Cloaking is very interesting, for magnetic fields as well as light, but standard approaches are held back by technical challenges. This work shows that a diamagnetic active-current boundary combined with a high-permeability magnetic shell can be precisely engineered to overcome the usual permeability and frequency-band limits, for a robust cloak covering the entire quasistatic frequency region, without using superconductors—no cryogenics required. These results provides an efficient way to circumvent the traditional limits of metamaterials and realize magnetic cloaks for ultralow frequencies, and could be generalized to yield other artificial magnetic systems as well.

Using only Deutsch gates, one could construct a quantum circuit to accomplish $any$ feasible quantum computation, but unfortunately a working Deutsch gate has remained out of reach, due to lack of a protocol. This study proposes an easily realizable Deutsch-gate protocol, based on the blockade interactions in $e.g.$ neutral Rydberg atoms. This protocol can be extended to realize the CNOT gate, as well as the Toffoli gate, which can be used in quantum error correction. Given the very broad applicability of these gates, this result is a significant advance in quantum information science.

For direct interfacing of digital and quantum electronics in quantum computation, mature CMOS technology, with its potential for large-scale integration and data management, offers solutions for control, readout, and fast data processing with large numbers of qubits. The authors combine three likely components of an all-CMOS quantum computer—a quantum-dot transistor, a digital transistor for control, and an rf readout circuit—to yield a single-electron memory cell. The digital transistor locks charge on the quantum-dot gate and allows conditional readout via gate-based rf reflectometry, demonstrating the building blocks for time-multiplexed readout of multiqubit devices.

Elastic metasurfaces can manipulate the propagation of surface acoustic waves (SAWs) for applications as wave filters, including barriers for seismic surface waves—earthquake protection, that is. Efforts so far have focused on homogenous media, in crude approximation of the real world. The authors study the dynamics of an elastic metasurface in an unconsolidated granular medium, which presents an inhomogeneous stiffness profile. The metasurface’s resonance hybridizes the lowest-order SAW and down-converts all higher-order SAWs, preventing the surface-wave delocalization seen in homogeneous media. These findings could impact protective designs for inhomogeneous stratified soils.

During precision measurement, the quantity being measured is often perturbed by the measurement process itself. This includes precision frequency measurements for atomic-clock applications using Ramsey spectroscopy. To eliminate probe-induced perturbations, a generalized method is developed in which the frequency control loop is augmented with a second control loop that feeds back to a secondary clock variable, and can perfectly compensate for perturbations of the clock frequency caused by the measurements in the first loop. This universal technique can be used for atomic clocks, high-resolution molecular spectroscopy, two-photon probing schemes, Ramsey mass spectrometry, and beyond.

Converting infrared radiation from a thermal source into electrical power, via a $thermophotovoltaic$ device, is important for energy harvesting and micropower applications. The authors present a large-area broadband infrared antenna-coupled tunnel-diode rectifier that directly converts infrared radiation into electrical power. The antenna resonantly enhances and couples IR light to an extreme-subwavelength tunnel barrier, leading to large induced photon-assisted tunneling currents. Peak electrical power is observed when the load resistance is matched to that of the diode. This direct conversion of thermal IR to electrical power using a scalable CMOS process seems quite promising.

Using only Deutsch gates, one could construct a quantum circuit to accomplish $any$ feasible quantum computation, but unfortunately a working Deutsch gate has remained out of reach, due to lack of a protocol. This study proposes an easily realizable Deutsch-gate protocol, based on the blockade interactions in $e.g.$ neutral Rydberg atoms. This protocol can be extended to realize the CNOT gate, as well as the Toffoli gate, which can be used in quantum error correction. Given the very broad applicability of these gates, this result is a significant advance in quantum information science.

Brillouin spectroscopy and micromagnetic simulations are used to study the propagation of magnetostatic spin waves across adjacent nonidentical magnonic crystals. The characteristics of the spin-wave modes in such a structure depend on the geometry of the slabs, and efficient spin-wave coupling at the magnonic-band-gap frequency can be achieved. This combination of spatial filtering features and spin-wave coupling leads to the realization of a frequency-selective magnonic drop filter, which is expected to extend functionality in spin-wave demultiplexing applications.

A solar panel gets hot as it works up on the roof, yet photoinduced self-heating is often ignored when characterizing lab-sized samples. The authors present their understanding of the turnover effect in measurements of open-circuit voltage versus light intensity (Suns-$V_{OC}$ curves), which is identified as a unique feature of all semiconductor-based solar cells. This effect is explained in terms of electrothermal feedback arising when the incident irradiation heats up the device. The authors’ model fully explains the experimental data, and allows one to determine key device parameters such as the ideality factor and the band gap from a single measurement.

Tuning the electronic band gaps of semiconductors is at the core of current research on optoelectronic materials and devices. For ZnO, many studies have tried to enhance the band gap by chemical substitution, but lattice mismatch often thwarts that approach. Instead, this study looks to dimensional reduction. Experiments and first-principles calculations reveal that not only the thickness but also the $orientation$ of a film can affect quantum confinement, and thus ZnO’s optoelectronic properties. A suitably oriented and thinned film can boost the band gap to the “solar-blind” region above 4.6 eV, opening up a wide range of possibilities for applications.

The spin-dependent Seebeck effect, linking thermal energy and spin currents, attracts attention due to its potential significance in semiconductor spintronics. Conventional Si-based electronics faces a serious problem: the tremendous amount of waste heat generated by its devices. This study reports not only the recycling of waste heat into electrical signals in silicon using the spin-dependent Seebeck effect, but also thermal spin injection and transport in everyday Si (as opposed to a metal, or a magnetic insulator). The work demonstrates a means of saving energy in semiconductor-based spin-caloritronic information processing.

The hydromechanical anisotropy of clay soils has a significant impact on a number of geophysical processes, and on civil infrastructure design. This anisotropy arises in response to the ordering of microscopic platelike particles of clay, depending on deposition and strain history, but the mechanisms behind this process are not well understood. In this study of the microstructural anisotropy of clay soils, neutron diffraction allows direct observation of the evolution of structural fabric in response to deformation. A simple strain-structure relationship provides an avenue for integrating micromechanical detail into models when designing $e.g.$ dams, or nuclear waste facilities.

SrTiO${}_3$-based interfaces with high electron mobility continue to attract interest, due to the possibility of combining quantum phenomena with the many functionalities exhibited by SrTiO${}_3$. The origin of the high mobility, however, remains poorly understood. The authors investigate the scattering mechanisms limiting the high mobility (over 10${}^5$ cm${}^2$/Vs) in γ-Al${}_2$O${}_3$/SrTiO${}_3$ heterostructures. Broken symmetry at the spinel/perovskite interface seems to play a key role, in terms of the accumulation of oxygen vacancies and spatial separation between donors and electrons. This deeper understanding paves the way for designing high-mobility oxide electronics at the nanoscale.

Focusing light’s energy in the near field, beyond the diffractive limit, is key to progress in fields as diverse as microscopy, electronics, telecommunication, wireless power transfer, and ablation and tomography in medicine. The authors address the problem by considering a specific class of partially guided waves, known as “leaky” waves. Interestingly, only the subclass of backward leaky waves is found to be useful here. Qualitative and quantitative assessments furnish general design rules, and a simple but efficient leaky-wave device is proposed for generating limited-diffraction beams and pulses in the millimeter-wave range, where few experiments have been performed.

Microwaves (MWs) are at the heart of many technologies, $e.g.$ for communication, timekeeping, remote sensing, and quantum information processing. Masers can boost faint MW signals quite nicely, but not under real-world conditions. The authors demonstrate efficient population inversion of optically pumped vacancy-related spins in SiC, at room temperature, and find realistic operating conditions for a SiC maser to serve as a continuous-wave amplifier. These spins can also be coherently coupled to superconducting cavities efficiently at cryogenic temperatures. These findings suggest this SiC system as a promising platform for MW photonics and quantum electronics.

Microfluidic filters have a wide range of applications in the medical and biological industries, but suffer from clogging and low throughput. This study utilizes hydrodynamic interactions around trilobite-shaped filter units to separate and concentrate particles without clogging, while maximizing throughput and concentration ratios. Its high performance and applicability suggest the presented technology as an alternative to conventional methods, and the work sheds light on some of the advantages of employing hydrodynamics for particle manipulation.

At first sight, quantum key distribution (QKD) working at single-photon levels seems very different from classical optical communication using strong laser pulses. Surprisingly, in the simultaneous QKD and classical communication (SQCC) protocol, a single, coherent system can both transmit classical information and distribute a quantum key, but implementation has been impeded by its low tolerance of phase noise. The authors identify “trusted” noise from the coherent receiver, which cannot be accessed by an eavesdropper, and greatly improve the phase-noise tolerance of the SQCC protocol. Their findings suggest that the SQCC protocol could be truly practical for secure communication.

Finding sound-absorbing materials and structures is a key task in noise control. Engineered structures that can absorb sound are usually based on local resonance, but this study uses the collective response of individual building blocks to design a metasurface that soaks up acoustic energy. It is revealed that multiple internal reflections inside the blocks can increase their interaction with acoustic waves, and a little loss can lead to broadband absorption with better uniformity. This nonresonant approach to designing structures for noise control might also inspire the pursuit of other systems with enhanced wave-matter interactions.

Spin-transfer-torque magnetoresistive random-access memory (STT-MRAM) is a very promising approach to ultralow-power, nonvolatile storage devices. While prototypes are already being manufactured, a known problem with this technology is “back-hopping” (undesired high-frequency switching) in the information-carrying layer during writing. The authors investigate the destabilization of the reference layer as the possible origin of back-hopping, and propose measures to avoid it. Device miniaturization will increase the impact of the mechanism described here, making these insights significant to surmounting what could become an important obstacle to the success of STT-MRAM.

Microwave-assisted magnetization switching uses ferromagnetic resonance (FMR) excitation for magnetization reversal in a highly anisotropic magnetic material, for writing in next-generation magnetic recording. Because FMR is a nonlinear phenomenon, the resonance frequency changes with the amplitude of FMR excitation. This study uses a microwave magnetic field with a time-varying frequency that follows the nonlinear shift of the resonance frequency. Compared to a constant-frequency microwave field, the variable-frequency field can induce larger FMR excitation and thus enhance switching. These experiments show how to optimize the use of spin-torque oscillators in advanced data storage.

The continued miniaturization of electronics depends on our capability to engineer nanoscale heterostructures. This study focuses on the energetic response of anisotropic silicene upon injection or extraction of an electron, using density functional theory on varying armchair and zigzag edge lengths of rectangular silicene structures. The findings show that these finite structures are prone to behave as $n$-type semiconductors that could be used in low-dimensional heterojunctions. Moreover, a possible connection between the material’s energetic response and the distortion of its buckled lattice is discovered, which would be attractive in designing such devices.

The accuracy of optical qubit readout is fundamentally limited by the likelihood of a qubit flip induced by the optical excitation. Experiments here show that cavity quantum electrodynamics can break this limit and significantly enhance qubit readout. For a spin in a single InAs quantum dot plus a photonic-crystal cavity, selectively coupling an optical transition of the dot to the cavity mode yields spin-dependent cavity reflectivity, enabling spin readout via the reflected optical-field intensity. This work addresses a longstanding issue in solid-state quantum information processing, and is applicable to a variety of qubit systems that lack a good cycling transition for readout.

Since they can fundamentally affect a semiconductor’s electrical and optical properties, carrier-recombination processes are of great technological relevance, in that they largely determine the performance of minority-carrier devices such as light-emitting diodes and solar cells. Comprehensive measurements of photoluminescence, electron paramagnetic resonance, and magneto-optics show that intercenter charge transfer can be surprisingly efficient in ZnO, despite the much smaller Bohr radii of its free carriers and excitons than those in silicon. This represents unambiguous proof of efficient bound-excitonic Auger capture between two defect centers in a semiconductor.

This article presents technology to control the size of sessile gas bubbles in porous microchannels, which is of particular interest for microfluidic lab-on-a-chip applications using silicone-based materials. It shows how such bubbles can be removed, enlarged, dwindled, or maintained at constant size in an automated way, using a proportional-integral-derivative (PID) algorithm for pressure control. The technique is used to show how in a microchannel the streaming flow close to an acoustically actuated bubble changes in topology and strength, as a function of bubble size. This approach could be useful in potentially any experiment involving bubbles in porous microchannels.

For direct interfacing of digital and quantum electronics in quantum computation, mature CMOS technology, with its potential for large-scale integration and data management, offers solutions for control, readout, and fast data processing with large numbers of qubits. The authors combine three likely components of an all-CMOS quantum computer—a quantum-dot transistor, a digital transistor for control, and an rf readout circuit—to yield a single-electron memory cell. The digital transistor locks charge on the quantum-dot gate and allows conditional readout via gate-based rf reflectometry, demonstrating the building blocks for time-multiplexed readout of multiqubit devices.

Beyond conventional rf accelerators, dielectric laser accelerators (DLAs) are an emerging technology to generate high-energy electron beams at very short length scales, with numerous applications in medicine, manufacturing, and basic science. Current setups use a free-space laser beam to drive the accelerator, but an on-chip optical power-delivery system would give the precise control needed to dramatically scale up device length, for higher total energies. The authors investigate the challenges, constraints, and avenues for on-chip optical coupling, and verify that it is a promising route for moving DLAs from proof-of-principle to application.

To fill the “terahertz gap” in frequency, where many materials do not interact effectively with light, systems with tunable absorption are still needed. The authors design and demonstrate a Fano-type metasurface supporting controllable coherent perfect absorption—lasing in reverse, essentially—, the experimental results being well described by coupled-mode theory. A transition from single- to dual-band operation is effected by simply changing the angle of incidence. This system offers the potential to realize dynamic THz modulators in conjunction with electrically driven vibration devices, and absorptive interferometers for switches and imaging.

Worldwide research on devices for tomorrow’s spintronic logic and storage continues apace. The authors use $ab$ $initio$ calculations to investigate the electronic structures, magnetic properties, and tunneling magnetoresistance (TMR) effect of magnetic tunnel junctions based on the ferromagnetic nitrides $M_4$N ($M$ = Fe, Co, Ni). The case of iron presents a half-metallic state, and an ultrahigh TMR ratio greater than 24,000%—strong motivation for experimentalists.

The technology of x-ray and $\gamma$-ray detectors is important for advanced medical imaging that uses lower radiation doses and provides higher spatial resolution. The widespread application of these detectors is hindered by both material development and our understanding of the physics of charge collection after absorption of a high-energy photon. This study numerically models such charge collection, and develops a simple approach to understanding current waveforms in a coplanar-grid radiation detector, even without complex numerical simulations. It also shows the advantage of differential current sensing, which helps in overcoming material limitations.

For the promising class of micromachines that respond to a rotating magnetic field with linear motion, control of large groups has been largely limited to aggregate movement. Unfortunately, techniques to differentiate micromachines do not scale well to large groups, and are not practical for application $in$ $vivo$. This work presents an open-loop control scheme to sort magnetic micromachines, requiring neither localization of individuals nor a structured environment, and for which sorting time is independent of group size. This work is a step toward enabling magnetic microswimmers to be used in medical applications.

In the quest for efficient light-emitting devices, improving the efficiency of photoluminescence in carbon nanotubes (CNTs) by utilizing their defects has become widespread. Here is a detailed investigation of the defects, electron and hole transport, and photoluminescence intensity of individual suspended CNTs. The results quantify the relationship between the $D/G$ -band Raman intensity ratio and photoluminescence intensity, and point to a potential limitation in using brightly luminescent CNTs for optoelectronic applications.

Is there still “plenty of room at the bottom”? The controllable generation of atomic-scale gaps is key to fabricating single-molecule-based devices, but it is a great challenge to accurately fabricate an atom-sized gap between gold nanoelectrodes by lithography. Here a strategy termed field-aided atom migration (FAAM) is used to fabricate angstrom-scale gaps efficiently, with $ab$ $initio$ calculations illustrating the evolution of atomic motion upon applying an electric field. This strategy seems to have good potential for nanoscale circuit integration.

Coupled mechanical resonators have shown high potential for sensing applications and integrated signal-processing devices, in which strong and switchable coupling is essential. Realizing a coherent switch for motion transduction is still challenging, though, because optimal coherence requires that the system always work at resonance. The authors report an optomechanical switch for motion transduction at a coherent optimal point, based on dynamically localized modes. This approach to tuning the coupling strength between elastically coupled cantilevers at a degenerate point allows very efficient transduction at a fully controllable rate, with well-preserved phase coherence.

A metamaterial’s properties primarily originate from those of its component metamolecules, which makes single-metamolecule spectroscopy important for input in designing a metamaterial. However, detailed spectroscopic studies have been out of reach, due to the difficulties in manipulating individual micrometer-scale metamolecules. This work utilizes a coplanar waveguide to study the microwave transmission spectrum of a single magnetic chiral metamolecule, revealing its magnetic resonance and nonreciprocity signals. This approach could have real impact on high-throughput metamaterial engineering.

Elastic metasurfaces can manipulate the propagation of surface acoustic waves (SAWs) for applications as wave filters, including barriers for seismic surface waves—earthquake protection, that is. Efforts so far have focused on homogenous media, in crude approximation of the real world. The authors study the dynamics of an elastic metasurface in an unconsolidated granular medium, which presents an inhomogeneous stiffness profile. The metasurface’s resonance hybridizes the lowest-order SAW and down-converts all higher-order SAWs, preventing the surface-wave delocalization seen in homogeneous media. These findings could impact protective designs for inhomogeneous stratified soils.

Superconducting radiation detectors are promising for high sensitivity in the terahertz frequency range, as required for applications in astrophysics and quantum information processing. This study considers using a detector readout scheme based on a superconducting quantum interference proximity transistor (SQUIPT), with a detailed analysis of device physics and performance predictions. This interferometer-based scheme could also be used for other temperature-sensing purposes, and has the potential to improve the state of the art in ultrahigh-sensitivity superconducting single-photon detectors.

With zero net magnetic moment for ultrafast switching and high resistance to interference, antiferromagnets are of keen interest for next-generation data storage. This study demonstrates and analyzes current-induced switching of antiferromagnetic moment in Mn${}_2$Au films at room temperature, with different film orientations exhibiting various switching characteristics. This strong orientation dependence of switching adds another dimension to our thinking about spin-orbit torque, and makes this alloy a versatile basis for antiferromagnetic spintronics.

After more than 70 years of research and billions of dollars in losses to multiple industries, the physics of metal whiskers remains mysterious. The authors investigate a tin whisker’s structure at the nanoscale with transmission electron microscopy. A rich, nontrivial morphology is revealed, suggesting that metal whiskers consist of many filaments grown side by side. An electrostatic theory of field-induced nucleation explains such multifilament growth, bringing us a step closer to understanding, and possibly preventing, the formation of these nanostructure that can be so deleterious to electronics.

Temperature monitoring is critical for recently developed thermal-ablation procedures guided by MRI. In clinical practice, existing thermometry methods often fail, restricting the use of these procedures. The authors propose using magnetic particles as sensors for $T_2^{*}$ MRI temperature-sensitive contrast. The temperature-dependent local dipolar magnetic field of the particles modifies the static magnetic field of the MRI scanner, resulting in proton-gradient-echo images that are darker at lower and brighter at higher temperatures, with a resolution on the order of 1°C in tissue-mimicking phantoms.

Even after a decade of research, a fundamental controversy persists concerning the electronic structure of a suspended graphene monolayer, as revealed by scanning tunneling spectroscopy (STS). Is the spectrum truly gapped, or V-shaped? This study systematically shows that substrate interaction can affect the STS spectrum of graphene crucially, by suppressing out-of-plane phonons and thus eliminating any gap. Switching the tunneling spectrum of graphene with voltage pulses through the probe’s tip is also demonstrated. This sort of clear understanding is needed for successful engineering of related graphene devices.

Josephson junctions can be used to generate terahertz radiation, but achieving high emission power requires phase locking of many junctions in an array. The conventional cavity-resonance (standing-wave) mechanism of phase locking an array becomes ineffective when the extent of the array becomes larger than the wavelength. This study of coherent superradiant emission from such large arrays of Nb/NbSi/Nb Josephson junctions finds evidence for another, nonresonant mechanism of synchronization, via a traveling wave. That mechanism seems to offer the possibility of phase locking very large arrays of coupled oscillators, of any type.

Reconfigurability is a central theme for the next generation of microwave and millimeter-wave optical components. Microwave devices featuring electromagnetic response that can be changed on the fly enable us to reduce complexity, size, weight, power consumption, and cost in applications. To this end, the authors explore the use of spin-orbit interaction to tune the response of a helical tape from that of a transmission line (waveguide) to that of a leaky-wave antenna. This approach to reconfigurability might influence solutions in the next wave of agile technology.

During precision measurement, the quantity being measured is often perturbed by the measurement process itself. This includes precision frequency measurements for atomic-clock applications using Ramsey spectroscopy. To eliminate probe-induced perturbations, a generalized method is developed in which the frequency control loop is augmented with a second control loop that feeds back to a secondary clock variable, and can perfectly compensate for perturbations of the clock frequency caused by the measurements in the first loop. This universal technique can be used for atomic clocks, high-resolution molecular spectroscopy, two-photon probing schemes, Ramsey mass spectrometry, and beyond.

Spin-orbit torque, which can be dampinglike or fieldlike, draws much attention as a possible means of spin switching in devices. Accurate measurement of the fieldlike torque is quite important in identifying good spintronic materials for applications. The authors show that in a ferromagnetic resonance experiment, the magnitude of the current density—and therefore that of the fieldlike torque—can be obtained by measuring the shift in resonance frequency as it is tuned with different direct currents. This method could play a key role in characterizing materials for spin-orbit-torque devices.

The authors present an approach for identifying the point defects and defect complexes responsible for the technology-limiting absorption band in the UV-C range (100—280 nm), and the associated luminescence bands, in single-crystalline AlN. The approach employs analysis via density functional theory with the total-charge-balance constraint in the crystal, by incorporating impurity data from secondary-ion mass spectroscopy and power-dependent photoluminescence excitation spectroscopy, to determine the responsible defects and their thermodynamic transition levels. This methodology may help to solve a key problem in AlN-based optoelectronics, and could be extended to other materials.

The energy dissipation of a single-domain magnetic nanoparticle via its intrinsic damping can be used to achieve hyperthermia in biomedical applications. The authors propose a mechanism to convert the power of an oscillating magnetic field to heat through the energy dissipation of magnetically soft nanoparticles, based on their resonantly excited spin precession. The dissipation rate has its maximum at resonance, where the applied field rotates at the Larmor frequency. This mechanism provides specific loss power 10—100 times that of conventional means, with reliability and control.

Despite their crucial role in organic electronics, direct detection of spins and their correlations and interconversions has remained elusive. The authors show how a simple, commercially available chromophore with dual fluorescence-phosphorescence emission converts spin correlations into spectroscopically distinguishable signals, as monitored directly by single-photon imaging. This technique is used to investigate the magnetic-field effect in OLEDs, and to track singlet-triplet interconversion driven by the hyperfine fields. Such quantitative access to both singlet and triplet excited-state species will help to address the longest-standing questions in OLED photophysics.

Understanding the physics of defective amorphous semiconductors such as indium gallium zinc oxide ($a$-IGZO) is of paramount importance for engineering thin-film transistors for large-area electronics, like your television. This work introduces a first-principles approach for studying defects in amorphous semiconductors, rationalizing the nature of these defects and offering a coherent description of $a$-IGZO that explains its experimentally observed electrical instabilities. The methodology offers a basis for insightful device optimization.

Converting infrared radiation from a thermal source into electrical power, via a $thermophotovoltaic$ device, is important for energy harvesting and micropower applications. The authors present a large-area broadband infrared antenna-coupled tunnel-diode rectifier that directly converts infrared radiation into electrical power. The antenna resonantly enhances and couples IR light to an extreme-subwavelength tunnel barrier, leading to large induced photon-assisted tunneling currents. Peak electrical power is observed when the load resistance is matched to that of the diode. This direct conversion of thermal IR to electrical power using a scalable CMOS process seems quite promising.

Cloaking is very interesting, for magnetic fields as well as light, but standard approaches are held back by technical challenges. This work shows that a diamagnetic active-current boundary combined with a high-permeability magnetic shell can be precisely engineered to overcome the usual permeability and frequency-band limits, for a robust cloak covering the entire quasistatic frequency region, without using superconductors—no cryogenics required. These results provides an efficient way to circumvent the traditional limits of metamaterials and realize magnetic cloaks for ultralow frequencies, and could be generalized to yield other artificial magnetic systems as well.

Efficient, environmentally friendly magnetic refrigeration at room temperature requires a material with a large magnetocaloric effect (MCE) in the desired temperature range. Prototypes already outperform conventional refrigerators, but expanding the operating range remains a challenge, as often the MCE is narrowly limited to the vicinity of the magnetic transition temperature. This study shows that, for a magnetic insulator with two competing phases leading to different transition temperatures, varying thin-film growth parameters can significantly improve the range by tuning the proportion of both phases, leading to nearly temperature-independent MCE over a range of more than 100 K.

Real-time remote measurement of gas concentration is desirable in numerous applications, including environmental and industrial monitoring. Using optical methods, the gas’s absorption spectrum is usually analyzed by fitting with a model spectrum, but high precision and accuracy come at the cost of speed. This study exploits the unique characteristics of an optical frequency comb to obtain high-accuracy spectra of carbon dioxide over a broad range, plus an optimized fitting code, to yield the concentration in less than a second—without sacrificing accuracy (better than 1%) or precision (0.04%). The techniques developed here are applicable to any gas, for nearly universal measurements.

The evolution of atomic structure in phase-change materials is an essential ingredient for designing next-generation nonvolatile memory devices and memristors, yet has remained elusive for decades. This first-principles study offers quantitative analysis to understand the atomic bonding and dynamics in phase-change materials. Phonon softening initiates the dynamic instability, which triggers atomic disorder, and drastic softening of a specific optical mode with temperature destroys the $p$-bonding network. This deeper understanding of the phase-change process is expected to help in improving device performance, and in developing functional memristors.

In recent years, nanowire (NW) mechanical oscillators have emerged as a platform for force and mass sensing, with favorable properties that include a cantilever geometry, low mass, low mechanical dissipation, and high oscillation frequencies. The authors identify strong, highly tunable coupling between a NW’s two orthogonal modes, and use it to drive Rabi oscillations, showing the way to coherent two-mode control techniques for signal enhancement in force and mass sensing. Such techniques could be used to significantly reduce the frequency fluctuations of nanomechanical oscillators, or for innovative forms of noise spectroscopy.

Superconducting artificial atoms are an emerging platform for the study of coherent quantum physics, and for quantum computation. However, their level-transition properties are currently much simpler than those of true atoms, which limits the quantum systems that can be implemented with this hardware. Here researchers use a nonlinear coupling element to engineer the selection rules of a fluxonium circuit, and thus gain access to previously forbidden transitions. Such a technique allows us to expand the set of quantum operations possible using superconducting circuitry, and is necessary for controlling and measuring physically protected quantum systems.

Capacitance-based measurements are key to the study of defects in semiconductor devices, and are used extensively in the field of thin-film photovoltaics. In a multilayered thin-film device, however, the mere presence of the layers can result in capacitance spectra identical to those due to deep defects. This study demonstrates that bias- and illumination-dependent impedance spectra reveal differences between effects from layers and defects. For Cu(In,Ga)Se${}_2$ thin-film solar cells, the dominant “N1” signature is difficult to reconcile with the presence of deep defects, but follows naturally from considering a transport barrier caused by a buffer layer in the device.

The magnetic properties of ultrathin Fe layers are important in spintronics, but their complex electronic and temperature-dependent properties lead to intricate surface effects that are difficult to predict. The authors use a combination of first-principles and spin-dynamics calculations to study magnetism in MgO/Fe/MgO sandwiches. Surprisingly, they find a noncollinear surface spin structure, due to competition between surface antiferromagnetic coupling and Dzyaloshinskii-Moriya interactions. These results reveal the true complexity of MgO/Fe/MgO ultrathin films, with implications for skyrmionic devices.

Tunable transport properties and extremely large magnetoresistance (MR) are the basic requirements of magnetosensor and magnetoactuator applications. Through defect engineering during the growth process, the authors tailor the level of Te deficiency in HfTe${}_{5-\delta }$, a nonmagnetic layered chalcogenide, to obtain a series of compositions with varying electrical and magnetotransport properties. HfTe${}_{4.92}$ presents a MR value of 2.63×10${}^4$%, at 2 K in a 9 T magnetic field. These experiments suggest a viable approach to tuning the MR of similar compounds via defect engineering, to enable superior devices.

Angular dispersion is an intrinsic property of metasurfaces, but its physical origin remains obscure, hindering metasurface design. This work establishes a theory to quantitatively describe such intriguing effects, and verifies it for a typical terahertz metasurface by means of experiments and numerical simulations. With this understanding, the authors propose a design strategy for meta-devices exhibiting incidence-angle-dependent multifunctionalities. As an illustration, they describe a polarization-control device that can behave as a half- or quarter-wave plate under different excitation angles.

In free space, electric and magnetic fields display $duality$ $symmetry$; a system exhibits this symmetry when its responses to electric and magnetic fields are equivalent. This symmetry is broken in most natural materials, but we can create artificial ones that preserve it. This study designs a microscopic core-shell particle with duality symmetry an order of magnitude better than in previous efforts. The improved symmetry persists when several particles are ganged up, so they could serve as elements for effective bulk materials, for $e.g.$ transformation optics, chiral transparency, coherent control of light, or topologically protected propagation of edge states.