A topological insulator (TI) does not conduct electrons in its interior, but guarantees robust metallic conduction on its surface, with potential applications in spintronics and quantum computing. This study shows that by introducing a layer of strong surface disorder, and patterning that layer’s depth, a TI’s surface conduction can be focused, directed along particular channels, and switched—all of the requirements for a “topological integrated circuit”. The key is tuning the surface-disorder potential by electrically modulating the Fermi level.

The importance of concrete and cement to civil infrastructure cannot be overstated, yet there are still surprising gaps in our knowledge of these systems, which are unexpectedly complex. One basic example is that cement paste shrinks when water is removed from its multiscale pore network, and swells when water is added, but the mechanism is unclear. In a complete reassessment of this phenomenon, the authors differentiate the physical behavior of water in the layered nanostructures of cement hydrates from that in larger gel and capillary pores, linking microstructure to macroscopic material properties.

Concrete is the most used human-made material on earth, the backbone of civil infrastructure, yet the link between the composition of cement paste and its thermophysical properties remains rather obscure. This multiscale study combines statistical physics and mean-field homogenization theory to unravel the relationship between the chemistry of the calcium silicate phases present in the paste and its macroscopic heat capacity and thermal conductivity. These results offer a robust physical basis for engineering thermal and acoustic isolation of buildings, and more durable ceramics and other construction materials.

Biological functional systems generally have been optimized for high performance at low energy cost. Inspired by natural cilia, the authors propose an elastic valve based on induced-charge electro-osmotic flow, and simulate its outstanding performance. This biomimetic device would offer microfluidics applications a short closing time of 5–10 ms at low voltage, and a design that passively blocks reverse flow, reminiscent of the action in veins of the circulatory system.

Chip-based micromechanical devices, already used as compasses and accelerometers in smartphones, are being developed for molecular sensing applications, from point-of-care medical diagnostics to airport luggage screening. The vibrational frequency of a nanomechanical resonator changes when it adsorbs mass (molecules), but when the adsorption is not uniform, the proportionality constant connecting frequency to deposited mass is obscured. The authors solve this problem by taking time-dependent measurements of multiple frequencies, and in proof of principle accurately measure the molecular mass of the explosive RDX.

Room-temperature thermoelectric applications have been impeded by a shortage of suitable materials. This study indicates that electronic anisotropy, in particular a one-dimensional feature of the Fermi surface, allows decoupling of the factors that largely determine electrical transport, elevating the dimensionless figure of merit ZT. The authors predict this electronic structure to occur specifically in the valence band of hexagonal AgBiSe${}_2$, but the concept is quite general and can guide the rational search for even better materials that will drive applications.

In condensed matter physics, the fractional quantum Hall effect is still keenly studied, particularly the $\nu =\frac{5}{2}$ state, long believed to harbor exotic non-Abelian anyon quasiparticles that could be used for quantum computation. Such study, however, requires an ultrahigh-quality two-dimensional electron gas (2DEG) for a test bed. The authors show that careful design and device processing of the host semiconductor heterostructure yields a 2DEG whose density can be widely tuned while preserving a very strong $\nu =\frac{5}{2}$ state, which to date has been a great challenge.

A positive fixed charge at the interface between crystalline silicon and amorphous silicon nitride attracts electrons and repels holes. Although this phenomenon is extensively exploited in high-efficiency solar cells, its atomic-scale origin is not precisely understood. The authors’ high-throughput calculations allow a statistically meaningful analysis of many sample structures, relating the fixed charge to the abundance of $K^{+}$ defects featuring threefold-coordinated Si at the interface. This understanding furthers the quest to improve solar-cell efficiency.

A topological insulator (TI) does not conduct electrons in its interior, but guarantees robust metallic conduction on its surface, with potential applications in spintronics and quantum computing. This study shows that by introducing a layer of strong surface disorder, and patterning that layer’s depth, a TI’s surface conduction can be focused, directed along particular channels, and switched—all of the requirements for a “topological integrated circuit”. The key is tuning the surface-disorder potential by electrically modulating the Fermi level.

Technology to efficiently handle light in the terahertz range is sought for a wide range of applications, including nondestructive testing, medical diagnostics, and weapons detection. Liquid-crystal devices are cheap and promising, but currently suffer from high operating voltage and long response times. The authors exploit coupling between periodically arranged patch resonators and external fields to propose a low-voltage absorber whose reflectance can be modulated from nearly 0 to 90%, with switching times around 50 ms–a dramatic improvement over existing technology that stands to enable new, low-cost, highly tunable terahertz devices.

At the nanoscale, random distribution of impurities causes pronounced device-to-device variability (DDV) in short-channel transistors. This study investigates quantum transport with multiple-impurity scattering in silicon nanoFETs as a function of channel length, doping concentration, and doping profile. The results reveal the microscopic physics of DDV and provide insight for rectifying it, to yield better electronics.

The importance of concrete and cement to civil infrastructure cannot be overstated, yet there are still surprising gaps in our knowledge of these systems, which are unexpectedly complex. One basic example is that cement paste shrinks when water is removed from its multiscale pore network, and swells when water is added, but the mechanism is unclear. In a complete reassessment of this phenomenon, the authors differentiate the physical behavior of water in the layered nanostructures of cement hydrates from that in larger gel and capillary pores, linking microstructure to macroscopic material properties.

Concrete is the most used human-made material on earth, the backbone of civil infrastructure, yet the link between the composition of cement paste and its thermophysical properties remains rather obscure. This multiscale study combines statistical physics and mean-field homogenization theory to unravel the relationship between the chemistry of the calcium silicate phases present in the paste and its macroscopic heat capacity and thermal conductivity. These results offer a robust physical basis for engineering thermal and acoustic isolation of buildings, and more durable ceramics and other construction materials.

Anatase TiO${}_2$ is of great interest for applications spanning photovoltaics, memristor electronics, and catalysis. Among its interesting properties is $d^0$ ferromagnetism: Undoped samples can be intrinsically, strongly magnetic at room temperature, despite a nominally diamagnetic electronic structure. The authors’ refined calculations reveal that cation vacancies in TiO${}_2$ can yield molecular oxygen, which itself carries a magnetic moment and is probably the source of the phenomenon. This explanation may extend to other oxides, and the insight about these defects informs the processing and band-gap engineering of titania.

The recently discovered spin-orbit torque could be used to flip bits in spintronic devices. The authors investigate its less-studied component, the fieldlike torque, and find that with the help of this piece, there is no need for an external magnetic field to achieve the deterministic reversal of magnetization. In principle this solves one of the critical issues in spintronics, and is promising for applications in terms of speed and energy efficiency.

Advanced mesoscale devices require tight tolerances of very fine features. Laser ablation is widely used for this purpose, and extreme-ultraviolet (EUV) lasers in particular allow for a tighter focus and lower-temperature machining than traditional lasers. The authors perform simulations and experiments of laser ablation using an EUV laser, producing high-aspect-ratio mesoscale (100 nm—1µm) features with a high level of control over the ablation profile. This work provides an effective technique to investigate and promote capillary-discharge laser technology for industrial processes.

Acoustic diodes (ADs) have applications ranging from medical imaging to nondestructive testing, but existing ADs change wave frequencies, often leading to poor efficiency. The authors propose a nonlinear AD that preserves frequencies with good forward power transmission, breaking the classical reciprocal-transmission law for linear devices. Its forward, reverse, and breakdown characteristics echo those of electrical diodes, hinting that designs from electronics could be used in acoustics.

Finding solar absorbers that are chemically stable and made of abundant elements will promote the improvement of photovoltaic technology. Perhaps surprisingly, the prototypical Mott insulator LaVO${}_3$ is promising in this arena, as it strongly absorbs visible light and has a suitable band gap. The authors study the optical and transport properties of this material in a working device, discussing the advantages and challenges of using a Mott insulator, and showing the way for further solar cells based on strongly correlated electron systems.

Polariton lasers, which provide low-threshold lasing, are sought as energy-efficient light sources for photonic devices. The authors create deep photonic wires in organic microcavities, altering their angle-dependent emission to facilitate polariton dispersions with coexisting localized and extended states. Selective excitation precisely controls the build-up of coherent modes, providing tunable lasing on the microscale. These experiments are performed at room temperature under ambient conditions, and thus are clearly adaptable to real-world applications.

Molecular junctions have been predicted to be excellent thermoelectric energy converters, but actual devices show poor efficiency and low thermopower. The authors’ model indicates that a different sort of hybrid junction, between a single molecule and a semiconducting nanoparticle, could offer 1000 times the performance, and furthermore would be tunable, offering a pathway to efficient recovery of energy at the nanoscale.

Applications ranging from microphones to AFM transducers exploit the interplay between mechanical stress and electric field arising from structural transitions in certain perovskite oxides. The classic material contains lead, however, and efficient, environmentally friendly alternatives have long been sought. The authors use a challenging technique to visualize the real-time microstructural evolution of a lead-free ferroelectric system, and the multidomain states and transitions they find are expected to inform the search for additional systems, as well as the applications that will rely on them.

Concepts from optics aimed at creating “invisibility cloaks” are being extended to acoustics for soundproofing and stealth technology, with the same goal of passing waves around an object with no perceptible perturbation. The authors employ transformation acoustics to design and demonstrate a broadband, three-dimensional cloak that renders an object in an open cavity with hard boundaries nearly undetectable. Their actual cloak is an acoustic metamaterial made simply of stacked sheets of common acrylic plastic, with millimeter-scale features.

Heterostructures of group-III nitride semiconductors attract considerable interest for optoelectronics applications requiring wavelengths anywhere from infrared to ultraviolet. The authors analyze the electronic structure of nitride-based quantum dots placed inside a nitride-based quantum well, grown along different crystallographic directions. Certain orientations offer reduced intrinsic electrostatic fields and seem promising for devices, with improved carrier capture and radiative-recombination rates.