Uniform rain does not penetrate homogeneously into dry sandy soil, but rather forms narrow channels that can leave much of the granular bed dry. The authors study this process in detail for both hydrophilic and hydrophobic soils, identify distinct dynamical behaviors, and demonstrate mitigation strategies that effectively improve fluid infiltration and retention. This study provides guidance for control over channeling and wetting under natural as well as artificial conditions.

The authors study the optical properties of self-organized three-dimensional metamaterials, and explain their observations with a simple analytical model. These systems exhibit three outstanding features: a tunable plasmonic response orders of magnitude stronger than previously reported; operation in the visible, rather than infrared or microwave, spectrum; and fabrication via self-assembly, rather than a complicated multistage process such as lithography. Thus such metamaterials potentially provide a practical and attractive avenue for future applications.

In planar ferromagnetic nanowires, domain walls take the form of circulating vortices of magnetism. The authors use micromagnetic simulations to show that if binary data are encoded using the two states of vortex circulation (clockwise or counterclockwise), short nanowire sections and junctions can be made to perform a full range of logic operations. This offers a novel platform for spintronic devices.

The authors study the optical properties of self-organized three-dimensional metamaterials, and explain their observations with a simple analytical model. These systems exhibit three outstanding features: a tunable plasmonic response orders of magnitude stronger than previously reported; operation in the visible, rather than infrared or microwave, spectrum; and fabrication via self-assembly, rather than a complicated multistage process such as lithography. Thus such metamaterials potentially provide a practical and attractive avenue for future applications.

Exotic properties of topological insulators (TIs) and graphene have captivated many condensed matter physicists, but are practical outcomes actually within reach? The authors propose an efficient, beyond-CMOS spin logic platform exploiting the strong exchange coupling between a ferromagnet and the Dirac fermion states of a TI and of graphene. A detailed theoretical analysis illustrates the desired ultralow-power performance under realistic conditions, leading to predictions for practical devices to be controlled by signals as small as atto- (10${}^{-18}$) Joules!

Uniform rain does not penetrate homogeneously into dry sandy soil, but rather forms narrow channels that can leave much of the granular bed dry. The authors study this process in detail for both hydrophilic and hydrophobic soils, identify distinct dynamical behaviors, and demonstrate mitigation strategies that effectively improve fluid infiltration and retention. This study provides guidance for control over channeling and wetting under natural as well as artificial conditions.

Advanced applications often require advanced materials, and predicting the composition-function relationship is a major theme of current research. The authors extend the dilute-defect model to study the effects on band structure and electrical properties of simultaneously incorporating both aliovalent metal cations and isovalent chalcogenide anions into the cuprite structure of semiconducting Cu${}_2$O. A wide range of properties is predicted, including conversion from $p-$ to $n-$type via cadmium doping.

Leakage currents are the bugaboo of thin-film electronics. In this study field-effect transistors, made of either ZnO film or bilayers of tellurium and organic oligomer, are machined using picosecond laser pulses, leaving the SiO${}_2$ substrate beneath unharmed, due to its different optical absorption. This approach (1) provides almost no variation in structure or performance from device to device, (2) enables patterning of novel materials that may be soft or easily damaged, and, critically, (3) drastically reduces gate leakage compared to traditional fabrication techniques.

As the width of a copper interconnect approaches an electron’s mean free path length, its resistivity increases dramatically–a real problem in nanoelectronics. A main cause is electron scattering at grain boundaries (GBs), yet the specific resistivity of a $single$ GB remains unclear in general. The authors develop a fully atomistic first-principles technique to calculate this property, and the result for a coherent twin GB matches experiment well. They furthermore predict the resistivities of other GBs for which experimental data are lacking, and suggest a way to improve interconnect conductivity.

Spintronic devices based on graphene are keenly anticipated for a new generation of technologies operating at higher speeds and smaller scales. Unfortunately, the low efficiency of spin injection from ferromagnetic electrodes into the carbon sheet poses a formidable challenge. Inspired by tunneling transport in everyday semiconductors, the authors calculate that a barrier layer of hexagonal boron nitride, unlike other materials, suppresses graphene’s minority spin channel. This reduces the spin-conductance mismatch between electrodes and graphene, enabling high spin polarization for efficient injection.

To improve the performance of the cerium-activated scintillators widely used in cutting-edge medical imaging, a comprehensive understanding of the role of cerium valence states (especially Ce${}^{4+}$) in the scintillation mechanism is essential. The authors use theory and experiment to establish a model for stable Ce${}^{4+}$ emission under high-energy excitation, thus clarifying how to engineer the response time of detectors for better positron emission tomography (“PET scans”).

Due to the broadband nature of sunlight, solar cell efficiency is restricted by the well-known Shockley-Queisser limit (33% for a single $p-n$ junction illuminated by “1 sun”). A way around this limitation would be lossless conversion of sunlight into monochromatic light. The authors propose a rigorous thermodynamic model for this approach and obtain limits for several cases, including a 1-sun photovoltaic efficiency of 45% with an optimized band gap.

Two-dimensional metamaterials (metasurfaces) are desirable for their ability to modify electromagnetic fields–for example, producing perfect absorbers for stealth technologies. Metasurfaces reported to date, however, have demonstrated only limited control over the polarization and phase of these fields. In this work, the authors incorporate metamaterial anisotropy and chirality into calculations to derive simple, general, closed-form expressions relating constituent parameters of an arbitrary bianisotropic metasurface to its reflection and transmission behaviors.

Metamaterial surfaces (metasurfaces) can generate tailored electromagnetic wavefronts with spatially varying phase and polarization profiles. In this study a common Gaussian beam is transformed into a vector Bessel beam, which is useful for particle trapping, near-field probes, and laser machining. Going forward, the design and fabrication methodology presented here could be used to create efficient metasurfaces even at optical wavelengths, enabling applications such as tractor beams in compact nanoscale devices.

The time-reversal mirror (TRM) has been studied extensively as a means of focusing acoustic or electromagnetic waves in both space and time, without any $a$ $priori$ information about the propagation channel. The authors experimentally demonstrate $nonlinear$ time reversal for efficiently concentrating electromagnetic energy in a complex, lossy model environment. Potential applications include wireless power transfer, secure communications, sensors, and medical therapies.

Magnetic anisotropy is a central concept in spintronics, and while in-plane and perpendicular anisotropies dominate discussion, an arbitrarily tilted anisotropy would allow greater flexibility in designing magnetic storage and logic technologies. The authors directly measure the magnetization tilt angle in a so-called “exchange spring”, and tune the angle by exploiting the system’s competing anisotropies. This level of understanding is a significant step in developing e.g. spin transfer torque MRAM and spin torque oscillators.

Fine control of coherent electron beams is required for experimental progress in quantum optics and electron microscopy, and chip-based technology permits interfacing of various components needed for e.g. quantum information processing. The authors provide a strategy for direct injection of electrons into low-lying motional quantum states of a linear electron guide, an important step toward guided matter-wave interferometry.

Terahertz-frequency light has recently attracted interest across many fields, including biology and medical diagnostics, security and nondestructive testing. Synchronized stacks of superconducting Josephson junctions provide a high-intensity potential source of terahertz radiation; however, designing powerful sources requires a strategy to overcome local overheating that destroys the superconducting property. The authors here find that hot spots can be minimized in these devices, thus substantially increasing the intensity of resulting coherent terahertz waves. They also observe two qualitatively different emission regimes at low and high bias.

Photovoltaic cells are not the only means to harness solar energy. In this study, a model turbine is driven not by motion of a fluid, but by contraction of photoactive liquid-crystal elastomers. By exposing one part of an elastic band to light and keeping the other in shadow, the authors develop both fundamental understanding and practical designs for turbines, and show that in principle realistic systems can directly convert light to mechanical energy at up to 40% efficiency.

The impact velocity of a liquid droplet can be deduced from its spatter pattern, but there has been controversy concerning details of how droplets respond after impact. The authors show which forces play a role in impact and spreading, and obtain an expression relating impact velocity to droplet volume and maximum diameter of the resulting pattern. This offers important insight for the analysis of bloodstains in forensic science, as well as for inkjet printing and other applications.