Most countries allow international inspectors to access their nuclear facilities, to verify that plutonium in spent nuclear fuel is not being diverted to a clandestine weapon program. Unfortunately, the heavily shielded storage casks that protect workers and the public from highly radioactive spent fuel also prevent typical radiographic probes, such as photons or neutrons, from verifying that fuel is actually present in the cask. This paper shows that measurements of the scattering of cosmic-ray muons by spent fuel in casks are sensitive to the removal of fuel, potentially solving a longstanding problem in international nuclear safeguards.

Using an optical frequency comb to directly convert a frequency reference to a useable time output will enable tight synchronization of tomorrow’s optical-clock networks. Previous frequency-comb systems experienced cycle slips too often to provide true optical timescales. This study combines innovative fiber-comb designs, digital phase locking, and precise phase-determination methods to demonstrate optical clockwork with no phase slips for over a $month$. Fault-free timekeeping at the femtosecond level over months is a vital step in supplanting the current microwave time standard with an optical standard, and could improve $e.g.$ gravitational geodesy and GPS technology.

A flexible thin-film silicon solar cell with power conversion efficiency approaching 30% would be a game-changer for the photovoltaics industry. This dream has been considered unattainable, though, due to Si’s indirect band gap. Also, silicon solar cells are typically thick, inflexible, and limited in efficiency by nonradiative charge-carrier losses in the large bulk volume of the cell. This research demonstrates how light trapping based on wave interference in photonic crystals could raise conversion efficiency to ~28%, over a large wavelength range of 300—1100 nm. This would set a new record for silicon-based photovoltaic technology.

Although quantum key distribution (QKD) promises information-theoretic security that can never be hacked, several studies have investigated how its security can be compromised by targeting the detectors in the system. Have they found truly fundamental problems with the physics of the scheme, or merely sloppy implementation? This study seeks to define best-practice criteria for these detectors, to distinguish between genuine loopholes and incorrect operation. The authors show that if these directions are followed, many of the attacks previously demonstrated simply do not work—bringing this technology a big step closer to everyday life.

Strong photon-photon interactions are a key requirement for numerous protocols in quantum computing and communication. Generation of these interactions mediated by atomic vapor in a hollow waveguide has shown great promise, with efficiency enhanced by the tight transverse confinement and extended interaction length in the optical fiber. The authors investigate the strength of such interactions in a series of hollow-core photonic-crystal fibers, and show that they scale only with optical-mode diameter, not mode area (as might be expected). This insight allows targeting of specific photon-photon interaction strengths in waveguide design.

Using an optical frequency comb to directly convert a frequency reference to a useable time output will enable tight synchronization of tomorrow’s optical-clock networks. Previous frequency-comb systems experienced cycle slips too often to provide true optical timescales. This study combines innovative fiber-comb designs, digital phase locking, and precise phase-determination methods to demonstrate optical clockwork with no phase slips for over a $month$. Fault-free timekeeping at the femtosecond level over months is a vital step in supplanting the current microwave time standard with an optical standard, and could improve $e.g.$ gravitational geodesy and GPS technology.

Topological insulators (TIs), ideally insulating in the bulk yet wrapped in metallic surface states, show great promise for $e.g.$ spintronics, aside from hosting exotic fundamental phenomena. Many of their unique properties are fully realized only in the bulk-insulating regime, but most real materials have band structures complicated by defect doping. The authors tailor carrier concentration in Bi${}_2$Se${}_3$ films using $nonvolatile$ electrostatic gating, via the corona charging of electret polymers. Being able to reproducibly shift the chemical potential, without a need for continuous gating, opens vistas for incorporating TI films into devices.

Electric current flowing through a metal can permanently move its atoms, and this $electromigration$ (EM) is important in ultrasmall electronics. To prolong the life of a device, deeper insight into the interplay of multiple transport mechanisms during EM-induced failure is essential. Phase-field modeling is employed to unravel the roles of surfaces and grain boundaries during EM-mediated formation of grain-boundary grooves in nanoscale interconnects. Simulations capture the entire sequence of microstructural evolution of the sample. The scaling laws derived here provide useful guidelines for predicting the lifetimes of polycrystalline interconnects, and for efficient design.

Scattering effects, from $e.g.$ grain boundaries and surface roughness, are significant in ultraminiaturized copper interconnects. $Ab$ $initio$ methods cannot handle structures with thousands of atoms, yet detailed atomistic analyses are needed. This study utilizes semi-empirical tight-binding methods to simulate interconnects with over 10,000 atoms (current industrial scale). Grain-boundary scattering is not assumed to be local, but rather is found to be nonlocal, and strongly dependent on the grain’s crystallographic orientation. Moreover, this work shows that machine-learning methods can create fast, compact models to predict the conductance of a disordered system.

Fundamental research aside, studying three-dimensional topological insulators (3D TIs) is important for realizing advanced electronics, such as dissipationless transistors and spintronic devices. In characterizing 3D TIs, one difficulty comes from the fact that both surface and bulk conduction exist, and are markedly different. When surface conduction dominates, as in SmB${}_6$, characterizing the bulk value is impeded. This study offers a technique to access the bulk conductivity in the surface-dominated regime. This method is expected to see wide use for in-depth transport studies of 3D TIs.

Spin-transfer nano-oscillators (STNOs) have a lot of potential as memory devices or microwave sources, but low output power limits their application. Reliable synchronization of a collection of many STNOs would improve a device’s power. The authors propose a dumbbell-shaped STNO in which the repulsive force between a magnetic domain wall and a skyrmion provides mechanical synchronization, rather than the traditional phase-locking approach. In principle, there is no limit to the number of STNOs that can be synchronized via this mechanism.

Energy harvesting offers the appealing possibility to extract electrical work from ambient energy sources, like waves or human locomotion, but limitations in efficiency, practicality, and generality have thwarted its widespread adoption. This study demonstrates energy harvesting based on a microfluidic realization of an electrostatic $influence$ $machine$—a modern take on an old idea. This technique could reach power efficiencies well beyond those of existing technologies, and the microfluidic context enables straightforward scaling and parallelization. The device concept and initial results presented here might well open a path to practical, battery-free, low-power electronics.

A flexible thin-film silicon solar cell with power conversion efficiency approaching 30% would be a game-changer for the photovoltaics industry. This dream has been considered unattainable, though, due to Si’s indirect band gap. Also, silicon solar cells are typically thick, inflexible, and limited in efficiency by nonradiative charge-carrier losses in the large bulk volume of the cell. This research demonstrates how light trapping based on wave interference in photonic crystals could raise conversion efficiency to ~28%, over a large wavelength range of 300—1100 nm. This would set a new record for silicon-based photovoltaic technology.

Vector vortex beams, a form of structured light, have been applied in many fields, from imaging to materials processing, $e.g.$ for tighter focusing and smaller spots than allowed by diffraction-limited beams. The latter application requires high-power vector beams of good purity (vector quality), a little-explored topic. This work presents a toolbox for studying the amplification of vector beams, outlining how to create these beams with high power and high purity, including the exciting discovery that the vector nature can be not just preserved, but even enhanced, by a judicious choice of medium. This study is a crucial step toward seeing vector vortex beams deployed in industry.

In devices, being able to control magnetism with electric rather than magnetic fields is very appealing, and magnetoelectric multiferroic materials are key to this functionality. ε-FeO is a room-temperature multiferroic featuring both strong magnetization and strong ferroelectricity simultaneously, but the origin of this multiferroicity is unclear. This work shows that the compound’s ferrimagnetism arises from strong spin frustration, and that a low-energy transition state is responsible for switchable ferroelectricity. At a higher level, these results also suggest how to search for other high-performance multiferroics, for use in next-generation data storage.

Integrating plasmonic nanowires into optical fibers yields hybrid fibers with nanophotonic elements, but so far the inevitable imperfections along the nanowires have prevented plasmon propagation over practical distances. This study reveals that the long-range surface plasmon of a gold nanowire inside a step-index fiber does $not$ suffer from significant scattering losses, thanks to a self-recovery mechanism provided by the surrounding fiber core. The ultrafast nonlinearity of gold in the near infrared is measured over propagation distances of more than 1 cm. This advance points to improved fiber-based plasmonics for nanoscale nonlinear light sources.

Most countries allow international inspectors to access their nuclear facilities, to verify that plutonium in spent nuclear fuel is not being diverted to a clandestine weapon program. Unfortunately, the heavily shielded storage casks that protect workers and the public from highly radioactive spent fuel also prevent typical radiographic probes, such as photons or neutrons, from verifying that fuel is actually present in the cask. This paper shows that measurements of the scattering of cosmic-ray muons by spent fuel in casks are sensitive to the removal of fuel, potentially solving a longstanding problem in international nuclear safeguards.

Strain is known to impact the properties of spin-based quantum devices, altering spin-resonance frequencies and potentially affecting device reproducibility. Few studies have been performed to understand the effect of device strains on spins located near the micro- and nanostructures of a practical quantum device. This work uses high-sensitivity superconducting microresonators to measure the spin-resonance spectra of a small ensemble of bismuth donors in silicon. The observed spectrum is unlike that of the bulk, and is reproduced by finite-element strain modeling of the resonator, illustrating the importance of considering strain in device design.

Progress in alkali-ion batteries based on graphite intercalation compounds (GICs) is hindered by a lack of microscopic understanding of the intercalation/de-intercalation process. To fill this gap, the authors use $ab$ $initio$ molecular dynamics calculations to investigate the dynamics of potassium intercalation in finite flakes of graphite. K atoms diffuse rapidly on a flake’s surface, then make an intriguing “U-turn” at the edge and enter the interlayer region, which widens substantially. This atomic-level insight is expected to benefit the development of potassium-ion batteries with high output voltage, energy density, and charge/discharge rate, plus low cost.

Nanostructured materials are key to a variety of applications in electrochemistry, catalysis, and energy storage and conversion. Unfortunately, the microscopic mechanisms of the growth and structuring of cluster-assembled films, which could be used to engineer their structural and functional properties, are poorly understood. In particular, the transition from submonolayer to thin-film conditions has remained relatively unexplored. This quantitative study of the evolution of cluster-assembled zirconia films provides an experimental basis for a deeper theoretical understanding of bottom-up growth processes based on nanoparticle assembly.

Measuring space-charge-limited current (SCLC) is a prevalent method for estimating the charge-carrier mobility in an organic semiconductor. Extracting meaningful results can be complicated, though, since here charge transport is usually limited by traps and injection issues, leading to activation energies that must be accounted for. Typical fitting equations cannot handle this, but this study uses a drift-diffusion model to show that the mobility in a sample system is independent of temperature, field, and charge-carrier density. Alongside numerical results, an analytical model for trap-limited current in a single-carrier device is presented, for high accuracy in SCLC analysis.

Achieving low-emittance electron bunches is a key enabling technology for advanced x-ray free-electron lasers (XFELs), ultrafast electron microscopy and diffraction, and the imaging of vacuum nanoelectronic devices. The authors construct a velocity-map imaging spectrometer that enables measurement of the complete transverse phase space of electron bunches emitted from an arbitrary photocathode. Combining velocity and spatial mapping reveals a cathode’s rms normalized emittance with a remarkable energy resolution of 0.2 meV. This work spans basic physics, engineering, large-scale scientific facilities, and tomorrow’s nanoelectronics.

Routing an electromagnetic pulse around sharp corners without scattering is in great demand for photonic on-chip communication, but is fundamentally difficult, because of the drastic momentum mismatch before and after the pulse turns the corner. In experiments using a special type of photonic crystal on a single metal surface, the authors send surface-wave pulses around multiple 90° bends, with no perceptible scattering. This approach overcomes the longstanding bottleneck of pulse routing, and may find use in large-scale integrated photonic circuits requiring suppressed crosstalk between channels.

Voltage control of magnetism has been advancing rapidly, with ongoing discovery of a family of magnetoelectric phenomena. The authors describe and benchmark an equivalent-circuit model that is applicable to a wide variety of these phenomena. They then use this model to suggest a different mode of operation, in which logic states are represented not by physical states of net magnetization (like $m_x$, $m_y$ or $m_z$), but rather by different ferromagnetic easy axes ($m_x^2-m_y^2$). Here readout is enabled by the reciprocal back-voltage, without the use of magnetic tunnel junctions or magnetic fields. This points to spintronic devices for nonvolatile memory and neuromorphic computing.

Having a wide and robust phononic band gap is important for controlling mechanical waves in a real device that is imperfect or deformable. Band gaps due to Bragg scattering can be wide, but are intrinsically sensitive to imperfections and deformations, while those due to local resonances are robust, but typically narrow. By combining these two mechanisms (with the latter dominating), this study realizes simultaneously large and resilient band gaps, showing that suppression of instability is essential for robustness against deformation. This approach can drive applications such as devices for wave modulation, and thin-layer materials to control noise or vibration.

The high-temperature superconducting (HTS) flux pump, as a wireless superconducting dc power supply, would slash the cost of cryogenics for HTS magnets. This technology is held back, though, by our limited understanding of how ac magnetic-field stimulation induces superconducting dc output voltage. The authors reveal a macroscopic phenomenon in which magnetic poles couple and transport millions of magnetic vortices in the superconductor. This explains the physical origin of the HTS flux pump, and will impact the engineering of HTS magnets for applications such as NMR and MRI instruments, wind turbines, fusion power generators, particle accelerators, and magnetically levitated trains.

“Squeezed” states of the microwave field, which allow one to beat the Heisenberg uncertainty limit for some phase values, find diverse applications in improving quantum measurements. These states can be created using a Josephson parametric amplifier (JPA) operated with a single current pump, but nonlinearities limit the available squeezing. The authors show how to choose pump-tone parameters to reduce this distortion, so that anyone with such a JPA can squeeze harder for better measurements, without needing extra hardware or a more complicated scheme.

The waveguide is an indispensable transmission line for signal processing in high-frequency electronics, such as 5G wireless communication systems in the millimeter-wave region, with the merits of low loss and low crosstalk. It is quite challenging to integrate lumped components ($e.g.$ inductors, capacitors, and resistors) directly inside a waveguide. Inspired by the physics of optical lumped nanocircuits, the authors design a subwavelength-sized integrated circuit residing completely inside a waveguide, using the concept of waveguide metamaterials. Classical circuit theory for low frequencies or dc can be transferred to much higher frequencies, with simple design rules.

Hybrid inorganic/organic semiconductor devices combine and enhance the optoelectronic features of their constituents. To ideally exploit the advantageous properties of this class of materials, extensive parameter studies are needed to explore the operating regimes for optimized device performance. This theoretical study investigates the influence of system preparation and geometry on nonradiative excitation-transfer efficiency across the hybrid interface. The coupling efficiency depends strongly, and not intuitively, on multiple parameters such as resonance-energy detuning, molecular coverage, and charge-carrier concentration and temperature.

Several graphene-based active plasmonic optoelectronic switches have been demonstrated, but have been limited to a single wavelength. This study uses transmission and reflection to propose a multiwavelength optical switch based on a graphene/dielectric/graphene multilayered corrugated structure, exploiting the electrical tunability of graphene. By adjusting the Fermi energy of the graphene layer, high modulation depth at four different wavelengths is possible, with significant implications for active devices such as high-speed modulators for optical telecommunication.

Although quantum key distribution (QKD) promises information-theoretic security that can never be hacked, several studies have investigated how its security can be compromised by targeting the detectors in the system. Have they found truly fundamental problems with the physics of the scheme, or merely sloppy implementation? This study seeks to define best-practice criteria for these detectors, to distinguish between genuine loopholes and incorrect operation. The authors show that if these directions are followed, many of the attacks previously demonstrated simply do not work—bringing this technology a big step closer to everyday life.

Nonvolatile magnetic random-access memory (MRAM) based on perpendicular magnetic tunnel junctions (PMTJs) has been proposed as a universal memory solution for computing, where high thermal stability and low energy consumption are key. Unfortunately, PMTJs using materials with interfacial perpendicular magnetic anisotropy (PMA) cannot provide the required stability and scalability. This study presents a perpendicular structure using the synthetic antiferromagnet Fe-Pd, which possesses an ultralow damping constant and high PMA. This allows PMTJs with 25% room-temperature magnetoresistance that are compatible with 400°C thermal treatment, for integration with CMOS technology.

Fully electrical control of devices is keenly pursued for applications in optoelectronics and spintronics. Inspired by the continual discovery of interesting features in two-dimensional semiconductors, the authors investigate the electronic structure and properties of monolayer GaSe in a transverse electric field. Density functional theory predicts a giant Stark effect, switchable optical anisotropy, and controllable spin-orbit coupling and spin polarization in this group-IIIA monochalcogenide. This study provides welcome guidelines for exploiting GaSe in various realms of device physics.

Compact metasurface devices herald an exciting revolution in optics technology. Their design complexity and functionality has been restricted to intuitive by-hand designs for single-layered devices. This study proposes a large-scale approach known as topology optimization, applied to multiple, closely spaced device layers, which greatly expands the scope and functionality of metadevices. In particular, the authors demonstrate angular phase control, the ability to encode arbitrary information using different angles of incidence, which enables $e.g.$ the design of a one-piece, aberration-corrected metalens, and of an angle-convergent metalens.

In photonics, devices that break time-reversal symmetry are at the leading edge of technology for communication, imaging, and quantum information. Knowing the fundamental rules that lead to nonreciprocal wave transport (NWRT) in the case of periodic (Floquet) driving would allow the design of driving schemes with narrow- or broadband NWRT at predefined frequencies. To this end, the authors provide a theoretical framework that lays out the rules and provides a general recipe for designing these schemes. This development will impact applications ranging from frequency converters to reconfigurable optical, microwave, and acoustic isolators and circulators, and beyond.

The assembly of ultrathin, metastable phosphorus nanotubes into two-dimensional arrays provides many low-energy allotropes, most being energetically more favorable than the experimentally viable “black” and “blue” phosphorenes. In this study, calculations show that two forms in particular are pure elementary piezoelectric materials, with remarkable piezoelectricity greater than that of well-known $h$-BN and MoS${}_2$. These engineered varieties of two-dimensional phosphorus are interesting for potential piezotronic applications, such as nanoscale sensors and energy harvesters.

Light at terahertz frequencies is seen as key to future applications, but the related technology is still at a developmental stage. In contrast to the usual magnon (magnetic collective excitation) possessing an ac magnetic response in the GHz range, magnetoelectric coupling in a multiferroic material provides a resonance with electrical activity in the coveted THz region—an $electromagnon$. This study of multiferroic Y-type hexaferrites demonstrates robust electromagnon resonances and their versatile magnetic control, even at room temperature, thus providing an avenue to THz applications.

A magnetic tunnel junction with a $single$ magnetic electrode, rather than the usual two, would be interesting for magnetoresistance (MR) -based devices in spintronics, but a low MR ratio remains as one of the main obstacles. This study uses first-principles calculations to investigate MPT-TMR, the change in tunneling magnetoresistance due to a magnetic phase transition of the electrode, in an α’-FeRh/MgO/Cu tunnel junction. In this case the MPT-TMR ratio is predicted to be high—up to hundreds of percents. This plus structural simplicity, tunable spin injection with nearly complete spin polarization, and electric-field-driven switching make this system quite promising for applications.

Applications involving magnetostrictive materials require an accurate approach to characterizing their properties. The dynamic deformation induced by an alternating magnetic field—the piezomagnetic effect—is often defined by the strain derivative of the quasistatic magnetostrictive curve. However, this can be inaccurate for “semihard” magnetic materials, such as cobalt ferrite. This study shows that measurements of dynamic magnetostriction are more reliable than strain derivatives in characterizing the piezomagnetic effect. The results give a better understanding of $e.g.$ the magnetoelectric effect in composites, which could be exploited in sensors and energy-harvesting devices.

Physical systems with strongly correlated electrons (fermions) are notoriously difficult to study using traditional computers. Meanwhile, as programmable $quantum$ computers become a reality, efficient (and preferably general) quantum algorithms are needed, in the face of limited qubit-qubit connectivity in the near term. Addressing both problems, the authors develop quantum algorithms that run on two-dimensional qubit lattices with nearest-neighbor interactions, to simulate strongly correlated fermions. Their approach avoids the parity problem in mapping fermionic operators to qubit operators, with hardly any overhead, and can be used for a whole class of problems.

Semiconductor quantum dots (QDs) hold great potential for solid-state light-emitting devices and optical and spin-based quantum information processing, but the efficiency of QD-based spin LEDs remains limited, and their physics not fully understood. Using tunable laser spectroscopy, the authors discover that here an intrinsic obstacle to spin generation is $phonon$ $bottleneck$, a lack of phonon-mediated relaxation pathways that can arise in a zero-dimensional system. This insight provides design rules for energy-level engineering in spin-optoelectronic applications of QDs, yielding about a threefold increase in spin generation efficiency, even at elevated temperatures.

Nano- and microelectromechanical systems (NEMS and MEMS) are interesting as switches for next-generation computing electronics. Unfortunately, repeated switching causes the formation of gunk known as $tribopolymer$ in the junction, which spoils device performance. To understand the mechanism behind this phenomenon at the intersection of physics, chemistry, and electronics, the authors use density functional theory to study the switching of a RuO${}_2$ NEMS in the presence of benzene. Tribopolymer formation involves a chain reaction of an oxidized biphenyl-like structure, with abstraction of surface oxygen, leading to strongly chemisorbed species that continue to polymerize.

Controlling physical properties using light or electric field, rather than magnetic field or electrical current, is of keen interest for tomorrow’s energy-efficient, high-density, nonvolatile memory applications. The authors combine the effects of electric-field-induced ferroelastic strain and photoexcited delocalization for significant tuning of electronic transport in manganite-based multiferroic heterostructures. The strong correlation of these effects is essentially driven by electronic phase separation. These results provide a framework for realizing multifield tuning of phase separation in complex oxides, for advanced device physics.

Should you trust what you see? The authors show that the potentials recovered using distance-based techniques in scanning probe spectroscopy deviate more than previously assumed. To facilitate accurate measurement with efficient data acquisition, they propose modulating the drive amplitude at a constant height from the surface, while monitoring both oscillation amplitude and phase. This approach enables quicker scans with better accuracy, compared to standard techniques, with the further advantage that since no feedback loop is active during the amplitude sweep, the interaction potential can be consistently recovered deep into the repulsive regime.

A central challenge in the burgeoning field of antiferromagnetic spintronics is to find materials with an antiferromagnetic state that can be both controlled and read out. To address this challenge, the authors create artificial La${}_{0.67}$Sr${}_{0.33}$MnO${}_3$/LaNiO${}_3$ superlattices that are $nearly$ antiferromagnetic, featuring a state that can be controlled with a small external magnetic field and read out via resistance measurements. This approach introduces ferromagnetlike controllability in an antiferromagnetic system, in a tunable manner, without sacrificing the advantageous properties (such as low stray fields) of antiferromagnetism.

One of the fundamental limitations in photonics is the lack of a transducer for interconverting optical and electronic information. The experiments in this study demonstrate just such a bidirectional transducer, for measuring and modulating optical fields. The authors go on to propose an approach for reference-free holographic imaging by showing the unique determination of the pattern that maximizes focused intensity at a point as an optical phase conjugation, using the time-reversal symmetry of light scattering. Furthermore, broadband applicability of this approach is demonstrated, at visible and infrared wavelengths.

The nonlinear optical absorbance of conventional materials is very weak, yet its magnitude dominates device performance in, for example, optical limiting and pulse shaping. Therefore, achieving a strong nonlinear optical response is a longstanding goal. The authors propose charge transfer between donor and acceptor materials as a means to greatly enhance nonlinear response, toward the realization of high-performance optical limiters. Excellent agreement between experiment and theory for a test hybrid material validates the idea, and guides the design and fabrication of an actual liquid-cell-based absorptive optical limiter, which outperforms benchmark devices.

The interfacial Dzyaloshinskii-Moriya interaction (IDMI) provides a very important energy contribution in ultrathin-film magnetism, as it favors chiral magnetic structures and nonreciprocity in spin-wave propagation—features that open prospects for spintronic devices, such as magnetic random-access memory. Thus the authors use Brillouin light scattering to measure spin-wave nonreciprocity and investigate the dependence of the IDMI on thickness and annealing in ultrathin films of a Heusler alloy, with various buffer-layer materials. It turns out that the sign and strength of the effective IDMI constant depend on stacking order as well.