To realize functionality similar to that of their biological inspirations, advanced neuromorphic systems require massive interconnectivity, extreme energy efficiency, and complex signaling mechanisms. The authors propose an integrated optoelectronic platform combining superconducting electronics with photonic signaling, to enable neuromorphic computing beyond the scale of the human brain.

Microscopic imaging of local magnetic fields provides a window into the inner workings of complex and technologically relevant materials. The authors present an extremely sensitive scanning-probe microscope for imaging charge transport in strongly correlated and topologically nontrivial systems, from ambient down to liquid-helium temperature. This scanning quantum cryogenic atom microscope (SQCRAMscope) will change research spanning disciplines from basic condensed matter physics to the engineering of hybrid quantum systems.

Modes are the universal language of resonant metamaterials, yet our understanding of them is limited. It is surprisingly difficult to define them for such strongly scattering objects, as there are practical difficulties in performing calculations with diverging fields. This work shows how to calculate these modes in a robust manner, and how they describe the physics of the dielectric element. Interference between modes is key to understanding, and engineering, the spectra of such meta-atoms.

Profound control over the absorption and emission properties of structured materials is of interest for a range of applications, from photodetectors to infrared sources and radiative cooling. The authors present a broadband, nearly unidirectional absorber/emitter platform that relies on highly asymmetric coupling to cascaded phonon-polariton resonances of SiC micropyramid building blocks. Though reststrahlen-band materials have been little explored due to their near-perfect reflecting properties in bulk form, this study suggests they could be promising for photonic applications.

Thermally activated delayed-fluorescence emitters attract considerable attention for highly efficient, low-cost organic light-emitting diodes that do not rely on heavy metals. The decay mechanism of excited states in these emitters is generally understood in terms of intersystem crossings between the lowest singlet and triplet excited states. However, the authors show that the second-lowest triplet excited state also plays an important role, and that the singlet-triplet energy gap cannot be determined accurately without considering it.

Harvesting waste heat could yield a $tremendous$ amount of useful energy. Unlike a thermoelectric module, a thermophotovoltaic device converts heat to electrical energy through photon emission and capture. Starting from the viewpoint of coupled-mode theory, this study proposes device configurations that more than double the near-field radiative power transfer from emitter to photovoltaic cell.

Magnetic tunnel junctions (MTJs) are leading candidates for memory cells in spin-transfer-torque magnetic random-access memory, and to that end larger perpendicular magnetic anisotropy and smaller Gilbert damping $\alpha$ are sought. The authors’ calculations show that in Ag/MgO/Fe MTJs the decrease in $\alpha$ with increasing MgO thickness, previously observed in experiments, is due to MgO’s suppression of spin pumping upon insertion at the Ag/Fe interface. Other interfacial effects are also investigated.

Random lasing occurs in disordered systems with optical gain, without a need for periodic geometry or a carefully aligned cavity. The authors exploit this phenomenon to demonstrate a sensing scheme based on modification of the gain by the environment’s $p$H. This technique’s simplicity and sensitivity (200 times that of an otherwise identical fluorescence-based sensor) make it promising for biosensing applications, as it opens a path from nanophotonics to medicine.

Modes are the universal language of resonant metamaterials, yet our understanding of them is limited. It is surprisingly difficult to define them for such strongly scattering objects, as there are practical difficulties in performing calculations with diverging fields. This work shows how to calculate these modes in a robust manner, and how they describe the physics of the dielectric element. Interference between modes is key to understanding, and engineering, the spectra of such meta-atoms.

At base, creating better lithium-ion batteries is all about understanding the diffusion of Li${}^{+}$, particularly its reversible intercalation in materials like olivine phosphates. This study shows that for a given diffusion path for Li${}^{+}$, the forward and backward activation barriers can be manipulated simply by choosing the level of doping $x$ in LiFe${}_{1-x}{M}_x$PO${}_4$. This simple knob can tune three of the most important electrochemical parameters: diffusivity, operating voltage, and band gap. The insight from this study should also apply to other transition-metal oxide cathode materials.

Finding ways to tune their states is the key to using correlated electron systems in device applications. The authors show how to cleanly generate an extremely oxygen-starved environment, which leads to the collapse of the insulating ground state in VO${}_2$, a prototypical correlated oxide. A nonvolatile metallic phase with unique optical properties is stabilized down to 2 K by oxygen vacancies, and the insulating phase plus intermediate-resistance states can be accessed reversibly via “oxygen breathing”.

The special electronic properties of a two-dimensional material are sensitive to its environment. Thermal annealing (on the path to making a device, say) can change not only the monolayer itself, but also its bonding to the substrate—yet what actually goes on during this process is rarely known. The authors continuously monitor $in$ $situ$ the changes in the optical properties of monolayer MoS${}_2$ on SiO${}_2$ during consecutive thermal cycles, revealing structural evolutions at different stages, as well as the spatial inhomogeneity of the material in response to annealing.

In an optical vortex beam, each photon carries orbital angular momentum (OAM), a useful resource for applications in optical communication and quantum information processing. The authors demonstrate a diffractive bifocal vortex lens that generates and sorts light beams of different OAM, by means of polarization control. Placing the lens inside a cavity, a vortex-beam laser with a chosen OAM can be realized. Moreover, the lens’s OAM sorting can be used in a multifocal optical-trapping system that facilitates the manipulation of nanoparticles, molecules, and biological samples.

Electronic applications of transition-metal dichalcogenides (TMDs) might require $p$-type or $n$-type doping. Heavy doping of TMDs also facilitates the formation of low-resistance ohmic contacts, which is essential for both device applications and fundamental study of electrical transport in these materials. The authors succeed in synthesizing WSe${}_2$ samples substituted with Nb ($p$-doped) and Re ($n$-doped), systematically characterize their electrical properties, and finally combine them to demonstrate an unconventional $p-n$ diode.

Conventional perovskite oxides such as Pb(Zr,Ti)O${}_3$ feature octahedrally arranged oxygen atoms. Ferroelectricity in these materials is well studied, while in compounds with $tetrahedral$ oxygens it is little explored, even though these compounds are naturally abundant and could be the basis for environmentally friendly ferroelectric devices. Through experiment and calculations, the authors explain how the improper ferroelectricity in such an oxide offers excellent performance for pyroelectric energy harvesting.

To realize functionality similar to that of their biological inspirations, advanced neuromorphic systems require massive interconnectivity, extreme energy efficiency, and complex signaling mechanisms. The authors propose an integrated optoelectronic platform combining superconducting electronics with photonic signaling, to enable neuromorphic computing beyond the scale of the human brain.

Despite the prevalence of superconducting nanowire single-photon detectors (SNSPDs) in quantum information processing, optics, and plasmonics, their modeling is based on phenomenological assumptions. This study presents a theory of single-photon detection by a superconducting strip with impurities, based on the dynamics of the electron and phonon distributions. Depending on the strip’s material parameters and width, either a hot belt or hot spot forms, and for the latter single-photon detection is possible even in a wide strip at high current, pointing toward even faster devices.

Voltage control of magnetism is a promising route to energy-efficient memory and logic devices. Cr${}_2$O${}_3$ is a magnetoelectric antiferromagnet that allows for voltage control of magnetic states, and when it is included in layered structures, exchange bias can abruptly disappear. Experiments reveal that this is due to competition between interfacial coupling and the tendency of spins in chromia to align along the crystallographic $c$-axis. This insight allows the tuning of exchange bias at conditions favorable for spintronic applications.

Optical aberrations in light-pulse atom interferometers are a major limitation in the accuracy and stability of these quantum sensors. In a proof-of-principle experiment, the authors use a deformable mirror to actively control the laser wave fronts in a cold-atom gravimeter, compensating for the distortions induced by optical elements. This fine control of wave fronts opens perspectives for atom interferometry, with applications in navigation, space physics, and high-precision metrology.

Building on recent theoretical insight, the authors predict that ultrasound can be used to pattern and manipulate solute concentration fields, with excellent spatiotemporal control. This technique could be used, for example, to create layers of solution stratified by concentration, or for acoustic tweezing of a region of high concentration in a relatively dilute medium. In the context of lab-on-a-chip engineering, these results pave the way for a class of manipulation methods analogous to those already developed for suspended microparticles.

To produce better solar cells, deeper knowledge of the physics of photovoltaic absorbers (particularly polymer-fullerene systems) and refined understanding of the experimental characterization of these systems are needed. The authors present a simple model to interpret capacitance-voltage measurements of organic solar cells under illumination. Their discussion of the model’s assumptions, limitations, and pitfalls, and demonstration that the charge-carrier mobility can be obtained from data taken under reverse bias, should be of great practical interest to engineers and physicists alike.

An exciton-polariton is a hybrid quasiparticle, a photon plus an electron-hole pair in a semiconductor, that combines propagation at nearly the speed of light with strong interactions. The authors give experimental proof of principle for the guided motion of polaritons through $III$-nitride heterostructures, which could lead to active all-optical devices operating in the strong-coupling regime $at$ $room$ $temperature$. As such, this study is a promising step toward the practical realization of integrated polaritonic circuits.

Doping is a means to tune the switching properties of oxide-based RRAM and CBRAM nonvolatile memory. The authors calculate the effects of 50 dopant elements on the resistive switching of hafnium oxide, then synthesize a model for predicting dopant behavior. Their general guidelines for engineering transition-metal oxides will inform the development of tomorrow’s digital storage devices.

The authors show that for $any$ hollow photonic crystal fiber filled with a gas, stimulated Raman scattering can be impaired by coherent gain suppression, due to strong coupling between the interacting optical fields. This effect is worse at higher pumping intensities and, counterintuitively, higher frequencies. These results are important for designing fiber-based Raman shifters, amplifiers, and frequency combs, especially in the ultraviolet region, where the Raman gain is high.

Spin-transfer-torque magnetic random-access memory (STT-MRAM) is one of the leading emerging technologies for nonvolatile memory. Key to its development is identifying magnetic materials with sufficient perpendicular magnetic anisotropy (PMA). The authors describe the general mechanism of tetragonal distortion in Heusler compounds, a class of promising materials for STT-MRAM, with over 2000 members. This study offers guidance for finding tetragonal Heusler alloys with high PMA.

Perpendicular magnetic anisotropy at metal/oxide interfaces continues to receive great interest, from the viewpoints of both fundamental research and spintronic applications. In a detailed investigation of the magnetic properties of Pt/Co/AlO${}_x$ sandwiches, unusual phenomena are observed, including reorientation of anisotropy as a function of temperature, and the onset of exchange bias associated with very large coercivity and anisotropy field at low temperature. These are interpreted in terms of chemical bonding and varying material phases in the trilayers.

Ultrasmall-angle x-ray scattering (USAXS) is sensitive to microstructures that are smaller than the imaging system’s resolution, and therefore inaccessible via other contrast mechanisms. This comparison of two phase-contrast imaging techniques focuses on their differences in the retrieved absorption and USAXS signals. The results indicate that the retrieval method for speckle tracking needs revision, while beam tracking extracts the correct signal. This is a key result for implementing the beam-tracking method with laboratory (not beamline) x-ray sources, for tomographic applications from materials science to medicine.

The lack of a means for $direct$ characterization of the pyroelectric and electrocaloric effects, particularly in ferroic thin films, limits our understanding of the electrothermal physics essential for applying these materials in areas such as energy conversion, solid state cooling, and sensing. The authors develop a direct approach to probe polarization-heat interactions in pyroelectric and electrocaloric thin-film capacitors, including measurement of pyrocurrents < 10 pA and electrocaloric temperature changes < 2 mK.

Microscopic imaging of local magnetic fields provides a window into the inner workings of complex and technologically relevant materials. The authors present an extremely sensitive scanning-probe microscope for imaging charge transport in strongly correlated and topologically nontrivial systems, from ambient down to liquid-helium temperature. This scanning quantum cryogenic atom microscope (SQCRAMscope) will change research spanning disciplines from basic condensed matter physics to the engineering of hybrid quantum systems.

Ferromagnetic resonance and spin waves have been used in miniaturized devices for rf and microwave applications, but large magnetic bias fields are required for the high frequencies at which most satellites operate, for example. Exploiting perpendicular standing spin waves in an innovative way, the authors excite spin-wave resonance at frequencies above 20 GHz with bias fields below 100 Oe, which can be realized with small, lightweight permanent magnets, or electromagnets with current sources that would not require cooling.

Semiconductor nanowires provide some of the smallest designs for laser emission into both photonic and plasmonic modes, but many aspects of their lasing dynamics and overall performance can only be accessed indirectly in experiments. The authors offer an approach to simulating lasing and light-matter interaction in semiconductor nanowires, with significantly enhanced numerical efficiency that allows them to investigate the influence of nanowire geometry, material properties, and excitation on lasing, and to explain some of the effects seen in experiments.

Achieving a transparent interface between a superconductor and a two-dimensional (2D) semiconductor is a longstanding challenge in mesoscopic physics, and has received renewed interest as the basis of a scalable approach to topological quantum computing. This study reports nearly perfect transmission in a gateable Josephson junction formed in a 2D InAs quantum well with epitaxial aluminum. The results shed light on the proximity effect in semiconductors through a transparent interface, and on the behavior of semiconductor-based Josephson junctions.

Graphene’s exceptional thermal conductivity is of great interest for heat management in advanced electronic devices, yet there are still fundamental issues regarding substrate effects. Using state-of-the-art simulations, the authors study collective phonon excitations in graphene on on partially hydroxylated silica surfaces, and show that certain vibrational modes can be influenced to increase thermal conductivity beyond that seen with an untreated substrate. This insight opens a path for systematic control of heat flow in this important system.

Detection and efficient readout of weak signals in the quantum regime are critical aspects of quantum information processing. Current measurement protocols rely on amplifiers plus signal routers (circulators or isolators), but bulk is problematic for any scalable architecture. The authors present minimal, efficient schemes to design nonreciprocal quantum-limited amplifiers for simultaneous routing and boosting of signals. This could lead to significantly simpler measurement chains, which is especially relevant in the face of increasingly complex multi-qubit experiments.

In the past decade, metamaterials and metasurfaces have enabled many unusual applications, among which electromagnetic invisibility (cloaking) is arguably one of the most fascinating. Going farther, cloaking can be seen as a particular case of a more complex, general transformation afforded by metasurfaces. The authors propose a systematic methodology for advanced scattering manipulation, showing how to make an object appear larger or thinner, or even to be made of a different material.

When a standard, isotropic electrolyte is replaced with a liquid crystal, a nonlinear mechanism of electrophoresis is introduced, and one unresolved issue is how the electrophoretic velocity depends on the crucial properties of the liquid crystal: the anisotropy of its dielectric permittivity and conductivity. The authors establish these dependencies with experiments and simulations, and demonstrate how to use them for controlled $reversal$ of electrokinetics, by changing either temperature or composition. This work embraces a wide range of problems, from electrokinetic phenomena and microfluidics to soft matter physics.

Field-effect transistors (FETs) employing transition-metal dichalcogenides, at the atomic limit of thickness, are particularly vulnerable to environmental influences, leading to strong fluctuations in output current. Attempts to understand the physics behind low-frequency noise in such devices have led to a blizzard of conclusions. The authors use photodoping to identify the dominant carrier-noise mechanisms in FETs with one to a few layers of MoS${}_2$, reconciling previous ideas. Additional conduction channels generated by the strong light-matter interactions lead to a percolative crossover between mechanisms, based on the stochastic nature of electron transport.

Chalcogenide glasses as phase-change materials are important in data storage and processing applications, including rewritable high-density media like optical discs, and now nonvolatile memory. Technological progress with these materials may hinge on understanding their glass physics, a notoriously difficult subject. The authors discuss how metal-semiconductor transitions hidden below the liquidus temperature—by also being liquid-liquid “fragility” transitions—could explain the propensity of certain chalcogenide alloys for phase-change functionality.