Airborne nanoparticles may be manipulated with light via $photophoresis$, a thermal force due to uneven illumination of a particle. The authors use a funnel-shaped hollow-core laser beam to trap graphite particles at different heights, depending on their masses. This allows for calibration, and prediction of the trajectories of objects in the optical funnel. Such precise, touch-free positioning could be used to place a biomolecule in the focus of an x-ray free electron laser, for the recording of its time-varying structure in a “molecular movie”.

Every little bit counts toward energy efficiency, including catching what would be lost to stray mechanical vibrations. Current research into harvesting of noisy vibrational energy aims to exploit nonlinear rather than linear system resonances, but identifying a priori the relevant modes for a given system is guesswork. Rather than focusing on specific nonlinearities, the authors study the fundamental limits, develop a framework for simple calculation of these limits, and offer a universal buy-low-sell-high strategy that guarantees the maximum rate of energy harvesting for a generic system. Their approach furthermore opens avenues for future work by connecting this field to control science, information theory, and statistical physics.

Airborne nanoparticles may be manipulated with light via $photophoresis$, a thermal force due to uneven illumination of a particle. The authors use a funnel-shaped hollow-core laser beam to trap graphite particles at different heights, depending on their masses. This allows for calibration, and prediction of the trajectories of objects in the optical funnel. Such precise, touch-free positioning could be used to place a biomolecule in the focus of an x-ray free electron laser, for the recording of its time-varying structure in a “molecular movie”.

To improve battery technology, cathodes based on intercalated Na${}^{+}$ rather than Li${}^{+}$ are closely studied, as sodium is much cheaper, and sodium-ion batteries can be fully drained yet still remain viable. Unfortunately, during cycling Na cathode materials present reversible vacancy-ordering phase transformations that can reduce performance. The authors use high-throughput calculations to study the nature of these transformations in all known Na${}_x$$M$O${}_2$ layered compounds, and their findings may also apply to systems with other alkali ions.

Complex oxides present a rich spectrum of electronic and magnetic properties, of interest for both functional devices and fundamental research. The authors utilize the two-dimensional electron liquid at the LaAlO${}_3$/SrTiO${}_3$ interface as a drain-source channel, plus advanced electron-beam lithography, to create field-effect transistors (FETs) with gate lengths down to 60 nm. Most of their properties are attributable to short-channel effects, as in semiconductor FETs, yet there are several characteristic differences as well.

Applications in spintronics require sources of spin-polarized current that exhibit sharp magnetization reversal, but real electrodes do not always comply—an unmentioned elephant in the scientific room. The authors use magnetic anisotropy to tune a popular oxide system from sharp to gradual switching, their quantitative analysis allowing device output to be evaluated under these two scenarios. The implications of switching mechanism for device performance are shown to be dramatic, and by “naming and shaming the elephant” this work sets the standard for verifying, rather than assuming, electrode behavior.

Generally solar-cell efficiency is spoiled by the grain boundaries (GBs) in polycrystalline absorber materials, but not for the high-performing alloy Cu(In,Ga)Se${}_2$. Stranger yet, in pure CuInSe${}_2$ the $\mathrm{\Sigma }$3 GBs are harmless, but not in pure CuGaSe${}_2$. Subtle differences in electronic structure explain why, and offer clues to improve device fabrication and efficiency.

A laser is a very sharp tool, so to speak, for carving materials at the nanoscale, but the carving involves many poorly understood, nonequilibrium processes. The authors’ investigation combines an advanced experimental technique, using a UV laser to realize much finer structures than possible for a visible beam, with an advanced atomistic-continuum computational method that allows direct comparison of simulations to empirical results. This provides both the tools and understanding needed to develop nanotechnology for diverse ends in e.g. metamaterials, optoelectronics, coatings, and manufacturing.

What if gears could mesh perfectly smoothly and without friction or wear? Allowing adjacent magnetic dipoles to rotate about axes orthogonal to their dipole moments produces special orientations, for which the ground state is a degenerate continuum. In these cases, when one dipole spins around its axis the next dipole follows, without any counterforce, creating a gear train. Different multipoles allow various gear ratios. This is of immediate practical relevance, from micromachines to lab-on-a-chip actuators driven by an external magnetic dipole.

Hafnia is a high-$\kappa$ dielectric material poised to replace silica in next-generation electronics. The authors propose a mechanism for formation of oxygen vacancies and interstitial ions in $m$-HfO${}_2$, aided by electron injection. They find that a preexisting oxygen vacancy can act as an electron trap and facilitate the formation of an O-vacancy/O-interstitial ion pair nearby. This means that injecting electrons into metal oxides can yield correlated oxygen-vacancy clusters, with implications for systems beyond just hafnia, as oxygen vacancies and their aggregates are a recurring theme in current research.

Every little bit counts toward energy efficiency, including catching what would be lost to stray mechanical vibrations. Current research into harvesting of noisy vibrational energy aims to exploit nonlinear rather than linear system resonances, but identifying a priori the relevant modes for a given system is guesswork. Rather than focusing on specific nonlinearities, the authors study the fundamental limits, develop a framework for simple calculation of these limits, and offer a universal buy-low-sell-high strategy that guarantees the maximum rate of energy harvesting for a generic system. Their approach furthermore opens avenues for future work by connecting this field to control science, information theory, and statistical physics.

In metal-insulator-metal devices, such as those sought for use in next-generation resistive RAM (ReRAM), rough electrodes reduce the breakdown voltage. This effect is generally attributed to electric-field enhancement at asperities. The authors show that such enhancement is rapidly mitigated by the generation and migration of charged defects near an asperity, but find that the effect of field enhancement on dielectric breakdown is much smaller than one would expect from electrode geometry alone. This study is a significant step in understanding the role of electrode texture in ReRAM properties, including the scaling effects essential to attaining ultimate performance.

A hybrid quantum network is built of nodes based on different physical systems, each good for a particular task. The trick is, they must all play together nicely, even though their requirements for photonic linking generally do not match. The authors solve this problem by using a conjoined double cavity to create nondegenerate, narrow-band photon pairs, to accommodate the different frequencies and bandwidths that must be meshed. This setup is robust to environmental noise and can link an ion-trap computing node to a solid-state memory node, for example​.

A new microfabrication technique creates bubble-based acoustic swimmers at the 10-µm scale—the size of a red blood cell. The authors use experiments and a three-dimensional analytical model to analyze the efficient propulsion mechanism of these artificial microswimmers. Such active matter could be used to transport a payload within the bloodstream, or to mix fluids in a lab-on-chip device.