover in its power law from an exponent of 1 to ½, the photoluminescence crosses from 2 to 1½. The authors discuss phenomenological scenarios leading to this unusual situation, and propose an analytical model of competing carrier diffusion and recombination.

Spin Hall nano-oscillators (SHNOs) generate highly tunable microwave signals at room temperature, and beyond that, the ability of constriction-based SHNOs to phase lock with each other makes them candidates for neuromorphic computing devices. However, most practical applications require operation in only a weak magnetic field, or none at all, so a deeper understanding of SHNO dynamics in weak fields is needed. Here angle-resolved measurements of constriction-based SHNOs under weak in-plane fields reveal both a linearlike spin-wave mode and a bullet soliton, and intrinsic frequency doubling allows output signals above 9 GHz, in fields as low as 40 mT.

In magnetism, Brillouin light scattering spectroscopy (BLS) is the most popular method for probing the Dzyaloshinskii-Moriya interaction (DMI) in ferromagnetic thin films, but for a very narrow magnetic strip, BLS is limited by the spatial resolution of the light. This work proposes a magnonic counterpart to BLS to overcome that technical difficulty. A thorough study of spin-wave propagation and interaction in a thin film reveals interesting DMI phenomena, such as noncollinearity of wave vector and group velocity, negative refraction, and stimulated three-magnon splitting. This extension of our ability to probe the DMI is expected to facilitate the design of chiral magnonic devices.

The charge-carrier mobility and the built-in potential are important device parameters for optimizing the performance of thin-film solar cells. Charge extraction by a linearly increasing voltage (CELIV) is a commonly used technique for mobility measurements, but the original CELIV method applies only to devices with blocking contact—a severe limitation, since most operating devices have Ohmic contacts. Here the CELIV method is extended to determine these parameters in thin-film metal-insulator-metal ($m-i-m$) or $p-i-n$ diodes, rendering it a powerful tool for such devices.

Quantum-limited Josephson parametric amplifiers are a key component in many precision microwave measurement setups, such as for the readout of superconducting qubits in a quantum computer. As qubit setups scale up, these amplifiers must be optimized to handle input signals of ever-larger power. The authors design a quantum-limited parametric amplifier based on an array of superconducting nonlinear asymmetric inductive elements. This “SNAIL” is optimized to handle large input signals without sacrificing other desirable characteristics. The method can be extended to improve all forms of parametrically induced mixing in quantum information applications.

Capacitors and inductors, those familiar basic circuit elements, have very different shapes and structures: Capacitors are made of parallel metallic plates, while inductors are formed from wound metallic wires. Inspired by $optical$ $metatronics$ (metamaterial-based nanocircuitry for light), the authors propose a parallel-plate structure filled with a negative-permittivity medium that nevertheless behaves as an inductive circuit element, and may exhibit a higher $Q$ factor than conventional inductors, as well as other advantages. This suggests exciting possibilities in the design of low-loss microwave lumped circuits and electrically small antennas with subwavelength dimensions.

Heusler compounds are fascinating alloys that present a plethora of physical phenomena. They have been used for various applications, including spintronics, and their functional properties are quite sensitive to their structural order. The authors undertake a theoretical and experimental study of the effects of Cr doping on the structural, magnetic, transport, and electronic properties of Co${}_2$FeSi alloys. The improved understanding of the interplay of intrinsic defects that they attain should help to enhance the performance of devices based on this class of materials.

An enormous amount of kinetic energy is stored in the Earth’s rotation. According to a recent proposal, this can be used for the generation of electricity, by means of a special antenna. Experiments with such an antenna show that the energy harvested is more than 3 orders of magnitude smaller than predicted. The reason for this discrepancy is revealed, and the potential for improvement is discussed.

Improving the performance of organic field-effect transistors is of interest for organic (and especially flexible) electronics, but so far their transconductance—the change in output current per voltage difference at input—remains too low. Here a general approach to obtain giant transconductance in semiconducting thin films in multiple microelectric fields is proposed, and experimentally verified in organic films. The giant transconductance achieved with optimized thickness and doping concentration is compatible with temperatures up to 400 K and with low intrinsic mobility, for use in high-performance circuits and sensors.

The dynamics of mobile oxygen vacancies depend on electric field and temperature, and this is key to controlling interfacial resistive switching in BiFeO${}_3$ memristive devices, which are interesting especially for neuromorphic computing. The authors use impedance spectroscopy and quasistatic state measurements to reveal the dynamics of resistance changes in such devices, and relate these changes to the redistribution of oxygen vacancies via modeling. This work will also impact the use of other oxides with mobile oxygen vacancies in similar devices.

Recent progress on increasingly complex devices based on semiconductor spin qubits shows that automated methods are indispensable for efficient device characterization and further scale-up. The authors present fast, fully automated procedures to extract key device parameters, as well as various readout and initialization points for operating a two-electron qubit. These efficient, quantitative methods can be the basis for automatic tune-up, in which gate voltages are iteratively adjusted based on the deviation of measured parameters from targets. This would enable operation of devices with more and more qubits, and systematically optimize fabrication procedures.

The dynamic motion of particles in time- or space-varying light fields is important for applications in photophoretic optical manipulation, but the temporal behavior of the photophoretic force is poorly understood. This study uses an intensity-modulated photophoretic trap of $\mu$m-sized absorbing particles to demonstrate that the temporal change of force in response to changing brightness is remarkably slow, due to the slow change in particle temperature. Surprisingly, when the trapping beam is turned off, the trapped particle is pulled toward the light source by the residual photophoretic force.

Spin-torque-operated magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy are established charge-to-spin transducers. Their switching speed is still fundamentally limited by thermal activation, however, leading to slower switching in memory applications. In this study a canted magnetization state is engineered to overcome the incubation delay, for faster switching times. Comparison to perpendicular MTJs is performed, with direct time-resolved observation of switching dynamics in both systems. These results should facilitate optimization of MTJ speed for spintronic computing.

High-speed optical modulation based on photonic integrated circuits is important for various applications in all-optical signal processing, yet conventional strategies suffer from a kHz-level rate bottleneck. This study describes an optically driven Mach-Zehnder interferometer for integrated silicon-on-insulator platforms, where differential phase shift is achieved by the instantaneous nonlinear Kerr effect. Calculations show that, by choosing a suitable pulsed pump, the intrinsic loss in crystalline silicon can be greatly mitigated, which is especially relevant for emerging on-chip quantum interference applications operating at the telecommunication wavelength of 1.55 $\mu$m.

Repeatable and reversible switching of magnetization by an electric field at room temperature is a prime goal of multiferroics research, and was recently achieved in BiFeO${}_3$/Co${}_{0.9}$Fe${}_{0.1}$ heterostructures. Unfortunately, the mechanism of this magnetoelectric switching process remain largely unexplored, as accessing the buried BiFeO${}_3$ domain state is difficult. The authors use a nondestructive optical method plus magnetic force microscopy to access the details of the system’s magnetoelectric poling dynamics $in$ $operando$, an essential step toward studying the ultrafast switching dynamics that are so important for device applications.

Acoustic metamaterials have been utilized to realize miniaturized sound sources with enhanced emission, yet we still lack a rigorous analytical model for enhanced multipole directional emission using a metamaterial enclosure. The authors develop such a model, and its analytical results show that enhanced multipole radiation can be achieved with a lone monopole source encircled by metamaterial. This model’s reliability is verified by full wave simulations and experiments, and thus it could provide an efficient tool in engineering devices with enhanced emission of sound.

Optical and thermal metamaterial-based structures have been extensively explored, yielding many interesting functionalities; in what other ways can they be used? This study, following the basic rules of transformation optics (here thermotics), demonstrates another application: thermal encoding, based on binary states of heat flux for bit values of 0 and 1, by means of thermal energy shielding and harvesting units. This approach is a fresh alternative to existing implementations for thermal memory or computing.

Catalysis at the nanoscale plays an increasingly important role in subjects ranging from energy and environment to medical sciences, but our physical understanding of the dynamics and effectiveness of nanocatalysts in porous media is lacking. The authors theoretically investigate the factors needed for success in $in$ $situ$ nanocatalyst applications, such as heavy-oil upgrading and soil remediation, and they discuss the physical mechanisms behind the dynamics of mixing during nanocatalytic miscible displacement in porous media. This insight will aid the optimal design and utilization of nanocatalysts.

The conventional electronic-impurity doping required for Si-based electronics faces serious challenges below the 14-nm Si technology node in very-large-scale integration, and for Si nanocrystals up to ~10 nm in size. Expanding on successful modulation acceptor doping of SiO${}_2$, the authors investigate additional acceptor candidates, elucidating the role of atomistic parameters in allowing or blocking such doping. Some acceptors work in bulk Si but not in SiO${}_2$, while others ignored for bulk Si become very attractive for SiO${}_2$, due to their particular orbital configurations. This work points the way to advanced field-effect transistors and solar cells.

Tuning the properties of transition-metal oxides by controlling oxygen content has value in many applications. $Topotactic$ oxides, which can vary their crystal structure as oxygen content changes, are promising materials for reversible manipulation, since they do not create defects in random fashion. Real-time x-ray scattering is used to monitor the topotactic phase transition and its reversibility in SrFe${}_{0.8}$Co${}_{0.2}$O${}_{3-x}$. Reversible, redox-driven topotactic transformations at low temperature and atmospheric pressure indicate that these materials may be of use in electrochemical devices, such as solid oxide fuel cells.

A quantum network consisting of more than one physical system can combine the advantages and avoid the inherent drawbacks of those different systems. However, a compatible quantum interface is needed to connect them and form a larger quantum network. The authors use nondegenerate narrow-band polarization-entangled photon pairs to entangle different nodes, creating a universal photonic quantum interface that will significantly aid in the development of more complex networks, for quantum communication or distributed quantum computing.

Permalloy (iron dissolved in nickel, in about a 1:4 ratio) is an important material for magnetic recording, due to its negligible magnetostriction and relatively low damping. This study of ultrafast magnetization dynamics investigates the correlation between phenomena occurring on time scales from femtoseconds to nanoseconds, following irradiation of a permalloy film by femtosecond laser pulses. The intrinsic Gilbert damping experiences a transient change with varying pump fluence, due to the rising ratio of system temperature to Curie temperature. The insight from this work is valuable for developing several varieties of magnetic data-storage technology.

The advanced computational method presented in this study is important for a variety of effects studied in spintronics that involve interplay between spin-transfer torque, spin pumping, and the damping of magnetization dynamics. Other approaches either use a purely time-dependent classical scheme, such as micromagnetics, or combine it with a steady-state quantum description that cannot take into account the impact of time-dependent fields due to evolving magnetic moments on electrons. This numerically exact, and thus nonperturbative, framework will impact the computational design of spintronic nanodevices utilizing magnetic domain walls or skyrmions for digital and bioinspired computing.

Voltage control of magnetic properties, such as magnetic anisotropy and exchange interaction, is a key phenomenon for low-power spin manipulation. Based on micromagnetic simulations, magnetization switching in a triangular magnetic element using voltage control of magnetic anisotropy and the Dzyaloshinskii-Moriya interaction under an in-plane magnetic field is proposed. This scheme is not toggle switching, but a deterministic switching in which the magnetic state is determined by the polarity of the applied voltage pulse. The results suggest a fast, low-power writing method for magnetoresistive random-access memory.

Electromagnetic cloaking is important in various microwave applications, such as stealth technology, interference reduction in multiantenna communication systems, and efficient energy harvesting. Fundamental limitations, though, hinder the operation bandwidth of passive cloaks as the size of the hidden object increases. Inspired by the concept of parity-time symmetry, researchers propose a realistic cloaking scheme based on $active$ circuitry, which enables a practically realizable, broadband microwave cloak that can satisfy the requirements of real-life applications.

Digital holographic tomography (DHT) using refractive index for imaging contrast has been explored as a minimally invasive, label-free tool for microscopic specimens. Extending an interferometry-based DHT, this study demonstrates “4D” (three spatial dimensions plus one spectral) absorption and refractive imaging of weakly absorbing specimens in the visible wavelength range (450—700 nm). Its results demonstrate the high sensitivity of the proposed method, as well as the potential of DHT using absorbance for imaging contrast.

Plasma instabilities, spokes, and flares are important in low-temperature cross-field devices (think magnetron sputtering, or Hall thrusters for spacecraft), yet their roles in discharge maintenance and charged-particle transport are not fully understood. The authors use high-speed photography to investigate the formation of plasma flares, and low-frequency oscillations in the plasma potential. At high power there is a drastic change in plasma characteristics, and the oscillations become chaotic. The instabilities assist particle transport far from the cathode, which may be of considerable technological importance.

The dream of graphene modified to have a spin-polarized semiconducting band structure has remained elusive for over a decade. This study uses spin-polarized scanning tunneling microscopy to reveal that the strong interaction between graphene and Ni(111) substrate generates a large gap in graphene, and simultaneously leads to a relative shift of majority- and minority-spin bands. This band structure persists even at room temperature, which is promising for graphene-based spintronic information technologies.

The spin-diffusion equation (Valet-Fert model) is widely used to describe spin transport in normal metals, using two separate electrochemical potentials for “up” and “down” spin states. The authors extend this model to a broad class of materials with spin-orbit coupling that exhibit spin-momentum locking (SML). They deduce simple expressions for charge-spin interconversion, in good agreement with existing experiments on diverse materials, and predict interesting effects related to spin-charge separation in the presence of SML, observable with current technology. The transmission-line model they provide is suitable for analyzing complex device geometries within a standard circuit solver.

The ability to fill interdigitated microchannels with closed ends is important for various acoustofluidic applications. Currently, technical difficulties plus a lack of physical understanding of the filling mechanism impede the progress. This study demonstrates the complete filling of such microchannels both with deionized water and with liquid metal, and elucidates the physical mechanism. Also, traveling surface acoustic waves are successfully generated using liquid-metal-filled channels, which will be useful in applications.

Heat-assisted magnetic recording (HAMR) employs a laser to heat the recording medium nearly to its Curie temperature $T_C$, to ease magnetization switching. HAMR hard drives have been promised for the near future, yet an understanding of magnetic damping in HAMR media at such temperatures has not been realized. This study of the near-$T_C$ damping properties of FePt thin films on MgO through ferromagnetic resonance (FMR) suggests that just below $T_C$, two-magnon scattering and spin-flip magnon-electron scattering make comparable contributions to the FMR linewidth, while at lower temperatures two-magnon scattering dominates.

Near-field radiative heat transfer between dissimilar materials is important in thermal management and energy-harvesting, but an in-depth physical analysis is needed. Using fluctuational electrodynamics, this study shows that the spectral redshift or blueshift of thermally generated surface phonon-polaritons (SPhPs) can be mediated by a nonresonant layer. The apparent shift is due to multiple reflections within the subwavelength gap separating resonant and nonresonant layers, which generate gap modes and decrease thermal emission around the SPhP resonance—valuable insight for applications based on near-field thermal radiation, such as thermophotovoltaics and thermal rectification.

The authors present a highly noninvasive and fast nanoscale thermometer that outperforms those in common use. It is based on the superconducting proximity effect of a normal metal, yielding a temperature-dependent zero-bias anomaly (ZBA) in charge transport. Operating at very low temperatures, it opens the way to calorimetry at ultralow energies, and a road map for detecting single microwave quanta.

Suppressing or manipulating elastic waves over a broad band is a dream for applications in fields such as mechanics, acoustics, metamaterials, and dynamics. The recently reported “chaotic band” mechanism offers possibilities for achieving this goal, but the physics of generating and controlling the chaotic band is not understood. This work reports an interesting remote-interaction regime, $bridging$ $coupling$ $of$ $band$ $gaps$, in which the gaps behave as piers of a bridge that support the bridge floor, $i.e.$ the chaotic band. This result also provides a solution for designing nonlinear acoustic metamaterials.

In edge-illumination x-ray tomography, which yields phase contrast in addition to attenuation contrast, spatial resolution is decoupled from the size of the x-ray source’s focal spot, and of the detector pixels, as the primary x-ray beam is structured in an array of narrow beamlets. This study provides a theoretical framework to explain how beamlet width sets spatial resolution in a tomographic phase-contrast image, and examines the effect of sampling during tomographic scans. The results will inform the design of advanced experimental setups and acquisition schemes, and help us understand how resolution is affected by constraints on scan time or dose.

Nonadiabatic geometric phases have important applications in quantum computation, as they depend only on the global properties of the evolution paths, and thus are robust against certain types of local noise. Experimental implementation is challenging, though, due to the need for complex control of multilevel or multiple quantum systems. This study proposes a delicate gate implementation to solve the problem, based on parametrically tunable resonant coupling between two superconducting transmon qubits, without introducing any auxiliary state. This approach is promising for high-fidelity geometric manipulation and robust solid-state quantum computing.

In electronics, current-induced mass depletion at the cathode is often seen to decrease with decreasing sample length; this is called the $Blech$ $length$ $effect$. However, in the presence of a self-induced temperature gradient in copper film with a geometry that allows current crowding, the authors observe strong coupling between thermomigration and electromigration, which $increases$ mass depletion with decreasing sample length. This $inverse$ $Blech$ $length$ phenomenon warrants a revision of design practices currently used to avoid current-induced damage in multilevel microelectronic devices.

According to the authors’ calculations, the overall transmission through a graphene-loaded nanogap in a waveguide can be completely suppressed, leading to extremely high modulation efficiency, by slightly shifting the Fermi level of the graphene. The active length of the device is about 2% of the free-space wavelength, so its effective volume could be 1/1000 of the diffraction-limited volume. By reducing device capacitance, the extremely small active area is beneficial for not only high-density integration, but also high-speed, low-energy operation. This work has implications for many areas of active plasmonics and graphene photonics research, especially on-chip applications.

Top-pinned perpendicular magnetic tunnel junctions featuring a synthetic antiferromagnet (SAF) have been extensively studied for spin-transfer-torque magnetic random-access memory (STT-MRAM). However, degraded device performance after annealing remains a problem for this design, and hinders integration with CMOS technology. This study instead uses a synthetic $ferromagnetic$ (SFM) pinning layer to enable high thermal tolerance in STT-MRAM stacks. Experiment and simulation confirm functionality in 20-nm devices, including current switching. The proposed design contributes significantly to the scalability of STT-MRAM, and enables future applications requiring a top-pinned stack.

Measurements of the two-photon correlation function $g^{(2)}(0)$ provide a means to verify coherent emission in high-$\beta$ nanolasers that operate in or close to the thresholdless regime. By deriving an analytic expression for $g^{(2)}(0)$, the authors identify lasing regimes in which the onset of coherent emission occurs at significantly higher pumping than is required to reach the intensity jump in the input-output curve. Surprisingly, for extended active media such as quantum wells or monolayer transition-metal dichalcogenides, this coherence threshold is insensitive to the $\beta$ factor. This insight is significant for developing efficient nanolasers for “green photonics”.

For giant- (GMR) and tunneling- (TMR) magnetoresistive sensors, noise and sensitivity are crucial in determining the smallest magnetic field that can be detected. Both parameters are analyzed for a sensor design comprising a circular free layer that features a flux-closed vortex magnetization state. The authors find that in the low-frequency regime, where noise is predominantly pink, the detectivity depends $only$ on the active sensor area, not on the magnetoresistance ratio or bias voltage, and is the same for both GMR and TMR devices. As such sensors are widely used in $e.g.$ biomedical, automotive, and aerospace applications, this insight is expected to be far-reaching.

The dynamics of superconducting vortices is important in accelerator physics, for its influence on the quality of resonant superconducting rf accelerating cavities, yet a theoretical description of the regime of non-Ohmic power dissipation observed in recent experiments has remained elusive. This study uses ideas from collective weak pinning to solve for the dynamics of a vortex line subject to a disordered potential landscape, and an ac magnetic field parallel to a superconductor’s surface. The solutions describe hysteretic power losses and crossover behavior, and can be used to guide the tuning of material parameters to optimize cavity performance.

$Hyperentanglement$ (simultaneous entanglement of a system in several degrees of freedom) is an interesting quantum phenomena that attracts much attention for use in high-capacity quantum networks, but it is difficult to faithfully distribute hyperentanglement between distant network nodes. This study presents an efficient protocol for hyperentanglement distillation by designing fidelity-robust quantum gates, $i.e.$ parity-check quantum nondemolitions (QNDs) and SWAP gates, which guarantee that the protocol works faithfully and with high performance. Furthermore, these quantum gates can find application in faithful optical quantum information processing.

Dynamical decoupling sequences of multiple pulses offer filter functions for the time evolution of a qubit superposition state, and thus are indispensable tools for quantum sensing. The authors investigate the effects of sequences of finite-width pulses on ac magnetometry based on nitrogen-vacancy centers in isotopically controlled diamond. This experimental and theoretical study shows that finite pulse width alters the optimum acquisition time, and shows how to correct the resulting frequency shift, providing a guideline for accurate measurement of the frequency, linewidth, and amplitude of an ac field.

In the past two decades, phononic crystals and metamaterials have led to advanced materials with exceptional acoustic and elastic characteristics, such as negative effective mass and stiffness. The design of such materials has always been divided between manipulating either airborne sound or elastic vibrations, but not both. In this work, the authors realize a universal metamaterial capable of simultaneously attenuating airborne noise $and$ mechanical vibrations, over broad frequency ranges. Their design harnesses both scattering and resonances, is load bearing, and operates in arbitrary directions for all wave polarizations, covering more than 60% of audible frequencies.

Ultrafast electron diffraction (UED) has emerged as a powerful technique to explore transient structural changes in materials at the atomic scale, enabling work in solid-state physics, physical chemistry, and biology. However, UED is typically performed using a laser pump-probe method, and therefore is dogged by problems with multishot measurements and time synchronization. This study presents an advanced technique called $compressed$ $ultrafast$ $electron$ $diffraction$ $imaging$ (CUEDI) to overcome the technical limitations of UED. This approach can also be extended to the transmission electron microscopy or x-ray regimes for ultrafast structural imaging.

Superconducting qubits now offer coherence times exceeding 100 $\mu$s and single-qubit gate fidelities above 99.9%, but realizing a large-scale quantum computer is still impeded by the quality of two-qubit gates, with typical fidelities of 95–99%. Here a simple, broadly applicable scheme to improve two-qubit gates using a tunable coupler is proposed. It simplifies the coupling scheme, and reduces unwanted interactions typically present in schemes in which the interactions are always on. Numerical simulations show that this approach should allow two-qubit gate fidelities greater than 99.9%, when implemented using state-of-the-art superconducting qubits.

To explain observations of Meyer-Neldel compensation for the temperature-activated charge transport in thin-film organic field-effect transistors, the authors reexamine different jump-rate models and multiphonon excitation processes in hopping transport in disordered organic semiconductors. Using an effective-medium approach, they show that averaging over the individual jump rates in a conventional Gaussian disordered system erodes the genuine, thermodynamically determined compensation effect, and therefore the macroscopic transport no longer reflects the microscopic rates. This has important implications for interpreting experiments on organic devices.

Custom-made stacks of two-dimensional materials can be constructed to achieve specific optoelectronic properties in devices. This work uses density functional theory to investigate the electronic structures and transport properties of phosphorene–tin-dichalcogenide van der Waals heterostructures in an external electric field. The results show that these systems exhibit rare type-III (broken-gap) band alignment, which will facilitate the development of tunnel field-effect transistors. Moreover, the band alignment can be tuned effectively by the applied electric field.

The stabilization of gold nanoparticles by biocompatible agents is crucial to their use in theranostics and biomedical imaging. Viral capsids can efficiently encapsulate gold nanoparticles, but the kinetic pathways of this multiscale, spatiotemporal process remain elusive. The authors use time-resolved absorbance spectroscopy and x-ray scattering to elucidate a three-step mechanism with timescales that span four orders of magnitude. This work highlights the robustness, adaptability, and utility of viral proteins, and should promote further studies of bioinspired systems.

Fixed properties are one thing, but the design of $tunable$ metasurfaces is significant for real-world applications, as an integrated, active control mechanism is much more resilient to the actual requirements of an ever-changing environment. However, design is often limited by the difficulty of finding a suitable tuning mechanism. The authors here design a tunable acoustic metasurface consisting of helical cylinders screwed into a circular plate. This screw-and-nut setup is used to obtain arbitrary phase profiles over a wide frequency range, and furthermore may inspire other sorts of designs.

As device sizes in micro- and optoelectronics continue to shrink, accurate control over defect distributions in semiconductor thin films becomes ever more important—particularly for extended defects that spoil the crystal quality of important material systems such as Ge/Si heterostructures. The authors combine state-of-the-art characterization techniques with advanced modeling to understand the impact of misfit dislocations on the lattice of Si-Ge/Si layers. Strong agreement between the predicted and measured distribution of tilt angles is obtained, shedding further light on plastic relaxation in semiconductor heterostructures.

Engineering heat transport at the nanoscale is important for thermal management in modern technology, but precisely how thermal resistance is affected by geometry and length scale has been a topic of debate. This study identifies a key parameter, the spatial frequency of a heating pattern, that governs thermal transport at the nanoscale. This result provides a means to quantitatively manipulate thermal dissipation at the nanoscale, simply by considering the spatial frequencies of the heating pattern.

Guiding waves at scales shorter than the wavelength is crucial for many applications, including compact signal-processing systems and concentration of wave energy. High sensitivity to geometrical imperfections and disorder-induced backscattering, however, pose major problems. This study proposes using a chiral metamaterial, in which the waves guided at the subwavelength scale are strongly protected by the chirality against various types of disorder. Through rigorous statistical studies, the authors demonstrate that this scheme is more robust than other waveguiding solutions, including recently proposed topological designs.

Chalcogenide phase-change materials, which exhibit an amorphous-to-crystalline structural transition, are useful for applications in digital data storage, random-access memory, displays, and transmission windows for photonic devices. Here researchers dope the important chalcogenide Ge${}_2$Sb${}_2$Te${}_5$ with silver to lower its switching temperature, by drastically altering its electronic structure. This is of particular interest for developing photodetectors operating at near-infrared wavelengths.

A spin-orbit gap is necessary for a variety of proposed topological quantum computing designs and semiconductor spintronic devices. Usually such a gap is achieved using materials with strong spin-orbit coupling, such as InAs or InSb. Considering the mature technology, it would be advantageous to use Si, which unfortunately has only weak intrinsic spin-orbit coupling. Nonetheless, an $artificial$ spin-orbit gap can be created in Si nanowires using nanomagnet arrays. The authors carefully study a variety of realistic designs, arriving at practical rules and guidance for using this approach to create an artificial spin-orbit gap in Si.

The study of low-damping multiferroic materials with strong magnetodielectric coupling is important for advancing several areas in electronics and spintronics; because of the scarcity of these materials, though, they have not seen industrial application. This study investigates a high-temperature multiferroic oxide with exceptionally low damping, and reveals an overlooked checkerboard ordering of oxygen vacancies in the structure, which could shed light on the physical aspects that rule the material’s functional applicability.