Conventional solar cells cannot collect energy from abundant midinfrared light, because there are no suitable semiconductors of the right band gap. Hyperbolic metamaterials (HMMs) are proposed to realize broadband, omnidirectional, sensitive diodes operating in this spectral region. The authors show how the slow-light modes supported by HMMs can trap incident radiation in metal-insulator-metal tunnel junctions, for ultrafast optical rectification and photon-to-electron energy conversion that is orders of magnitude more efficient than in conventional optical rectennas.

Thin-film solar cells are at the forefront of current energy research. A detailed theory to define how charges recombine—and so how performance is reduced—at electrical contacts to these films is lacking. This study derives analytic expressions that directly relate the open-circuit voltage to contact, material, and device properties. Drift-diffusion simulations verify their analytical expressions, and agree well with experiments.

Conventional solar cells cannot collect energy from abundant midinfrared light, because there are no suitable semiconductors of the right band gap. Hyperbolic metamaterials (HMMs) are proposed to realize broadband, omnidirectional, sensitive diodes operating in this spectral region. The authors show how the slow-light modes supported by HMMs can trap incident radiation in metal-insulator-metal tunnel junctions, for ultrafast optical rectification and photon-to-electron energy conversion that is orders of magnitude more efficient than in conventional optical rectennas.

The authors show that at low temperature and high magnetic field, thermal spin polarization is the chief source of magnetophotocurrent in organic photovoltaic cells. They find that longer-lived charge-transfer excitons substantially increase magnetoconductance, and their clear explanation of the mechanisms for the high-field effects provides hints for device architecture. This understanding could help to boost the power-conversion efficiency of this class of solar cells beyond the current level of about 10%, and may also impact research on organic spintronics.

Controlling magnetic and electronic properties with electric fields, rather than magnetic fields, is of keen interest for a new generation of energy-efficient, high-speed, nonvolatile logic devices, among other applications. The authors exploit ferroelastic domain switching via the magnetoelectric effect to achieve control of electronic transport and magnetic properties in manganite-niobate-titanate heterostructures. The domain-switching-induced ferroelastic strain is also strongly correlated with magnetic field, which is mediated by an electronic phase separation. These results are important for a mechanistic understanding of the couplings in multiferroic systems.

Graphene is the wonder material hoped to enable the energy-efficient spin-based information processing that could replace today’s electronics. Actually making working devices from this stuff, however, is not trivial. This study presents a milestone in harnessing spins in graphene for computation: The authors have produced one of the building blocks needed to make spintronic integrated circuits.

Circuit quantum electrodynamics is a powerful approach for scaling up the number of qubits in quantum processors, but the standard planar superconducting microwave resonators are poorly compatible with electron-spin qubits. The authors produce resonators that address the two main hurdles impeding the adoption of such a scaling mechanism for spin qubits: They strongly suppress unwanted vortex generation in magnetic fields up to 6 T, and they are projected to yield ten times the coupling of conventional devices.

Thin-film solar cells are at the forefront of current energy research. A detailed theory to define how charges recombine—and so how performance is reduced—at electrical contacts to these films is lacking. This study derives analytic expressions that directly relate the open-circuit voltage to contact, material, and device properties. Drift-diffusion simulations verify their analytical expressions, and agree well with experiments.

Voltage control of magnetic anisotropy (VCMA) in an ultrathin ferromagnet metal layer is a promising approach for ultralow-power spin manipulation, but interface effects spoil its efficiency for applications such as memory devices. The authors study VCMA in the fully epitaxial ultrathin Fe layer of a magnetic tunnel junction, and achieve an efficiency of almost 300 fJ V${}^{-1}$ m${}^{-1}$ with high perpendicular anisotropy, which satisfies the requirements for gigabit-class spintronic memory.

Whispering galley mode resonators have many applications, including photonic circuitry, optical clocks, and sensing. They can be used as magnetometers that operate in ambient conditions and require no cryogenics, but so far they have been limited to sensitivities above 100 nT in the crucial 1–1000 Hz frequency range. The authors engineer a combination of optical and mechanical resonances in such a system to detect kHz magnetic fields with sensitivity around 100 pT. This advance of three orders of magnitude opens the door to applications in geological surveying, medical imaging, and other areas.

Detecting explosives without setting them off is, obviously, a prime goal in security applications. Avoiding detonation requires deep understanding of the complex, spatiotemporal interplay of chemistry and heat transport in heterogeneous solids. The authors combine three-dimensional structural data, dielectric data, and simulations to study hot-spot formation and thermal gradients in energetic mesostructures under electromagnetic radiation. They find that enhanced light absorption at hot spots, below the initiation threshold—“tickling the dragon”—could provide alternative signatures for stand-off detection.

The orbital angular momentum (OAM) of light can be exploited for applications, particularly in high-speed optical communication, and quantum information processing. The authors design and build a liquid-crystal Dammann vortex grating, which can generate a series of equal-energy OAM beams. This technology presents excellent polarization independence, electrical switchability, and tunability, and is promising for OAM generation, manipulation, and detection.

Color centers in diamond provide a promising platform for quantum optics in the solid state, but often their desirable properties are spoiled when they are incorporated into actual devices. The authors demonstrate a simple technique to create centers with uniformly high-quality optical properties, even inside nanoscale devices. Their method may well open the gates for quantum information processing based on this type of hardware.

Like high-rise apartments for bits, stacked structures offer a means to increase the areal density of components on computer chips, despite the physical limits encountered at the nanoscale. The authors demonstrate a series of back-to-back Fe/MgO magnetic tunnel junctions with tunable interlayer coupling and discrete layer-by-layer switching, opening a path to three-dimensional magnetic storage and logic devices.

Feedback control has been used for nearly a century to provide reliable operation of electronic systems. Quantum systems introduce new considerations; in particular, quantum devices intrinsically use very small signals and so are highly sensitive to noise, and they require coherence between an original signal and any controlling input. The author demonstrates an approach that allows minimum-noise amplification even with realistic imperfections, for improved applications in computing, secure communication, and metrology.

Microchannel corners are standard geometric features of planar microfluidic systems and lab-on-a-chip devices, so understanding their influence on flow within the channel is important for device design. The authors show that, depending on system parameters, ejection from a dielectric corner stems from competing nonlinear mechanisms: induced-charge electro-osmosis (ICEO) and electrothermal (ET) effects. Divergence from purely ICEO flow is a result of increasing ET effects due to Joule heating.

Advanced x-ray imaging is promising in a host of contexts, including $in$ $vivo$ biological scans that could dramatically advance many medical treatments. This study presents advances in x-ray computed tomography using speckle vectors, an especially attractive technique requiring minimal instrumentation for high sensitivity and resolution. With just a piece of abrasive paper or a filter membrane as optical modulator, multiple image modalities become available at no extra cost, for a given dose of radiation to the specimen. The schemes described here should promote phase-contrast and dark-field three-dimensional imaging, using either synchrotron or laboratory sources.

In mesoscopic devices, individual quirks plus quantum effects can lead to variable behavior, including the exact onset of the “0.7 anomaly”, an empirical feature seen around 7/10 of the quantum of conductance. The authors present a systematic study of the impact of gate size on electronic behavior, for a large ensemble of devices. A longer one-dimensional channel reduces the conductance value of the 0.7 feature, and, surprisingly, the background impurity potential is at least as significant as gate size in controlling device electrostatics. Such understanding is important for guaranteeing reliable performance of e.g. mass-produced computer chips.

Three satellites can be used for global positioning. Similarly, the authors propose using the spins of three nitrogen-vacancy centers in the tip of a diamond nanoprobe as a multipoint quantum sensor, for fast 3D magnetic resonance imaging with subnanometer resolution. This technique offers fast detection of individual spins under ambient conditions, which would advance single-molecule detection in analytical chemistry and molecular biology, for example.

Stacks of intrinsic Josephson junctions (JJs) in cuprate superconductors are promising emitters of terahertz light, which is sought for many applications in imaging, spectroscopy, and communication. The authors simulate JJ stacks in three dimensions, simultaneously solving a Fickian diffusion equation for heat and coupled sine-Gordon equations for current. They predict, and confirm experimentally, that applying a modest magnetic field to a stack’s long side enhances emission significantly—one more step toward efficient devices.

To optimize photovoltaics, we need a microscopic understanding of the electronic processes in their collector materials, which begins with empirical measurement. Surface photovoltage is of great interest for studying carrier dynamics and lifetime in semiconductors. The authors use pulsed illumination plus Kelvin probe force microscopy to measure surface photovoltage with an order of magnitude decrease in standard deviation. Their time-domain approach avoids crosstalk that could mislead, and offers progress more generally in molecular electronics and scanning-probe microscopy.

Cryogenic superconducting niobium resonators are key to accelerators for free-electron lasers and experiments in particle and nuclear physics. A resonator’s performance is often degraded by a magnetic field that is trapped in it during a quench event. Without a clear understanding of this field’s origin, the problem has been addressed by thermal cycling above the cavity’s critical temperature. Now the authors pinpoint the mechanism responsible for the trapped flux, and show how to fully recover the quality factor while staying $below$ the critical temperature, which is of great practical interest.

Deforming a porous material can squeeze out interstitial fluid, as we know from washing with a kitchen sponge. Many of these materials, including soils, gels, and biological tissues, are soft and can experience very large deformations, which require complex, highly nonlinear mathematical models. The authors discuss an exact kinematic model for large deformations, and show when and how these large deformations change the mechanics of two example problems. Their findings lend a deep physical perspective to applications ranging from biomedical engineering to hydrogeology.

Circuit quantum electrodynamics promises an efficient realization of the quantum computer, but what should the architecture be? The authors demonstrate a system that successfully marries the advantages of two approaches: the integration capacity of a two-dimensional (planar) layout, plus the high quantum coherence of a three-dimensional structure. This approach is practical for a range of applications, including hybrid systems incorporating semiconducting nanostructures or trapped atoms.