“Gummy” metals that are both soft and highly strain-hardening, like aluminum, iron, nickel, and stainless steels, are quite difficult to cut, owing to very large deformation forces and poor surface quality. The authors show how this difficulty, a consequence of unsteady plastic flow, can be overcome using a mechanochemical effect: changes in deformation promoted by a suitable coating. From glues to inks, applying any of a host of household media to the metal’s surface induces a change in flow mode via a local ductile-to-brittle transition, which strongly enhances cutting. These results have wide-ranging implications for industrial machining and forming processes.

The band gaps and band offsets of the transparent conducting oxides Ga${}_2$O${}_3$ and (Al${}_x$Ga${}_{1-x}$)${}_2$O${}_3$ are key parameters in the design of next-generation high-power transistors and solar-blind ultraviolet photodetectors and solar cells. Based on first-principles calculations, the authors determine the mixing enthalpies, band-gap bowing, and band offsets of (Al${}_x$Ga${}_{1-x}$)${}_2$O${}_3$ alloys. The band gap can be tuned across a wide range by changing Al composition, adding great flexibility in device design, while the offset arises mostly from discontinuity in the conduction band.

Quantum computation requires a qubit-specific measurement capability to read out the final states of individual qubits. Promising semiconductor architectures use external readout electrometers, but these could be replaced by more scalable $gate$ $sensors$, based on the dispersive coupling between an electrical resonator and the qubit. The authors present an optimized gate sensor and show that significantly better sensitivity arises from changing circuit topology to enhance the resonator’s $Q$ factor. Here CMOS-based quantum devices achieve charge sensitivity on par with that of the best single-electron electrometers.

Modeling a full photovoltaic device with first-principles simulations is such a tremendous computational task that it has remained out of reach—until now. This joint work between academia and industry combines multiple state-of-the-art methods to enable the simulation of phonon-assisted photocurrent in a realistic device under operating conditions. The fully atomistic calculations include the combined effects of electron-phonon and electron-photon coupling, as well as finite bias and temperature. Excellent agreement with experiment shows that this method could be widely useful for physicists and engineers alike to benchmark tomorrow’s optoelectronic devices.

ZnSnN${}_2$ attracts great interest and has been intensively researched as a nontoxic, earth-abundant photoabsorber for thin-film photovoltaics. However, ambiguity remains concerning the existence of profuse deep defect levels that hinder its energy-harvesting efficiency. Carefully revisiting the point defects, including native defects and impurities, in ZnSnN${}_2$ from first principles, the authors find that all of the defects with deep levels are energetically unfavorable. This makes the compound even more promising for high-performance thin-film solar cells than previously thought.

Although various wavefront-manipulation capabilities have been demonstrated with metasurfaces, both fundamental and practical difficulties remain. This study elaborates on a synergistic approach that combines metamaterials and gratings to achieve complete control of diffraction patterns. Unlike in a metasurface, in a metagrating the number of scatterers is significantly reduced, relaxing fabrication tolerance. Strong control of diffraction with simple excitation, ultrawide bandwidth, and significantly fewer scatterers is particularly interesting at optical and infrared frequencies, for $e.g.$ efficient, reconfigurable antennas in microwave communication systems.

The band gaps and band offsets of the transparent conducting oxides Ga${}_2$O${}_3$ and (Al${}_x$Ga${}_{1-x}$)${}_2$O${}_3$ are key parameters in the design of next-generation high-power transistors and solar-blind ultraviolet photodetectors and solar cells. Based on first-principles calculations, the authors determine the mixing enthalpies, band-gap bowing, and band offsets of (Al${}_x$Ga${}_{1-x}$)${}_2$O${}_3$ alloys. The band gap can be tuned across a wide range by changing Al composition, adding great flexibility in device design, while the offset arises mostly from discontinuity in the conduction band.

Designing mechanical metamaterials is key to $e.g.$ vibration mitigation, wave guiding, lensing, and cloaking, and is mostly based on modeling a single unit cell of an infinite periodic assembly. The resulting dispersion relation is limited to wavelengths longer than the unit cell, but spatial harmonics should arise as well. Using iterative enhancement of inhomogeneous wave correlation, this study predicts and experimentally validates the distribution of energy between wave modes at a given frequency. In $bending$ $wave$ $band$ $gaps$, energy is seen to shift from bending waves to longitudinal waves.

Microfabricated atomic magnetometers are important for fundamental research and numerous applications, including geophysical surveying and biological imaging, as they provide a sensitive, accurate, and portable alternative to standard magnetic sensing. The strong scalability and all-optical nature of the free-induction-decay (FID) magnetometer also benefit applications requiring sensor networks. This study shows the advantages of applying synchronous modulation and differential polarimetric detection in an FID setting, for measuring picotesla-level fields using a miniaturized Cs vapor cell.

A tabletop source of hard x rays with controllable pulse duration would be interesting $e.g.$ for time-resolved nuclear scattering studies, or for using x rays as photonic information carriers, without the need for synchrotron facilities. The authors’ analysis shows that, using an iron borate crystal illuminated by radioisotope emission, it should be possible to produce very short x-ray pulses, with duration determined solely by the magnetic switching time of the iron borate. The suggested technique could have real impact on the development of time-domain tabletop Mössbauer spectroscopy and x-ray quantum optics.

Voltage control of magnetoelectric properties is important for low-power spintronic devices, but voltage manipulation of pure-spin-current transport in spin-caloritronic devices has remained elusive, as there is no proper material system to achieve it. The authors show how to use a Pt/YIG/MgO/PMNT multiferroic heterostructure to control the sensitivity of the spin Seebeck effect by voltage. Electrostrain in the PMNT layer alters magnetic anisotropy in the YIG layer, without suppressing spin current or the inverse spin Hall effect in the Pt/YIG bilayer. This seems very promising for devices employing pure spin current, with no flow of charge.

SiN is an emerging semiconductor for integrated optoelectronics, due to its ultralow loss in the visible region. Developing a high-performance SiN metamaterial lens (metalens) is attractive for on-chip optical devices, but is held back by technical challenges in nanofabrication. The authors report the experimental realization of a SiN metalens that is 1 cm across and 695 nm thick, by means of CMOS-compatible fabrication. With high-quality, wide-angle visible imaging, these results point to the miniaturization of lenses for optical fibers, microendoscopes, and smart phones, as well as applications in all-sky telescopes, large-angle beam shaping, and near-eye imaging.

Exceptional-point (EP) optics allows for innovative devices, such as a sensor based on a whispering-gallery-mode (WGM) microresonator. Meanwhile, WGM-enabled optomechanics has led to important advances, including optomechanically induced transparency (OMIT). The authors show that unconventional EP features can be observed in OMIT: Tuning the relative angle between two external nanoparticles coupled to the same microresonator causes EPs to emerge periodically, strongly modifying both the transmission rate and group delay of the signal, for a slow-light-to-fast-light switch. This approach is a means to engineer optomechanical EP devices for optical communication and signal processing.

Electrical manipulation of magnetic textures, such as magnetic domain walls and skyrmions, is important in the context of spintronics, to use them as information carriers. While motion of a magnetic texture induces an electric current via the spin-motive force, it also inevitably suffers from energy loss via Joule heating. This theoretical study shows that in a magnetic Weyl semimetal, the dynamics of a magnetic texture pumps electric charge that can be captured as a current pulse, with suppressed Joule heating. This idea may foster the design of spintronic devices that are even more energy-efficient.

In multilayers with weak perpendicular magnetic anisotropy, nucleation of magnetic vortex-antivortex pairs at the surfaces is set by topological characteristics, yielding a robust means to control vortex motion in extended films. However, exploiting this effect in digital memory devices would require a suitably broad range of fields supporting guided vortex motion. X-ray microscopy reveals cycloidal domains at the top surface with a stability range of tens of mT, in which propagation of magnetic vortices is effectively guided by the pattern of parallel stripes. This finding is an essential step toward the possible application of moving magnetic vortices in unpatterned films.

“Gummy” metals that are both soft and highly strain-hardening, like aluminum, iron, nickel, and stainless steels, are quite difficult to cut, owing to very large deformation forces and poor surface quality. The authors show how this difficulty, a consequence of unsteady plastic flow, can be overcome using a mechanochemical effect: changes in deformation promoted by a suitable coating. From glues to inks, applying any of a host of household media to the metal’s surface induces a change in flow mode via a local ductile-to-brittle transition, which strongly enhances cutting. These results have wide-ranging implications for industrial machining and forming processes.

The $Casimir$ $force$ arises from vacuum and thermal fluctuations of the electromagnetic field, and becomes dominant at separations of a few hundred nanometers, which is relevant for next-generation devices. The authors study a cyclic process caused by the balance between the light pressure and Casimir forces in a Fabry-Perot microresonator. The suggested Casimir microresonator converts continuous laser light into pulses—much like a traditional chopper in function, yet simpler in form and far, far smaller.

Controlling polarization in ferroelectric oxides has enabled fast switching and low-power operation in metal-oxide-semiconductor field-effect transistors (MOSFETs). Parallel studies on the performance of ferroelectric-polymer-based organic FETs (OFETs), where the polarization direction is rotated by 90°, are lacking. This article examines the effects of external electrical poling on the performance of small-molecule OFETs. The overall transistor properties are enhanced when the ferroelectric layer is vertically poled, and are easily and reversibly controlled by switching the polarization direction. The results highlight further design principles for improving charge transport in OFETs.

Optical frequency combs are widely used in precision metrology, communication, and sensing. Though highly desired for biosensing and interconnection with atomic systems, on-chip generation of combs near visible-light frequencies is severely limited by high losses. This study uses coexisting optical nonlinearities in an AlN microring resonator to generate a near-visible frequency comb with surprising efficiency (up to 22%). The approach could be extended to other frequency ranges as well.

Besides direct applications in quantum computing, as a basic system in circuit quantum electrodynamics a qubit coupled to a resonator provides a platform for other technologies. The authors explore two possible applications: thermometry and memcapacitance. Monitoring the effective temperature here is important, because it may change during qubit manipulation or measurement. Moreover, the same approach enables the authors to address emergent memory devices, such as memristors, memcapacitors, and meminductors. A transmon, being a charge qubit, could be the basis of a memcapacitor for a quantum memory device.

Metasurfaces exploiting optical spin Hall effects present an efficient means to control light, and suggest new functionalities in integrated optics. The authors show that an array of metallic nanoantennas can be used not only to tailor the polarization of light extracted from a waveguide with the direction of the wave, but also to control the number of output directions and their polarizations. This general approach can be extended to various frequencies and applied to other systems like silicon waveguides or photonic platforms, and could be a building block for multiplexing, chiral sensing, and polarization-encoded optical quantum computing.

Filling the “terahertz gap” is a prime technological goal. In this study, a quantum point contact (QPC) operating in the tunneling regime changes its conductivity by more than two orders of magnitude in response to rather weak THz light. This change, caused by photon-assisted tunneling, shows that QPCs under these conditions are good candidates for detectors of THz and microwave radiation. Such detector elements do not require antennas, are small, and are easily fabricated by planar technology, and thus can be handily integrated into optoelectronic circuitry.

The acoustic asymmetric focusing effect attracts more and more attention, with its extensive applications in high-intensity focused ultrasound (HIFU) therapy and medical ultrasound imaging. Typical focusing devices have symmetric structures, though, which do not yield asymmetric transmission simultaneously. This work realizes a broadband acoustic lens using dual-layer metasurfaces for both subwavelength focusing and asymmetric transmission. Without changing the metasurface’s structures, horizontal and vertical focusing are manipulated by mechanically changing the distance between metasurfaces, and the angle of incidence.

Topologically nontrivial states in acoustic systems, characterized by one-way sound transport that is immune to backscattering, have been explored in recent years. This study experimentally demonstrates the acoustic Weyl points and surface Fermi arcs in a $three-dimensional$ phononic crystal. The topological robustness of the surface states is confirmed, and surface dispersions are engineered by tuning the boundary conditions. These findings pave the way for the further development of topological acoustics in three dimensions, and can be used for acoustic spatial filtering, wave guiding, and harvesting of sound energy.

Quantum computation requires a qubit-specific measurement capability to read out the final states of individual qubits. Promising semiconductor architectures use external readout electrometers, but these could be replaced by more scalable $gate$ $sensors$, based on the dispersive coupling between an electrical resonator and the qubit. The authors present an optimized gate sensor and show that significantly better sensitivity arises from changing circuit topology to enhance the resonator’s $Q$ factor. Here CMOS-based quantum devices achieve charge sensitivity on par with that of the best single-electron electrometers.

Inevitable crystallographic defects present one of the most serious problems in semiconductor manufacturing, as they yield undesirable states in the band gap. Knowing the exact position of a deep energy level in the band gap, and its trapping characteristics, can be crucial. Here photo-Hall-effect spectroscopy (PHES) is a straightforward technique, but has languished due to intractable issues like strong recombination of minority carriers. The authors use a more powerful white laser source to overcome these limitations, making a wide set of measurements sufficient for the unique identification of deep-level properties, and thus invigorating the potential of PHES.

The electro-optic Pockels effect, in which refractive index is altered by an electric field, is important for the realization of thin-film modulators in integrated optics and plasmonics, for high-speed signal processing in a small footprint. Progress is thwarted because single-crystalline anisotropic epitaxial materials are thought to be required, but this study demonstrates a significant Pockels effect at the interface of polycrystalline metal and an $amorphous$ spin-coated thin film of dielectric. This approach should have real impact on engineering low-cost CMOS-compatible plasmonic modulators.

Topological magnetic skyrmions are particlelike objects that can be created, moved, and annihilated, making them suitable for information storage and logic applications. Toward this goal, most effort is focused on metallic systems, but here the authors use neutron scattering to study electric field control of skyrmion states in a magnetoelectric insulator. Under electric fields, metastable skyrmion states are created, and switching between competing skyrmion and conical topological phases is demonstrated, as well as the antagonistic interplay of thermal agitation and remnant skyrmion states (important for applications near room temperature).

Two-dimensional (2D) materials are attractive candidates for high-frequency electronic devices, due to their appreciable band gaps and high carrier mobilities and low effective masses. Using a well-calibrated ballistic quantum transport simulator along with simple, intuitive device physics, this study analyzes a large set of 2D materials for rf performance and its dependence on both intrinsic (material) and extrinsic (device) parameters. Phosphorene stands out as the best 2D material in this context. This work not only guides the choice of materials for rf transistors, but also highlights the device parameters critical to experimentally realizing high rf performance.

Organic LEDs present pronounced electrothermal feedback, which is problematic for high-current-density applications such as lighting in the automobile sector, or optical pumping. A model that can describe device behavior over a wide range of current densities and temperatures is, therefore, highly desired. The authors’ approach splits the device into three physically meaningful components, explicitly taking recombination into account. This intuitive picture of the different operating regimes and voltage losses enables the authors to analyze the LED’s nonlinear self-heating, including the characteristic voltage turnover, and narrow down the origins of the strong electrothermal feedback.

Diamond is a promising material for innovative electronic devices, radiation detectors, and integrated platforms for quantum technologies, but with a major hurdle: Deep levels in diamond’s band gap act as charge-carrier traps, causing electric-field inhomogeneities and memory effects. Conventional techniques cannot provide a direct, unambiguous picture of the local field distribution in the defective material. This study use the sensitivity of the native nitrogen-vacancy defect itself to measure the local internal electric field, for a clear view of the inner workings of diamond devices.

Spintronics based on antiferromagnets (rather than ferromagnets) continues to garner intense interest, as antiferromagnets offer no stray fields and ultrafast spin dynamics. However, there is a significant performance gap between antiferromagnetic (AFM) insulators and metals. This study reports the remarkable electronic properties of a series of AFM doped iridates—findings that close the aforementioned performance gap, and furthermore unveil the physics of the coupling between magnetic and charge degrees of freedom in this family of transition-metal oxides.

Modeling a full photovoltaic device with first-principles simulations is such a tremendous computational task that it has remained out of reach—until now. This joint work between academia and industry combines multiple state-of-the-art methods to enable the simulation of phonon-assisted photocurrent in a realistic device under operating conditions. The fully atomistic calculations include the combined effects of electron-phonon and electron-photon coupling, as well as finite bias and temperature. Excellent agreement with experiment shows that this method could be widely useful for physicists and engineers alike to benchmark tomorrow’s optoelectronic devices.

Keen interest has arisen lately in hybrid topological-insulator–superconductor nanostructures, from both the theoretical and experimental points of view. Among recent advances, the proposal for a Doppler-like shift in the energies of the edge states in toplogical-insulator Josephson junctions leads to a peculiar structure of the Andreev bound states along the helical edges. To exploit this effect, the authors propose a structure consisting of a normal-metal probe tunnel-coupled to the center of such a junction, which would function as a sensitive magnetometer.

In attempting to extend the utility of liquid crystals (LCs) in optics, the authors show that the orientation of the electric-field-induced convection patterns of a cholesteric LC can be modified via ultraviolet (UV) irradiation. This tunability is due to a photosensitive chiral dopant, which can change its molecular shape upon illumination; this makes the pitch of the cholesteric helical structure dependent on the intensity of the UV light, while the pattern’s orientation is governed by the thickness-to-pitch ratio. This phenomenon allows the design of innovative photonic devices, such as light deflectors switchable by UV pulses.

Recent progress in superconducting spintronics has highlighted the potential of superconductors (SCs) in low-energy computing technologies. Most studies of quasiparticle spin transport in SCs have relied on dc transport measurements, but this one utilizes the inverse spin-Hall effect induced by spin pumping to investigate spin-orbit coupling and spin transport in Nb, the standard material for applications. The authors not only provide reliable values of spin-Hall angle and spin-diffusion length in Nb, but also suggest a better device geometry for electrical detection of this effect, whether in the normal state or deep into superconducting territory.

The notion of $demagnetization$ is encountered in many areas of science and engineering that involve interactions between fields and finite-sized objects. In magnetism, demagnetizing effects account for the distortion of the magnetic fields around a specimen, and connect induced magnetic moment to the applied magnetic field. The authors derive analytic expressions for the total magnetic moment, and then use numerical finite-element solutions of Maxwell’s equations to find the effective demagnetizing factors for various nonellipsoidal geometries commonly encountered in experiments. This work provides practical guidelines for data analysis for superconducting and magnetic samples.

The two-photon optical transition in rubidium offers a means to a simple, stable optical frequency reference. Here accuracy is limited by frequency drift due to fluctuations in light intensity, while stability is limited by the signal-to-noise ratio of detection. In its traditional implementation there is a tradeoff between accuracy and stability, set by the technical ability to control light intensity. This study uses a two-color excitation scheme for cancellation of light-induced frequency drifts, and replaces fluorescence with absorption in detection, allowing significant improvement of the standard’s stability and accuracy.

Yttria- and scandia-stabilized zirconia are the most promising electrolytes for solid-oxide fuel cells, but a key drawback hindering their wide application is low ionic conductivity at low operating temperatures. As ionic conductivity is closely associated with the dopants and defect interactions in an electrolyte material, the authors theoretically and experimentally investigate the influences of these structural perturbations in Y- and Sc-doped ZrO${}_2$. The atomistic insight obtained should help in engineering solutions to the ionic-conductivity problem at the desired operating temperatures.

Electric field control of magnetism in metallic ferromagnets continues to receive close scrutiny for ultralow-energy spintronic devices. This study uses thermally excited ferromagnetic resonance to investigate the electric field effect on sub-100-nm magnetic tunnel junctions. The characteristic appearance of saturation magnetization, exchange-stiffness constant, and uniaxial perpendicular magnetic anisotropy in the mode-frequency splitting provides an effective means to selectively analyze the electric field effect on the exchange-stiffness constant. This insight is useful for controlling domain size, $T_C$, and other magnetic properties with an electric field.

Being able to generate orbital angular momentum (OAM) of light enables infinite communication channels for either classical or quantum optical communication, in principle, but this neat trick is held back by technical challenges when integrated with other designs. The authors present a physical mechanism to produce OAM-carrying vortex waves via a point defect in a three-dimensional photonic crystal, plus an interesting scheme to convert guided waves in a line defect to localized waves in a point defect, and then to radiated vortex waves in free space. This approach will impact engineering solutions for scalable, on-chip OAM generation.