Nature has been developing nanoscale machines far longer than people have, but perhaps we are catching up a bit. To design high-performance nanomotors for our own purposes, mechanistic insight is needed. The authors present a synergistic mechanical-kinetic model for a modular DNA-based bipedal nanowalker, revealing counterintuitive properties. The model resolves the motor’s working mechanism and identifies a specific structure in the motor’s design that limits its performance, offering guidance for its improvement and for creating other motors from the same modular design principle.

The authors program a superconducting circuit $in$ $situ$ to operate as a microwave circulator, or a directional amplifier. The compact lumped element can be directly integrated with other superconducting circuitry for nearly lossless routing and measurement of quantum microwave signals. This work combines advanced understanding of parametric-coupling physics with innovative design and engineering, for fundamental impact on quantum measurements plus direct technological impact on current efforts to build scalable, on-chip infrastructure for quantum computing.

Microwave-frequency nonreciprocal devices such as $gyrators$ and $circulators$ allow unidirectional transmission of ac electrical signals, which is crucial for solid-state quantum computing. Here a scheme for highly miniaturized circulators that exploit the quantum Hall effect is explored, and regimes of operation with very low intrinsic impedance, suitable for practical realization, are identified.

Injecting charge or spin current can switch the magnetization in a spintronic device without an applied magnetic field, at a speed limited by spin precession. Optical switching can beat this limit, but has been achieved only in single magnetic layers, not full devices. The authors demonstrate switching in magnetic tunnel junctions (MTJs), the building blocks of spintronic technology, with 0.4-ps infrared laser pulses. Their junctions use Gd-Fe-Co alloy, which after being heated by a pulse spontaneously relaxes to the opposite magnetic state—at 100 times the record speed for MTJ switching.

MoS${}_2$ is a prominent candidate among two-dimensional materials for future nanoelectronics, in which device performance depends crucially on the defect properties of the material. The authors show that in MoS${}_2$, vacancy defects give rise to Raman-active vibrational modes that uniquely identify missing Mo or S atoms, or larger vacancy clusters. Thus, Raman spectroscopy plus first-principles calculation emerges as a powerful tool to observe and identify vacancy defects in layered electronic materials, for device optimization and quality assurance.

Using first-principles calculations, the authors show an enhancement in the thermoelectric response of rocksalt nitride/oxide multilayers grown along a polar direction. Beyond a critical thickness of nitride layers, these heterostructures host spatially separated two-dimensional electron and hole gases. Transport calculations predict that each subsystem has an enhanced thermoelectric response, which could serve as the basis for an innovative nanoscale device for energy recovery.

Pulses of high electric fields increase cell-membrane permeability, and this $electroporation$ can be used to introduce drugs or other molecules into cells, to trigger intracellular calcium release, or to shrink tumors, for example. Electroporation by nanosecond pulsed fields has been considered a nonthermal process, but huge temperature gradients could arise at the membrane, and the authors’ molecular dynamics simulations suggest an important role of these gradients in directing and enhancing biophysical responses.

The optical properties of polar semiconductors are affected by strain, which introduces specific manufacturing concerns into optoelectronics applications. The authors demonstrate that detrimental crystal polarization effects can be fully compensated via a sequence of reverse interfaces, while maintaining the most natural, polar direction for crystal growth. As a result, built-in electric fields become a viable tuning parameter—or they can be eliminated entirely, boosting excitonic decay rates by orders of magnitude.

Ferroelectric tunnel junctions (FTJs) are of keen interest for next-generation electronics. This work presents a theoretical approach to describe polarization-dependent tunneling electroresistance (TER), and to explain the controversy of the opposite signs of TER observed experimentally by various groups. The model also can be directly extended to explore possible mechanisms responsible for the memristor effect in FTJs. This work informs not only our basic understanding of these devices, but also the design of nonvolatile memory and memristor circuits based on FTJs.

Multifunctional oxides could provide the basis of many of tomorrow’s technologies, and understanding the physics of their oxygen defects is often the key to tailoring device performance. The authors study the electronic structure, magnetism, and magnetooptical properties of SrTi${}_{1-x}$Fe${}_x$O${}_{3-\delta }$, an important “non-dilute” magnetic semiconductor, and illustrate its application in a nonreciprocal photonic device. This insight into oxygen-vacancy-mediated magnetism will be relevant for engineering the properties of a whole range of materials by controlling their stoichiometry.

For contactless manipulation of microparticles, acoustic tweezers offer trapping forces five orders of magnitude greater than their optical counterparts. Establishing a general relation between electrode shape and the resulting acoustic field, the authors present selective, flat tweezers based on acoustic vortices generated by spiral electrodes, the shape of which encodes the phase of the field like a hologram. These devices can be fabricated via standard photolithography, enabling massive production of miniaturized, easily integrable systems for lab-on-a-chip applications.

Transducers bridging the optical and microwave domains could be used, for example, to link distant superconducting qubits via telecom fibers; to detect weak rf signals in astronomy, radar, or MRI; or to process rf signals riding an optical carrier. The authors couple localized strain fields to both rf and optical electromagnetic waves in nanoscale devices, with optical waves manipulating acoustic waves and vice versa. This optomechanical interaction provides dynamic on-chip control of acoustic waves, which is difficult to achieve through other means.

The authors present a comparative study of spin microstructure in Nd-Fe-B nanocomposites, promising materials for permanent-magnet applications. A combination of synchrotron x-ray and neutron data shows nanoscale inhomogeneities in the magnetization distribution, and a correlation between crystallographic and spin texture. This phenomenon and the physics behind it are important for designing and processing magnets, and the methodology is of wider interest for magnetism research.

$Asymmetric$ $transmission$ is an active subject in information processing, because of its utility in constructing electromagnetic and acoustic devices. The authors present a multiband, subwavelength acoustic device based on a “coded metasurface”, an alternating arrangement of elements corresponding to bit values of 0 or 1. By assembling these building blocks to encode patterns such as 0000…, 1111…, or 0101…, asymmetric transmission can be handily controlled to select “off” and “on” states.

The remarkable electronic properties of carbon allotropes enable a large magnetoresistance (MR) effect, which in a spin valve can be tuned by a gate voltage, via spin-orbit coupling (SOC). Unfortunately, the same SOC also spoils the long spin-relaxation time that is the great asset of carbon materials. This study explores an alternative: tuning MR in the absence of SOC, through careful engineering of the tunnel barriers at contacts. Surprisingly, asymmetry in barrier strength can significantly increase the MR effect, and its response to gate voltage.

Advances in additive manufacturing will enable the fabrication of metamaterials with ever more complex architectures, to realize unusual and superior physical properties. The authors design and build lattices of sinusoidal beams, yielding a class of metamaterials with unusual, extreme mechanical behavior: Poisson’s ratio for the system switches from negative to positive upon large deformation (the nonlinear regime). Potential applications include energy absorption, tunable acoustics, vibration control, responsive devices, soft robotics, and stretchable electronics.

Despite the current focus on newer materials, silicon-based solar cells are still being developed. The authors study amorphous/crystalline interfaces for solar cells based on $a$-Si:H/$c$-Si heterojunctions. Toward this end, they perform large-scale $ab$ $initio$ simulations of thousands of atoms to reveal the medium-range order in amorphous hydrated silicon. They find a highly complex, interconnected distribution of voids, showing structural details that evolve with H concentration, which is a key parameter in material growth and optoelectronic applications.

$Acoustoelectric$ $transducers$, which convert sound to electrical signals and vice versa, are all around us, from microphones and loudspeakers working in the audible range, to the ultrasonic devices used for nondestructive imaging in industry and medicine. This study presents a transducer that works at even higher frequencies, for electrical detection of transverse (shear) waves in the microwave range. Here the main mechanism for transforming dynamical strain to a microwave field is the ultrafast piezoelectric effect, which now can be harnessed for applications.

The hunt continues for ever more efficient photovoltaic absorbers, and lately NaSbS${}_2$ has captured attention because, unlike most materials, it shows efficient collection of charge carriers despite potentially high defect concentrations. To understand why, the authors calculate its electronic structure and find cross-gap hybridization, which yields a high dielectric constant that provides screening and defect tolerance. Thus this mechanism is seen to hold not just in BiI${}_3$ and the perovskite halides, but also in a non-perovskite chalcogenide.

InAs${}_{1-x}$Sb${}_x$ type-II superlattices (T2SLs) are of significant interest for next-generation infrared photodetectors, due to their long minority-carrier lifetime. However, little is known about hole transport along the growth axis, the direction of photocurrent flow. This study presents quantitative measurements of vertical hole transport in band-engineered T2SL transistors, which turns out to be strikingly similar to electronic transport in disordered bulk semiconductors like amorphous silicon. These results carry significant implications for optimizing T2SLs and devices based on them.

Designing piezoelectric composites with hierarchical structure is a key means of enhancing material performance in harvesting wasted mechanical energy. The authors propose a type of polymer-based meso-laminated piezoelectric composites that theoretically can offer better electromechanical properties than their component materials. Their findings demonstrate the potential of optimizing multiscale structure in this class of engineered compounds.

A $photocatalyst$ speeds a chemical reaction in the presence of light, and is not itself used up. This can be a good use of abundant solar energy—for, say, generating H${}_2$ from water, to fuel a vehicle. The authors explain why a ferroelectric oxide with a particular band gap should be a better photocatalyst when stuck on the verge of a structural instability, and prove it to be so. To trap this metastable structure, simply reduce the size of the catalyst’s crystallites, then watch it increase an example reaction’s rate by a factor of five.

Magnetic resonance force microscopy (MRFM) can be an important tool for nanoscale NMR measurements in condensed matter, but it is a challenging technique, especially at very low temperatures. The authors show how to mechanically generate rf magnetic fields with ultralow dissipation and without an external source, even at millikelvin temperatures. Their method uses the higher modes of a magnetically tipped cantilever, which simultaneously is used as an ultrasoft force sensor at its fundamental frequency. This constitutes a major contribution toward low-temperature MRFM.

Nature has been developing nanoscale machines far longer than people have, but perhaps we are catching up a bit. To design high-performance nanomotors for our own purposes, mechanistic insight is needed. The authors present a synergistic mechanical-kinetic model for a modular DNA-based bipedal nanowalker, revealing counterintuitive properties. The model resolves the motor’s working mechanism and identifies a specific structure in the motor’s design that limits its performance, offering guidance for its improvement and for creating other motors from the same modular design principle.

In hybrid atomic-optical systems for quantum information processing, there is an unfortunate mismatch: The electronic transitions in an atomic two-level system correspond to ultraviolet photons, while the “light pipes” for such photons transmit in the infrared range. To address this, the authors demonstrate quantum optical frequency conversion spanning more than 2.4 eV, and thus joining these spectral regions. This is a milestone on the path to integrating atomic-qubit manipulation with low-loss quantum information transfer over optical fibers.

Turbostratic graphene disks feature multiple layers with rotational stacking order, which ensures electronic decoupling of adjacent layers. Despite there being up to 100 graphene sheets per disk, the authors find the signature of $two-dimensional$ charge transport. With its central region well protected from environmental influence, this system combines the merits of single-layer graphene with a remarkable, consistent robustness, making it eminently suitable for device integration.

Deterministic switching of magnetization via spin-orbit torque, without an external magnetic field, is an attractive approach for nonvolatile memory and logic devices. This can be accomplished using in-plane exchange bias instead of an applied field. The authors study the mechanism of this switching, and expose the importance of Joule heating: Field-free current-driven switching is achieved in experiments, but care must be taken so that heating does not decrease exchange bias and spoil the switching.

An electric field aligns charges of individual atoms or molecules to be parallel to the field, allowing second-harmonic generation from a passing laser beam. The harmonic can be used to measure local electric field, for any gaseous species, with high spatial and temporal resolution. This capability opens possibilities for studying field-driven nonequilibrium phenomena, local field-enhanced processes, aerodynamic flow control, skin treatment, air purification, combustion, ignition, surface chemistry, and many other topics in physics, engineering, and medicine.

Various phases of MoO${}_3$ are interesting for a wide range of energy technologies, particularly in optoelectronics, given their wide band gaps and high work functions. However, a systematic view of their physico-chemical properties is needed. The authors work to bridge the gap between materials physics and device engineering with hybrid density-functional calculations of the electronic structures of these oxide polymorphs, with an eye toward their role as next-generation anode buffers in organic light-emitting diodes.

Photonic integrated circuits for linear, nonlinear, and quantum optics rely on microresonators, which enable complex filters and high-speed modulators, efficient parametric processes at low input power, and generation of squeezed light and correlated photons. Systematically measuring integrated Si${}_3$N${}_4$ optical microresonators, the authors identify the high impact of design-dependent losses on device performance. Fully three-dimensional simulations reveal the origin of this loss to be poor resonator-waveguide coupling, which could be minimized thanks to the insight from this study.

When constructed in monolithic fashion, a classic Mach-Zehnder interferometer with arms of different lengths becomes a highly stable frequency reference. The authors’ scheme, easily adaptable for various experiments, is promising for space-based laser interferometry to track the motion of test masses—prominently the Laser Interferometer Space Antenna (LISA) for detecting gravitational waves. The prototype already achieves LISA requirements at and below 1 Hz.

The authors program a superconducting circuit $in$ $situ$ to operate as a microwave circulator, or a directional amplifier. The compact lumped element can be directly integrated with other superconducting circuitry for nearly lossless routing and measurement of quantum microwave signals. This work combines advanced understanding of parametric-coupling physics with innovative design and engineering, for fundamental impact on quantum measurements plus direct technological impact on current efforts to build scalable, on-chip infrastructure for quantum computing.

Nonlinear (and in particular quantum) optical experiments performed in whispering-gallery-mode resonators could be considerably enhanced by the ability to independently tune the coupling strength to pump and signal modes. The authors demonstrate such a scheme, exploiting birefringence in either the coupling prism or the resonator. Both the refined theory for coupling and the technique presented in this manuscript bear the potential to optimize many experiments in nonlinear and quantum optics.

Microwave-frequency nonreciprocal devices such as $gyrators$ and $circulators$ allow unidirectional transmission of ac electrical signals, which is crucial for solid-state quantum computing. Here a scheme for highly miniaturized circulators that exploit the quantum Hall effect is explored, and regimes of operation with very low intrinsic impedance, suitable for practical realization, are identified.

Sources of individual photons have applications in quantum cryptography, computation, and metrology, but truly scalable sources are still needed. The authors couple silicon-vacancy centers to a high-$Q$ microcavity, yielding a room-temperature source with the potential for high efficiency, brightness, and spectral purity. This setup improves spectral density by more than two orders of magnitude and could offer single-photon rates above 1 GHz, as well as generation of indistinguishable photons.

No matter how small, fast, or efficient electronics become, they are useless when they fail. The authors show that thermally induced stress in a metallic interconnect can produce a surface instability along its interface with a dielectric, leading to breakage. In certain ranges of thermal stress, this could be the dominant failure mechanism, with the critical instability length becoming shorter than the Blech length for electromigration effects. For soft, low-$\kappa$ dielectrics, this phenomenon may be an additional consideration in designing and manufacturing reliable devices.

The authors present a detailed investigation of individual core-shell (In,Ga)N/GaN microrods, which are potential next-generation white LEDs. Combining complementary analytical approaches at various length scales, they show that all subshells in a rod thicken drastically toward the tip, and that the local indium content (the key parameter in optical performance) can be precisely determined. High structural and crystal quality are essential to efficient optical performance, and this study gives important new insight on the interplay of elastic strain and chemical composition in these LEDs.