They say you don’t get a second chance to make a first impression. At the tender age of two, PRApplied receives an official impact factor.

$Transformation$ $optics$ allows one to design electromagnetic media to modify the trajectory of light in complex ways, as in an invisibility cloak. In this approach, an electromagnetic field undergoes a rotation and scaling, which restricts the achievable functionalities. What if there were another way? The authors present an alternative transformation theory that $preserves$ a field’s orientation and amplitude, allowing different effects and applications, including devices that feel no local electromagnetic force, which would help to shield systems from damage at high energies.

In developing artificial swimmers for use in noninvasive medicine, control of multiple microbots is of paramount importance. This study exploits the nonlinear behavior of a magnetized helix driven by a rotating magnetic field, by considering motors in which helices of opposite handedness dynamically compete against one another. One can design a velocity profile that is nonnegligible only within a chosen interval of operating frequencies, thus providing a selective control mechanism for the active matter. Arbitrarily complex velocity-frequency relationships are possible.

Titanium dioxide is widely studied for multiple uses, including as a photocatalyst with a strong ability to split water. In this work, the (211) crystal facet of anatase-phase TiO${}_2$ is found to be highly reactive in the presence of water. Detailed investigation reveals fourfold-coordinated titanium atoms to be key players in this process. This shows that the (211) surface of TiO${}_2$ is promising for applications in catalysis and photocatalysis, including renewable H${}_2$ production to support fuel-cell technology and the nascent hydrogen economy.

The motion of excitons in organic semiconductors is critical to the performance of organic light-emitting diodes (OLEDs) and organic photovoltaic devices (OPVs). The authors quantify the contributions of energy transfer and diffusion to the migration of excitons through the thin organic film in an OLED. Their results emphasize that an exciton’s $effective$ and $intrinsic$ diffusion lengths can differ strongly, under some conditions. This thorough, comprehensive study will inform rational device design, particularly of white OLEDs for solid-state lighting.

Understanding the pinning of magnetic domain walls is critical to controlling their motion, for switching, logic, and memory applications. Here a ferromagnetic vortex core, monitored by magneto-optical microscopy, is used as an integrated scanning probe to map the pinning potential in a soft magnetic film, with nanoscale resolution, at room temperature. This provides an unprecedented view of the pinning landscape arising from material defects, and will enable the directed optimization of materials processing to achieve the desired domain-wall-pinning behavior.

For the next generation of electronic devices, researchers seek to harness the potential of two-dimensional materials, the most famous being graphene. Unlike graphene, which is a semimetal, black phosphorus is a natural semiconductor with a band gap, requiring no chemical modification or strain. The authors demonstrate voltage control of both carrier mobility and Schottky-barrier resistance in this material at room temperature, the combined effect of which could yield a high-performance velocity-modulated transistor. These results on “wave function engineering” of charge-carrier distribution are a necessary starting point for advanced applications.

Though metasurfaces continue to draw much attention, a comprehensive description of the properties of $magnetic$ metasurfaces is still needed. This is a daunting problem for micromagnetic modeling, due to its multiple length scales from nm to cm, so an analytical approach is sought. The authors develop a scattering-matrix formalism using boundary conditions independent of the coordinate system, and then study the example of efficient reflection from a metasurface inside a microwave waveguide. Their approach should enable much further work.

Controlling switching with an electric field, rather than a magnetic field, is keenly sought for next-generation applications in ultrahigh-density data storage and spintronics. The authors propose using multiferroic tunnel junctions with active magnetoelectric-antiferromagnetic barriers, which would exploit a change in $electroresistance$, rather than magnetoresistance. The underlying physical phenomenon, however, is $not$ the same as the conventional tunneling electroresistance effect seen in junctions with ferroelectric barriers.

Heat-assisted magnetic recording (HAMR) uses the tiny hot spot of a laser to flip bit states, and suitable materials are needed to enable this technology. The authors show that antidots of a ferrimagnetic alloy arrayed atop nonmagnetic holes present an effective bit size of 45 nm, and four magnetic states at room temperature. Key here is the antidots’ magnetic-anisotropy reorientation at just 350 K, whereas a typical HAMR material must be heated above its Curie temperature (~750 K, or higher) to work. These $ferrimagnetic$ nanostructures do not suffer from a superparamagnetic limit, and seem promising for ultrahigh-density data storage.

$Transformation$ $optics$ allows one to design electromagnetic media to modify the trajectory of light in complex ways, as in an invisibility cloak. In this approach, an electromagnetic field undergoes a rotation and scaling, which restricts the achievable functionalities. What if there were another way? The authors present an alternative transformation theory that $preserves$ a field’s orientation and amplitude, allowing different effects and applications, including devices that feel no local electromagnetic force, which would help to shield systems from damage at high energies.

The lineup of moments in an antiferromagnet yields no net magnetization, which means for such a material we expect no useful anomalous Hall effect (AHE) for applications. However, this study identifies an antiferromagnetic compound that, due to its exotic electronic structure, $does$ show a strikingly large AHE, at room temperature. This is significant for creating nonvolatile memory with minimal stray fields, which allows a much higher bit density than is currently possible, and is also of general interest in the study of correlated electron systems.

A typical optical limiter protects sensitive components (such as optoelectronic sensors—or eyes) from a high-power laser by transmitting only low-intensity light, at the cost of absorbing most of the energy itself, which can cause irreversible damage. The authors present a photonic limiter that instead reflects the high-power radiation backward to free space, across a broad frequency range. Their limiter has a much higher damage threshold, and provides much stronger laser suppression.

Understanding the physics behind electric-field-responsive fluids would open the way for advanced hydraulic components like valves, dampers, and clutches, but existing theories do not explain some observations. The authors study the increase in field-dependent yield stress of electrorheological fluids in pressure-driven flow. This increase is tied to the increase in local particle volume fraction, which is dynamically set by the balance of electrostatic forces and shear stresses. The authors’ model allows quantitative design and optimization of valve parameters for robotic systems.

To control one-way propagation of pressure waves in air or water, acoustic diodes typically employ a nonlinear medium to double frequency, or specially designed diffraction structures with which power loss is unavoidable. The authors present a three-stage device that instead takes advantage of the thermal interaction between gas particles and the walls of a porous material with a temperature gradient along the wave path. This thermoacoustic diode $amplifies$ the transmitted sound waves, while blocking backward propagation.

The bit rate for photonic quantum-state transmission can be raised by encoding more than one bit per photon, using multiple temporal, spatial, polarization, and frequency modes and preparing a single photon in a superposition of these modes as a $qudit$. As always, state dephasing limits the performance of such technologies. This study shows that the qudit approach cannot be ramped up to arbitrarily high quantum numbers of orbital angular momentum—but there is a path to passive linear optical elements that can mitigate the inevitable dephasing and keep fidelity high.

Permanent magnets are used to generate magnetic fields in many different contexts, from accelerator and beamline infrastructure to electrical motors to medical devices. How should a limited number of magnets be shaped and laid out to most efficiently create the desired magnetic field? The authors show that in many cases this problem maps to the much simpler one of piecewise-linear approximation of a plane curve by perimeter maximization. The power of this approach is demonstrated with a number of examples.

Light-matter interactions often depend on the light’s state of polarization (SOP), but actually measuring this SOP can be difficult, as the phase information between orthogonal polarization states is completely lost in conventional, intensity-based, detection. Using multiple birefringent plasmonic metasurfaces, the authors propose a compact polarimeter that is well suited for in-plane optical circuitry and allows facile determination of the SOP, as illustrated for the telecommunication wavelength of 1550 nm.

Progress in nanotechnology has led to sensors that can detect very small numbers of analyte particles—down to the level of individual molecules. However, the observed signal often depends on not just how many particles are adsorbed, but also exactly $where$ each has landed on a nanomechanical resonator. This study presents a general theoretical framework for quantitative interpretation of such results, to aid the design of robust sensor platforms for ultrahigh-resolution measurements of chemical concentration, particularly in biosensing applications.

Even as technology based on solution-processable organic semiconductors advances, questions remain about their charge transport. The hallmark of $band$ $transport$ is electron mobility that increases with decreasing temperature. Here transistors based on the polycyclic molecule Ph-BTBT-10, which crystallizes in domains that are hundreds of micrometers long, present mobility that is already high at room temperature and bandlike down to 80 K. This study demonstrates that four-probe measurements are essential to understanding intrinsic transport in these materials.

One of graphene’s interesting properties is that, when pumped by a laser at near-infrared and visible frequencies, it offers amplification at terahertz (THz) frequencies, which are useful for remote sensing in security applications. The authors explain how to take advantage of exotic parity-time ($PT$) symmetry in an active graphene metasurface that realizes reciprocal, unidirectional reflectionless propagation of THz waves. This suggests exciting prospects for detecting chemical and biological agents with ultrahigh sensitivity.

In developing artificial swimmers for use in noninvasive medicine, control of multiple microbots is of paramount importance. This study exploits the nonlinear behavior of a magnetized helix driven by a rotating magnetic field, by considering motors in which helices of opposite handedness dynamically compete against one another. One can design a velocity profile that is nonnegligible only within a chosen interval of operating frequencies, thus providing a selective control mechanism for the active matter. Arbitrarily complex velocity-frequency relationships are possible.

In conventional circuitry, current flows as a stream of electrons, but in $single$ $electronics$ we release just one drop at a time from the tap. The authors propose a single-electron source based on the interplay of Coulomb blockade and superconducting proximity effect that is controlled using magnetic flux, rather than electric field. This device can be termed a $superconducting$ $quantum$ $interference$ $single-electron$ $transistor$ (SQUISET). As a phase-coherent turnstile for individual electrons, it could serve as a building block for a range of quantum circuits

In the rational search for new materials with useful properties, first-principles calculations are used to suggest promising needles in an astronomical haystack of elemental combinations. Alloys can be especially hard to study, as complicated crystal structures with large unit cells mean many atoms must be considered, which can be computationally out of reach. The authors show that orbital-free density functional theory is an efficient, accurate tool for predicting the properties of $e.g.$ complex Mg-Al superalloys needed for advanced engineering.