Designing the next generation of lithium ion batteries requires fundamental information that surprisingly often is still unknown—for example, details of how ions actually move through a working device. The authors use state-of-the-art quasielastic neutron scattering to directly monitor the fast diffusion of Li${}^{+}$ in a promising solid electrolyte. Here the ions migrate between stable regions within a jump length $<l>$ = 4.3 Å along conduction pathways that thread stacks of tetrahedral motifs in the crystal structure. This understanding is key to improved energy storage for applications ranging from vehicles to future smart grids.

In quantum information processing one strives for quantum nondemolition (QND) measurements, which are least likely to perturb the subsequent evolution of the system. The authors show, however, that common superconducting-qubit readout schemes are in fact not QND. They identify a mechanism that induces an effective coherent rotation of the qubit, which means that such a scheme is QND only in the limit of infinite measurement time. This affects any protocol where measurement is used during computation, and therefore is of immediate importance to contemporary experiments. Thankfully, the authors also explain how to fix the problem.

Modern experiments using high-speed, high-impedance devices such as quantum dots can suffer from strong suppression of signal power, if components are not carefully impedance-matched. To overcome this problem, the authors show that a resonant impedance-matching circuit based on superconducting transmission lines can improve the signal to noise ratio by nearly three orders of magnitude. This technology is particularly important to noise studies of electron transport in quantum systems, which can provide information that is unattainable by other means.

The field of fluxtronics is based on transmitting and processing information using quanta of magnetic flux in a type-II superconductor. In this study, temperature is used to reversibly tune the energy landscape of flux vortices pinned in a patterned sample of YBa${}_2$Cu${}_3$O${}_7$, providing an avenue to thermally switchable device functionalities. This nanopatterning method could also be applied to other oxide materials, such as ferromagnets or ferroelectrics, to possibly extend the concept of temperature-dependent functionality.

Interest in the bulk photovoltaic effect (BPE), observed in polar materials such as ferroelectrics, has been rekindled lately, addressing both underlying physics and materials engineering. This study predicts that polar oxides crystallizing in the lithium niobate structure and bearing $d^{10}{s}^0$ cations, such as PbNiO${}_3$, should possess visible-light band gaps and delocalized orbitals that yield a significantly enhanced BPE. These results are promising grist for the mill of solar-energy technology.

Magnetization reversal based on exchange bias is key to the bulk of our digital memory and logic systems, yet the behavior of nanoscale magnetic structures is still incompletely understood. In particular, local exchange bias is important to emerging technologies and is strongly dependent on length scale. The authors combine experiment and modeling to analyze the switching behavior of patterned structures at the uncharted submicrometer scale, to enable the next generation of even denser magnetic media.

Quaternary semiconductors in the kesterite phase are of keen interest for next-generation photovoltaics. For a given chemical composition, several idealized crystal structures are available, with quite different electronic band structures and gaps. Also complicating the picture are imperfections in the actual crystal structure realized during processing. The authors study Cu${}_2$ZnGeSe${}_4$ via ellipsometry and density functional theory, identifying its major optical features and relating them to structural imperfections. This work points the way to improved alloying and band-gap engineering.

Over the past decade, detection of mechanical force has advanced to an astonishing degree, reaching sensitivities of zeptonewtons (10${}^{-21}$ N) or even yoctonewtons (10${}^{-24}$ N). These measurements have involved mechanical oscillations, which intrinsically have a limiting frequency, so very rapidly varying signals cannot be measured. In this work the authors use dipole coupling between spin states, allowing a much faster response of order 1 yN/$\sqrt{Hz}$, with no moving parts.

Designing the next generation of lithium ion batteries requires fundamental information that surprisingly often is still unknown—for example, details of how ions actually move through a working device. The authors use state-of-the-art quasielastic neutron scattering to directly monitor the fast diffusion of Li${}^{+}$ in a promising solid electrolyte. Here the ions migrate between stable regions within a jump length $<l>$ = 4.3 Å along conduction pathways that thread stacks of tetrahedral motifs in the crystal structure. This understanding is key to improved energy storage for applications ranging from vehicles to future smart grids.

When cooled sufficiently, trapped gaseous ions condense to form Coulomb crystals, which have applications in precision spectroscopy and cold-ion chemistry. Progress requires improved methods to simulate and characterize such ensembles, so the authors introduce a method to accurately determine the numbers of ions and their energies of micromotion in very large Coulomb crystals with over 10${}^3$ ions. The agreement of their simulations with experimental observations is compelling.

Optical microcavities are often associated with laser physics, but here the authors show how a Fabry-Pérot microresonator integrated with an atomic force microscope can function as a detector of individual nanoparticles such as semiconductor quantum dots, carbon nanotubes, or biomolecules. Key features of their system include broadband operation, tunable detection frequency, and lateral scanning. This instrument is also promising for applications in quantum optics, e.g. efficient single-photon sources or optomechanical oscillators.

Kelvin probe force microscopy, a variant of atomic force microscopy, is invaluable for measuring electric potential with nanometer-scale spatial resolution, which is important across fields such as nanoelectronics and single-molecule chemistry. The authors improve this technique’s sensitivity tenfold, by measuring not the resonant-frequency shift but the damping of the oscillating cantilever, which is induced by electrostatic force. This uses a much smaller ac voltage, enabling less invasive measurement, which could be vital in studying electrically active materials such as semiconductors.

Detectors in medical and high-energy physics depend on scintillation, the emission of energy from ionizing radiation as visible photons, and technological progress requires improvements in materials. The performance of garnets, for example, can be improved by co-doping, which circumvents defect trap states. This study explains how band-edge engineering via doping can independently tailor either the conduction or valence band. These ideas extend to other complex oxides, and other applications requiring precise control of band gaps and edges, such as the rational design of optoelectronic devices.

Efficient internal mixing of colliding droplets upon coalescence is critical to technologies ranging from ink-jet printing to the reaction of gelled propellants in rocket engines. The authors simulate the head-on collision and subsequent dynamics of equal-sized droplets of non-Newtonian fluids. Shear-thickening fluids allow extended time for mixing, and greater rheological differences between the droplets facilitate internal mixing. This level of understanding is essential for addressing real-world problems in the behavior of complex fluids.

Optical phase-array antennas can be used to control fluorescence from quantum emitters. The intensity distribution and polarization of the resulting emission are determined by the properties of the antenna and the emitters, and the strength of the antenna-emitter coupling. The authors show how Fourier polarimetry can be used to characterize and understand these three contributions, particularly the coupling, in an important step for engineering plasmonic systems.

Metamaterials researchers seek to build on past successes, such as cloaking, by adding control features to artificial electromagnetic media. This study presents a scheme whereby an active metamaterial featuring conventional structures plus a magnetically hard alloy can be “written” (much like the memory elements of a hard-disk drive) to tune its response in real time. The light-matter coupling between the constituents is also relevant to research in quantum information and magnon-based digital processing.

In mesoscopic motion, every little bit counts. The authors show that for microelectromechanical systems (MEMS) with nanoscale separations, surface roughness plays a crucial role in the nonlinear actuation dynamics. A combined analytical and numerical analysis reveals that Casimir-Lifshitz forces can cause chaotic motion and stiction, and thus device malfunction. Notably, the methodology does not require an analytical expression for the Casimir force—or any surface force one might choose to include. These results are of interest in fundamental physics as well as the design of microdevices.

Electronic transport in organic light-emitting diodes (OLEDs) has traditionally been described using one-dimensional models, despite evidence of intrinsic spatial current inhomogeneities. The authors present three-dimensional simulations of several OLEDs to assess the physical factors behind current inhomogeneity, and how this affects OLED efficiency and operational lifetime. For some devices these inhomogeneities can cut performance by an order of magnitude, which makes their understanding, characterization, and control an imperative for future engineering approaches.

The hunt is on for sources of photons to fill the so-called “terahertz gap” in the electromagnetic spectrum, for applications spanning medical diagnosis, imaging, security, ultrahigh-speed communication, and biotechnology. The authors suppress the inevitable self-heating of intrinsic Josephson junctions in Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$, realizing an emitter with strongly expanded tunability and a range from 0.3 to 1.63 THz—and potentially extending to 15 THz, which would cover almost the entire gap with one device.