The layered semiconductor tin selenide has recently attracted much interest, particularly as a very promising candidate for thermoelectric applications. Unfortunately, natural samples are far from optimal, so the focus has been on understanding the fundamental physics for improvement through guided materials design. This work exploits the rich chemical-bonding characters of SnSe, its unusual multivalley electronic structure, and its strong electron-lattice coupling to engineering the near-edge electronic structure, with an eye toward strain-mediated $active$ control of device properties.

While the power of quantum computers scales exponentially with the number of qubits, harnessing this power is challenging, due to complexity of controlling a large number of qubits simultaneously. The authors show how error correction and logical operations can be performed on an $indefinite$ number of superconducting qubits, by repetition of a unit cell and a fixed set of control components. This solution offers an integrated, basic building block for fault-tolerant quantum computation, and thus is a step forward in addressing the scalability issues in quantum-computer engineering.

It has been said that good solar cells should also be good emitters. Hence, one might expect that a large excitonic radiative quantum efficiency is necessarily beneficial to organic solar cells. However, this turns out not to be simply true. The refractive indices and thicknesses of the layers in the cell also matter—an effect that has been overlooked in the context of solar cells. The authors show that taking this Purcell-like effect into account leads to very different optimal geometries than if one neglects it. Moreover, interesting prospects for gains in efficiency are afforded by properly exploiting this effect.

Metasurfaces offer great opportunities to control light, but so far most “metadevices” work either in pure reflection or pure transmission mode, leaving half of electromagnetic (EM) space untapped. Thus the authors design meta-atoms with polarization-dependent transmission and reflection properties, to efficiently manipulate EM waves in either mode. They fabricate three ultrathin devices with multiple polarization-dependent functionalities and very high efficiencies on both transmission and reflection sides. These findings significantly expand the capabilities of metasurfaces for more demanding and diverse applications.

To meet the challenge of growing high-performance oxide coatings rapidly, plasma electrolytic oxidation (PEO) relies on the control of the size and duration of anodic microdischarges. Counterintuitively, this is done by adding a cathodic current, which has raised questions concerning the influence of electrical parameters, and the possibility of $cathodic$ discharges. Using an approach based on charge-carrier dynamics, the authors predict a threshold frequency above which cathodic microdischarges appear, which may open the door to more efficient PEO.

Over the years, a number of nanoscale electric-field sensors that can image single surface charges have been developed. Due to intrinsic limitations of their electrical or mechanical detection schemes, however, most operate at frequencies below 1 kHz. This study presents a scanning sensor based on an optically active semiconductor quantum dot, combining exquisite sensitivity and nanoscale resolution with high bandwidth. In addition to demonstrating the sensor’s feasibility, the authors lay out the improvements required to take its performance to the physical limit, to measure charging dynamics and individual tunneling events in few-electron and mesoscopic systems.

In this study, nitrogen-vacancy color centers in diamond are employed in a versatile and promising magnetometer, which is useful for sensing fields that are difficult to detect with alternatives. The authors place an untreated, off-the-shelf diamond in a resonant optical cavity with a split-ring microwave resonator to achieve a remarkable magnetic-field sensitivity of approximately 200 pT/$\sqrt{\text{Hz}}$, with room for improvement. These results emphasize enhancing performance through uniformity of spin polarization, rather than focusing solely on optimizing material properties of the diamond sample.

Levitating an object allows to physically decouple it from the environment, affording terrific levels of isolation. The authors show that a small magnet can be stably trapped atop a perforated superconducting sheet. Once levitated, the magnet is extremely sensitive to external perturbations, and thus could be used as an ultrasensitive sensor of force or acceleration. Analysis rooted in current technology indicates that forces and accelerations on the order of 10${}^{-23}$ N Hz${}^{-1/2}$ and 10${}^{-14}$ $g$ Hz${}^{-1/2}$ might be within reach.

Electrical control is preferred in spintronics, but typically does not work for metallic systems, into which an applied electric field cannot penetrate. This study takes a different approach and shows that a metallic system can be manipulated electrically through $ionic$ effects. A ferromagnetic interface so controlled can yield interesting effects, such as independent control of magnetic anisotropy and magnetization, enhanced spin injection, and reduced switching current. This control scheme may also extend to other interfacial systems involving superconductivity, metal-insulator transitions, or spin Hall or spin Seebeck effects.

Superconducting nanowire single-photon detectors (SNSPDs) are important in quantum optics, secure communication, and even medical imaging. An SNSPD exhibits higher detection efficiency when a photon is absorbed near the edge of its wire, rather than in the middle. Combining insight from superconductor physics with nanophotonics, the authors show how to concentrate incident light on the edges of the nanowire, to enhance the efficiency and speed of next-generation detectors. This is expected to be particularly useful in the infrared range, for which there is presently no alternative SPD technology.

The hallmark of (anti)neutrinos is their ability to pass unimpeded through matter, without interaction. Nuclear reactors produce these particles in copious numbers, which improves the odds of detecting them—to the point that they might be used for safe study of working reactor cores. This work shows that, via spectral measurements at several points during the irradiation cycle, a hypothetical antineutrino detector can distinguish between four major core compositions with high confidence. Thus one could verify, for example, the disposal of plutonium from decommissioned weapons.

Manipulating spin current rather than electric current is considered a promising route to ultralow-power “beyond CMOS” devices. The authors propose a graphene-based demultiplexer that enables voltage-controlled distribution and propagation of pure spin current. Gate-driven pure spin current would allow multiple logic functions to be cascaded without spin-charge conversion, for efficient long-distance transport, so this device could be a building block for reconfigurable spin-logic circuits.

The layered semiconductor tin selenide has recently attracted much interest, particularly as a very promising candidate for thermoelectric applications. Unfortunately, natural samples are far from optimal, so the focus has been on understanding the fundamental physics for improvement through guided materials design. This work exploits the rich chemical-bonding characters of SnSe, its unusual multivalley electronic structure, and its strong electron-lattice coupling to engineering the near-edge electronic structure, with an eye toward strain-mediated $active$ control of device properties.

For a complex semiconductor alloy like Cu${}_2$ZnSnSe${}_4$, an emerging photovoltaic absorber, samples prepared by different techniques may yield comparable efficiencies and appear similar when probed by conventional techniques, yet may actually be rather different at the microscopic scale. In this study, laser-induced modification spectroscopy with high spatial resolution and high-temperature capability is shown to be a simple, effective tool to reveal microscopic structural variations. This promotes the investigation and screening of complex semiconductors, to assess the potentials of various technological pathways.

Nonreciprocal devices such as circulators function as one-way gates for microwave light, and thus are necessary components in superconducting quantum circuitry. However, today’s circulators are bulky, lossy, and employ strong magnetic fields, and thus incompatible with scaling up. A proof-of-principle experiment demonstrates gyration in a Josephson circuit, in which microwave signals traveling in opposite directions acquire a 180° phase difference. Inserting this gyrator into one arm of a Mach-Zehnder interferometer would enable a lossless, on-chip circulator with no magnetic materials, which could be used in a variety of applications for quantum information processing.

Is this a new Bronze Age of functional materials? Despite many studies to date, a conclusive atomistic picture of surface phases in the highly miscible Sn/Cu(001) system is still needed. Using density functional theory, the authors focus on the thermodynamics of Sn/Cu(001) structures, comparing calculated STM images of the various phases to extant experimental measurements. The physical insight they provide allows better design and tuning of this alloy’s surface for applications in gas sensing and heterogeneous catalysis, including the electroreduction of carbon dioxide.

Suppressing afterglow is important for extending the application of high-light-yield, high-density metal halides in sensor technology, from homeland security to medical imaging, but is thwarted by our limited understanding of the photophysics involved. This study uses codoping to successfully diminish afterglow in a high-performance halide scintillator. Its experimental results and theoretical calculations show why this approach works: Introducing a small cation to intentionally form a positively charged interstitial can reduce the formation of halogen vacancies, which act as deep electron traps that promote the undesirable afterglow.

The structural and magnetic properties of the full Heusler alloy Co2FeSi are its key features for practical applications, but the exact structure-function relationship and underlying mechanism still remain unclear. In this detailed study, experiments and simulations clarify the precise correlations between magnetic parameters and the degree of structural ordering. Enhancement of both biaxial anisotropy and atomic ordering are realized with increasing substrate temperature. These findings are helpful for designing advanced spintronic devices with controllable magnetic anisotropy.

Micro- and nanoelectromechanical systems (MEMS and NEMS) have clear advantages for space-based sensing and timing applications, but their operation in the face of strong radiation is not well understood. This work describes a mechanism whereby space radiation can impact a semiconductor’s elasticity, and hence an electromechanical device’s operation. It is shown that radiation-induced changes in carrier concentration at the surface result in long-term modification of the material’s mechanical constants—an important source of error for MEMS or NEMS resonators operating in high-radiation environments.

Understanding the influence of microstructure on energy-storage performance in ferroelectric nanocomposites is key to improving the energy density and efficiency of powerful dielectric capacitors. The authors demonstrate that topological vortices in ferroelectric-polymer nanowire composites can yield small remnant polarization and a narrow hysteresis loop, due to the vortex-to-polarization transformation. Phase-field simulations show that this approach of exploiting vortices leads to a large enhancement of energy density and efficiency, even at relatively low electric fields.

Artificial spin ices (ASIs) are two-dimensional arrays of patterned magnetic islands that are promising realizations of reconfigurable magnonic crystals, in which spin-wave band structure can be actively modified for communication and logic applications at microwave frequencies. The authors’ model shows that topological bands exist in square ASIs, and can be toggled by changing the magnetization ground state. Spin-wave nonreciprocity leads to well-defined propagation directions when an in-plane field is applied. These results suggest that propagating edge modes with reduced backscattering can exist, as in topological insulators, and could yield more robust spin-wave-based technology.

Interaction of a micromechanical resonator with a supporting surface is usually seen as an intense source of loss, and thus detrimental. However, in analogy to plasmonics, one can imagine using phononic chains to carry elastic energy in subwavelength structures. The authors show that resonator-surface coupling can be used to manipulate mechanical vibrations, reporting strong confinement of elastic energy in cylindrical pillars with dimensions of 1/10 the excitation wavelength or less. Individual resonators in ensembles can be addressed independently as a function of driving frequency, opening the door to coherent, high-frequency distribution and confinement of microscale elastic waves.

Two-dimensional materials are all the rage, in part because they may be deposited monolayer by monolayer to create custom-built electronic materials not found in nature. For example, van der Waals heterostructures of MoS${}_2$ and ZrS${}_2$ are promising for tunnel field-effect transistors, but what happens to device performance if the loosely bound layers shift? This first-principles study reveals that layer misalignment has a significant impact on tunneling current. Its results stress the importance of considering not just band alignment, but also lattice parameters, when designing such multilayers.

Room-temperature ionic liquids in nanopores are important for a wide range of emerging applications—consider electrodes in batteries, fuel cells, and supercapacitors, for example. Using a coarse-grained model that captures the essential physics, this study finds that nanopores favor adsorption of small ions even without surface attraction, which differs qualitatively from the situation for aqueous electrolyte solutions. Moreover, the imbalanced distribution of cations and anions inside a pore yields a net surface charge, promoting further enrichment of small ions. This self-amplified ion partitioning may open further avenues of research on such systems.

Radio-frequency single-electron transistors (rf-SETs) are very sensitive charge sensors that are promising for spin-state readout in quantum information processing. It would be advantageous to directly couple the rf-SET to a donor atom in a quantum dot, but it is unclear how rf driving of the SET might affect the electronic tunneling dynamics. Combining full counting statistics, tunneling-rate analysis, and autocorrelations, this study shows that rf driving merely introduces an effective temperature broadening. Optimization of single-shot readout fidelity is explained, which keeps this readout technique viable for quantum computing architectures.

Cutting-edge field-effect and thin-film transistors (FETs and TFTs) reportedly offer very high carrier mobilities, but the reliability of these values is controversial. This study reveals the complicated evolution of band bending and actual carrier concentrations in three-terminal devices with non-Ohmic contacts, and its effect on the estimation of carrier mobility. The widely used method is shown to be unreliable: In FETs or TFTs with gated Schottky contacts, mobility can be overestimated by a factor of 10 or more, while in those with resistive contacts it can be underestimated. Fortunately, this analysis also shows how to determine mobilities accurately.

While the power of quantum computers scales exponentially with the number of qubits, harnessing this power is challenging, due to complexity of controlling a large number of qubits simultaneously. The authors show how error correction and logical operations can be performed on an $indefinite$ number of superconducting qubits, by repetition of a unit cell and a fixed set of control components. This solution offers an integrated, basic building block for fault-tolerant quantum computation, and thus is a step forward in addressing the scalability issues in quantum-computer engineering.

Long-range, high-speed spin transport and fast spin manipulation are the keystones of spintronic logic devices with high operation rates. These experiments demonstrate that the spin polarization can be removed from an overlying spin-transport layer in mere picoseconds, by optical manipulation of the degree of confinement of spin carriers in the adjacent layer of a $III-V$ semiconductor stack. The observed functionality of fast, periodic erasure and regeneration of spin polarization makes the proposed semiconductor structure promising for spin-logic applications.

Implementing noise-free quantum devices at room temperature is the key to bring quantum technology from the laboratory to the public. So far this has been impossible, due to the inherent noise in thermal systems. The authors superpose spin waves by mapping photonic polarization qubits onto collective excitations of rubidium atoms. By manipulating the resultant quantum coherence, they obtain a room-temperature high-fidelity quantum memory. Such devices could have great impact as quantum repeaters, the cornerstones of elementary quantum networks.

It has been said that good solar cells should also be good emitters. Hence, one might expect that a large excitonic radiative quantum efficiency is necessarily beneficial to organic solar cells. However, this turns out not to be simply true. The refractive indices and thicknesses of the layers in the cell also matter—an effect that has been overlooked in the context of solar cells. The authors show that taking this Purcell-like effect into account leads to very different optimal geometries than if one neglects it. Moreover, interesting prospects for gains in efficiency are afforded by properly exploiting this effect.

Superconducting microwave circuits are among the most powerful tools for quantum information science and sensor applications. To realize their full potential in hybrid quantum systems and spin-resonance detectors, strategies are needed to allow these circuits to operate unaffected by external magnetic fields. The authors implement two new approaches to reduce the sensitivity of superconducting coplanar microwave resonators to perpendicular magnetic fields. Combining built-in flux screening loops with reduction of geometry-induced field-focusing, the authors demonstrate a significant increase in magnetic-field resilience and stability of resonator properties.

This study presents unusual vacuum photodiodes with rich, interesting photoemission and photoinjection physics, and lots of possibilities for applications. For example, the authors demonstrate a prototype vacuum single-junction solar cell, which produces photocurrent across a wide wavelength range with no bias applied between its electrodes. Also, the injection of very-low-energy electrons into an anode with negative electron affinity yields polarized cathodoluminescence, which suggests using the system as a spatially resolved detector of spin-polarized electrons.

Microwave-stimulated superconductivity is a counterintuitive phenomenon, and whether it affects magnetic-flux penetration via abrupt avalanches—which is harmful to superconducting devices—has remained unclear. This study shows that vortex avalanches triggered in Pb films are efficiently inhibited when the frequency of the microwave stimulus is close to the vortex-depinning frequency. A simple model based on the nonlinear dependence of vortex size explains the effect. These results pave the way to avoiding avalanches in superconducting devices through their nonlinear response.

InAs/In(As,Sb) type-II superlattices are of increasing interest for their practical advantages in device applications, particularly IR detection. This comparative study of highest-quality superlattices grown by different techniques and groups establishes the utility of Raman spectroscopy as an efficient, nondestructive technique for characterizing these systems. It also provides a unified understanding of the mechanism of the appearance of forbidden optical-phonon modes in four quite different categories of superlattices, and even bulk alloys with unintended structural modulations.

Photonic integrated circuits (PIC) are assemblies of optical waveguides and microcavities acting together as sensors, filters, or nonlinear emitters. Proper light injection into or extraction from microcavities requires precise control of the evanescent coupling between the waves circulating in the cavities and waveguides. This research reveals that the strength of the coupling depends on the waveguide-cavity distance in a more complicated way than previously thought, which could affect optimal PIC design, and the coupling varies sharply with environmental parameters, suggesting a new sensing mechanism.

Energy-selective extraction of photoexcited charge carriers is the key feature of a hot-carrier solar cell, but results in detrimental feedback for the electrons and holes remaining in the absorber. This paper highlights the influence of carrier harvesting on the solar cell’s properties via a simple model inspired by evaporative cooling. This approach notably provides a straightforward path to unequivocal demonstration of hot-carrier collection by measuring the cooling—or heating—of the steady-state distribution under extraction. This physical insight should facilitate the engineering of next-generation photovoltaics for clean, renewable energy.

The transparent conducting oxide ZnO is one of the most versatile materials for nanoscale applications, and can be shaped into a variety of structures. Here finite-element modeling is used to characterize band-gap changes induced by elastic distortions in monolithic ZnO nanowires, obtaining good agreement with experiments. Zn-ZnO core-shell nanowires are also proposed as an attractive optoelectronic system, with their size, shape, and morphology optimized for maximum band-gap downshifts.

The so-called skin effect near the surface of a single crystal of relaxor ferroelectric may play an important role in piezoelectric micro- and nanoelectromechanical systems. The authors observe an unexpected transformation from a poling-induced, metastable tetragonal phase to a monoclinic $B$-type phase, which exhibits surprisingly weak bulk piezoelectricity. This discrepancy between microstructure and a macroscale property is understood in terms of a heterogeneous “surface-interior” structure, shedding light on the effects at play in tiny devices based on these materials.

Metasurfaces offer great opportunities to control light, but so far most “metadevices” work either in pure reflection or pure transmission mode, leaving half of electromagnetic (EM) space untapped. Thus the authors design meta-atoms with polarization-dependent transmission and reflection properties, to efficiently manipulate EM waves in either mode. They fabricate three ultrathin devices with multiple polarization-dependent functionalities and very high efficiencies on both transmission and reflection sides. These findings significantly expand the capabilities of metasurfaces for more demanding and diverse applications.

Ferroelectric oxides are studied for applications in solar-energy conversion, but unfortunately most have a band gap of 3 eV or more and thus primarily absorb in the ultraviolet region, which is just 8% of sunlight. Lowering the band gap without spoiling the pivotal ferroelectric properties is a promising, challenging route to devices with meaningful power-conversion efficiency. The authors’ calculations indicate that this goal can be realized in double perovskite ScFe${}_{1-x}$Cr${}_x$O${}_3$. Furthermore, the photocurrent it generates is predicted to be fully spin-polarized over nearly the entire solar spectrum, making this semiconductor intriguing for spintronics.

Ferroelectric nonvolatile memory is of keen interest for information and communication technologies requiring vast storage, but its development has been slowed by size effects that prevent the boosting of storage capacity. This study uses a multilayered structure with three polarization states to increase the storage capacity of a memory cell by at least 50%, without requiring major changes to existing technology. This approach could have a significant impact on the drive toward multibit memory devices based on simple ferroelectric capacitors.

The THz frequency band is in many respects the next technological frontier for ultrafast, broadband wireless communication and ultrasensitive biosensing. Challenging goals in these fields demand fresh approaches in electromagnetism research, such as metamaterials. This paper presents an $experimental$ demonstration of a fully metallic, epsilon-near-zero (ENZ) graded-index metalens working in the terahertz regime. With a narrow quasisymmetric focus, high power enhancement, and a large focal length, this proof-of-concept lens shows the way for ENZ metamaterial devices based on arrayed waveguides.

Neatness counts, even in optoelectronics. Controlling the orientation of dye molecules in an organic light-emitting diode (OLEDs) is a powerful tool to improve its light outcoupling. By aligning transition-dipole-moment vectors parallel to the substrate plane, up to 40% quantum efficiency can be achieved. The authors give a comprehensive review of the physics behind this phenomenon, and highlight prospects for future device designs.