Over short (near-field) distances, photon tunneling notably increases the flux of energy between two bodies, compared to what they would exchange over long distances. Because this effect is enhanced if passive relays are interposed, the authors propose a many-body heat engine that should deliver more useful power than its two-body counterpart. Beyond practical interest for energy harvesting, this work and its generalization to $N$-body systems offer a natural platform to investigate the thermodynamics of systems with long-range electromagnetic interactions.

Over short (near-field) distances, photon tunneling notably increases the flux of energy between two bodies, compared to what they would exchange over long distances. Because this effect is enhanced if passive relays are interposed, the authors propose a many-body heat engine that should deliver more useful power than its two-body counterpart. Beyond practical interest for energy harvesting, this work and its generalization to $N$-body systems offer a natural platform to investigate the thermodynamics of systems with long-range electromagnetic interactions.

Nanoparticles, particularly core-shell nanoparticles (CSNPs), offer a wide palette of customizable properties unlike those of their bulk counterparts. Here finite-element modeling shows how the size, morphology, and composition of a semiconducting CSNP can be tailored to tune its optical properties. Large misfit strains are created at the interface of core and shell, yielding a distribution of values for the optical band gap throughout the shell volume—a phenomenon that could be used to engineer solar cells or photocatalysts, for example.

In valleytronics, bits of information would be stored using the so-called valley degrees of freedom available in the electronic band structures of some semiconductors, such as the transition metal dichalcogenides. The authors find emergent two-dimensional excitons as well as quantum confinement of valley electrons to a single atomic layer in bulk noncentrosymmetric MoS${}_2$ with $3R$ stacking. This stacking-engineered single-layer confinement offers intriguing possibilities for dimension-dependent valley phenomena and robust carrier transport in devices.

Progress in spintronics and magnetism requires the ability to measure at the nanoscale, which is challenging. The authors use a single $NV$ center in diamond as an atomic-scale magnetometer to accurately detect the surface density of magnetic moments in thin microstructures with perpendicular magnetic anisotropy. This technique operates under ambient conditions and without an external magnetic field, so signals from magnetic impurities may be excluded, and its spatial resolution of 100$\times$100 nm${}^2$ is a 10,000-fold improvement over that of prior methods.

Free-electron lasers (FELs) have invigorated physics research, but novel sources require novel means of beam characterization as an integral part of any experiment. In this study, desorption imprints in plastic yield transverse intensity profiles of an x-ray FEL beam, from which the authors recover the complex electric-field profile and coherence properties of the focused beam. This approach will inform not only experiments in coherent diffraction imaging, x-ray microscopy, and high-energy-density physics, but also the development of tomorrow’s x-ray laser sources.

Rendering things invisible has turned out to be in the domain of science, not magic. The authors offer an approach to cloaking based on a simple conformal surface wrapped around the object of interest. Their theory shows that by covering the object with a stealth surface on the illuminated side, and a time-reversed version of the same surface on the shadowed side, one can realize ideal cloaking, independent of the size of the object, in a robust, broadband, ultrathin, and lossless technology.

Graphene’s remarkable properties include its outstanding thermal conductivity $\kappa$, and understanding heat flow at its interfaces is key to thermal management of graphene-based electronics. The authors study the ties between interfacial morphology and thermal conductivity and find that, rather than the magnitude of van der Waals interactions, it is uniformity (or lack thereof) in the contact-force distribution that primarily sets $\kappa$ in devices.

Titania (TiO${}_2$) continues to be closely investigated for a host of applications, including the photocatalytic splitting of water (a clean source of H${}_2$ for fuel cells and the “hydrogen economy”). This study reveals the special electronic structure of the (001) surface: The anisotropy of the anatase crystal structure leads to two-dimensional orbital interactions, localized surface states, and facilitated electron-hole separation. This physical insight may help to identify and engineer functionalities of other oxide surfaces as well.

Integrated magneto-optical components require magnetic thin films for tuning of their parameters, and for many applications yttrium iron garnet (YIG) is the material of choice. The authors study high-quality cerium-substituted YIG thin films grown by pulsed laser deposition, which possess tunable magnetic anisotropy and a greatly enhanced magneto-optic Kerr effect, compared to conventional films. This discovery presents new opportunities for engineering tomorrow’s magneto-optical and spintronic logic devices.

Measuring small magnetic moments at the scale of tomorrow’s logic devices poses a great challenge. Magnetic resonance of a spin-1 system (such as the $NV$ defect in diamond) can be used to measure both strength and orientation of an external magnetic field at the nanoscale under ambient conditions, i.e. without cryogenic cooling, but angle resolution decreases rapidly for a weak field. The authors demonstrate that using instead a spin-$\frac{3}{2}$ system (here the uniaxial Si vacancy in SiC) can significantly improve angle sensitivity, even for fields under 1 mT.

Coherent anti-Stokes Raman scattering (CARS) is a vibrational spectroscopic technique offering high sensitivity, chemical selectivity, and three-dimensional visualization—but with spatial resolution limited by the wave nature of light. The authors exploit the saturation of the CARS signal to break that limit, increasing both spatial and spectral resolution and reducing signal background, and thus extending the utility of CARS microscopy in materials science, biology, and beyond.

Although noise is usually undesirable, by means of stochastic resonance it can actually be exploited, for example to improve the performance and switch the functionality of nanoelectronic systems. The authors demonstrate a device that rectifies voltage fluctuations and changes its behavior controllably to act as any of four logic gates. These findings lay the physical groundwork for energy-efficient, noise-tolerant, and autonomous electronics, and the particular device is based on the mature GaAs/(Al,Ga)As system, suggesting straightforward integration with existing technology.

Phosphors and luminophores are essential for applications such as optical displays, white LEDs, fluorescent lamps, medical imaging, and radiation detection. The authors use first-principles calculations to study a family of next-generation scintillator materials and find remarkably different trends in Eu${}^{2+}$ distribution between crystals containing Sr versus Ba. These findings explain recent experimental observations, and the authors predict that mixing these cations will allow better crystal quality and electronic performance.

Any scalable, universal set of quantum photonic logic devices will require on-demand production of deterministic entanglement. A key ingredient to this entanglement is strong photon-photon interactions. Using exposed-core optical fiber, the authors demonstrate strong photon-photon interactions by utilizing a two-photon transition within rubidium. This architecture offers great promise for logic devices requiring two-photon absorption at low power levels.

Spin-orbital interactions are a dominant topic in modern magnetism, and of strong interest for creating and manipulating spin currents in spintronic devices. Here the authors use the planar Hall effect to rectify microwave signals, exciting both oscillatory magnetization dynamics and eddy currents in a thin film of magnetic metal. This permits detection of not only the spatially uniform magnetization precession, but also the dynamics of parametrically excited spin waves associated with phase-correlated magnon pairs. Applications of this effect could include detection and demodulation of microwave signals, and nondestructive testing.

Pulsed-power applications supporting research in fusion, plasma, and particle physics require delivery of high voltages over very short times (nanoseconds, or less)—no mean feat of engineering. The authors study current interruption in a step-recovery diode (SRD), which could be a fast-opening switch for high-voltage pulsed electronics, and find the main governing parameters to be the diode’s reverse current density and base doping concentration. This understanding allows the design of improved structures, taking advantage of modern semiconductor growth technology.

Single- and few-layer graphene have been used as electrodes in organic optoelectronic devices, however limited attention has been devoted to understanding and optimizing the transfer of excited electrons at the organic-graphene interface. The authors find that the charge-transfer rate depends on the number of graphene layers and their stacking, and can be explained by considering the electronic coupling between graphene layers. This work suggests clear ways to control the key process in this class of devices, which may see use as photodetectors or solar cells.

Spin-torque oscillators (STOs) are tunable high-frequency devices well suited for use as resonators, mixers, or magnetic-field sensors. Large-angle oscillations are desired for high output power and operation in small or no magnetic field, but minimizing phase noise is critical. The authors study the effect of current bias on device output in the large-angle regime and correlate the results with nonlinear oscillator theories. The output quality of an STO can depend strongly on the details of the excited orbital motion of the magnetization, providing a handle for optimization.

Double quantum dots (DQDs), sometimes referred to as “artificial molecules”, are highly controllable quantum systems that serve as building blocks for spin-based quantum computation. A fast, high-fidelity charge sensor is needed for determining the number of electrons trapped in the DQD, and for spin-state readout. Here the authors integrate a Josephson parametric amplifier into the readout chain, improving the signal to noise ratio by a factor of 2000 and enabling live tuning of the DQD potential well, without the slow, painstaking adjustment of voltages on several electrodes.

Applications in spintronics require components that create or preserve pure spin currents. The authors’ calculations show that a nanowire built from organometallic (η${}^5$- C${}_5$H${}_5$)Ti(η${}^8$-C${}_8$H${}_8$)Ti clusters is half-metallic, with 100% spin polarization near the Fermi level in its electronic structure. They predict that such a wire coupled to gold electrodes should be multifunctional, which seems quite promising for devices—especially considering that these clusters can actually be prepared in the laboratory.

To compare and improve solar cells, merely knowing their percent efficiencies is insufficient; we need to be able to understand and quantify their different physical mechanisms of loss. The authors use the reciprocity relation between light absorption and emission to explore theoretical and practical performance limits for emerging technologies based on organics and perovskites, and compare them to state-of-the-art systems based on GaAs, c-Si, and CIGS. This study indicates the potential of the newer technologies, and shows the factors that limit present-day performance.

Powerful electric motors like those in hybrid vehicles require strong permanent magnets, mostly based on rare-earth metals that are expensive and of uncertain production and supply. The family of manganese-bismuth compounds has been an attractive, but unstable, alternative—until now. The authors manage to stabilize a member of this family by alloying, showing a viable route for developing this class of rare-earth-free magnets.

Superconductivity at the nanoscale presents tantalizing possibilities, but progress demands actual devices for study. The authors develop a nanofabrication technique to obtain YBa${}_2$Cu${}_3$O${}_{7-\delta }$ structures with gaps as small as 35 nm, yet retaining properties close to those of bulk material. When such a gap is bridged with gold to form an SNS junction, the Josephson effect (current with zero voltage) is observed from 0.3 through 84 K. These results pave the way for more exotic hybrid devices with barriers of graphene, topological insulator, or semiconductor nanowire, to advance basic physics as well as applications.

Incandescent sources typically emit broadband light in all directions. Most of this radiation is lost for applications in the infrared region, such as spectroscopy or compositional analysis. Here the authors control both the spatial and temporal coherence of blackbody radiation with a plasmonic metasurface that emits a narrow band of frequencies in a small solid angle. This system operates reliably at 600 °C using CMOS-compatible materials, inviting the development of compact, efficient, and cheap infrared sources and gas detectors.