Ion exchange is commonly used to strengthen oxide glasses. However, the resulting stuffed glasses usually do not reach the molar volume of as-melted glasses of similar composition—a phenomenon known as the network dilation anomaly. This behavior seriously limits the potential for the chemical strengthening of glasses and its origin remains one of the mysteries of glass science. Here, based on molecular dynamics simulations of sodium silicate glasses coupled with topological constraint theory, we show that the topology of the atomic network controls the extent of ion-exchange-induced dilation. We demonstrate that isostatic glasses do not show any network dilation anomaly. This is found to arise from the combined absence of floppy modes of deformation and internal eigenstress in isostatic atomic networks.

We present detailed analytical modeling and in-depth investigation of wide-angle reflect-mode metagrating beam splitters. These recently introduced ultrathin devices are capable of implementing intricate diffraction engineering functionalities with only a single meta-atom per macroperiod, making them considerably simpler to synthesize than conventional metasurfaces. We extend recent work and focus on electrically polarizable metagratings, comprised of loaded conducting wires in front of a perfect electric conductor, excited by transverse-electric polarized fields, which are more practical for planar fabrication. The derivation further relates the metagrating performance parameters to the individual meta-atom load, facilitating an efficient semianalytical synthesis scheme to determine the required conductor geometry for achieving optimal beam splitting. Subsequently, we utilize the model to analyze the effects of realistic conductor losses, reactance deviations, and frequency shifts on the device performance, and reveal that metagratings feature preferable working points, in which the sensitivity to these nonidealities is rather low. The analytical relations shed light on the physical origin of this phenomenon, associating it with fundamental interference processes taking place in the device. These results, verified via full-wave simulations of realistic physical structures, yield a set of efficient engineering tools, as well as profound physical intuition, for devising future metagrating devices, with immense potential for microwave, terahertz, and optical beam-manipulation applications.

We present analytical relations for the dark recombination current of a $p\text{−}{n}^{+}$ junction with positively charged columnar grain boundaries in the high-defect-density regime. We consider two defect-state configurations relevant for positively charged grain boundaries: a single donor state and a continuum of both acceptors and donors. Compared to a continuum of $\mathrm{acceptor}+\mathrm{donor}$ states or to the previously studied single $\mathrm{acceptor}+\mathrm{donor}$ state, the grain-boundary recombination of a single donor state is suppressed by orders of magnitude. We show numerically that superposition holds near the open-circuit voltage $V_{\mathrm{OC}}$ so that our dark $J(V)$ relations determine $V_{\mathrm{OC}}$ for a given short-circuit current $J_{\mathrm{SC}}$. Finally, we explicitly show how $V_{\mathrm{OC}}$ depends on the these two grain-boundary defect-state configurations.

The internal deformation of the brain is far more complex than the rigid motion of the skull. An ultrasound imaging technique that we have developed has a combination of penetration, frame-rate, and motion-detection accuracy required to directly observe the formation and evolution of shear shock waves in the brain. Experiments at low impacts on the traumatic-brain-injury scale demonstrate that they are spontaneously generated and propagate within the porcine brain. Compared to the initially smooth impact, the acceleration at the shock front is amplified up to a factor of 8.5. This highly localized increase in acceleration suggests that shear shock waves are a previously unappreciated mechanism that could play a significant role in traumatic brain injury.

It is known that the eye’s scotopic photodetectors, rhodopsin molecules, and their associated phototransduction mechanism leading to light perception, are efficient single-photon counters. We here use the photon-counting principles of human rod vision to propose a secure quantum biometric identification based on the quantum-statistical properties of retinal photon detection. The photon path along the human eye until its detection by rod cells is modeled as a filter having a specific transmission coefficient. Precisely determining its value from the photodetection statistics registered by the conscious observer is a quantum parameter estimation problem that leads to a quantum secure identification method. The probabilities for false-positive and false-negative identification of this biometric technique can readily approach ${10}^{-10}$ and ${10}^{-4}$, respectively. The security of the biometric method can be further quantified by the physics of quantum measurements. An impostor must be able to perform quantum thermometry and quantum magnetometry with energy resolution better than ${10}^{-9}\hslash$, in order to foil the device by noninvasively monitoring the biometric activity of a user.

Imaging of surface topography and elasticity of living cells can provide insight into the roles played by the cells’ volumetric and mechanical properties and their response to external forces in regulating the essential cellular events and functions. Here, we report a unique technique of noncontact viscoelastic imaging of live cells using atomic force microscopy (AFM) with a long-needle glass probe. Because only the probe tip is placed in a liquid medium near the cell surface, the AFM cantilever in air functions well under dual-frequency modulation, retaining its high-quality resonant modes. The probe tip interacts with the cell surface through a minute hydrodynamic flow in the nanometer-thin gap region between them without physical contact. Quantitative measurements of the cell height, volume, and Young’s modulus are conducted simultaneously. The experiment demonstrates that the long-needle AFM has a wide range of applications in the study of cell mechanics.

Metasurfaces offer great opportunities to control electromagnetic (EM) waves, but currently most metadevices work either in pure reflection or pure transmission mode, leaving half of the EM space completely unexplored. Here, we propose an alternative type of metasurface, composed of specifically designed meta-atoms with polarization-dependent transmission and reflection properties, to efficiently manipulate EM waves in the full space. As a proof of concept, three microwave metadevices are designed, fabricated, and experimentally characterized. The first two metadevices can bend or focus EM waves at different sides (i.e., transmission and reflection sides) of the metasurfaces, depending on the incident polarization, while the third one changes from a wave bender for the reflected wave to a focusing lens for the transmitted wave as the excitation polarization is rotated, with all of these functionalities exhibiting very high efficiencies (in the range of 85%–91%) and total thickness $\sim \lambda /8$. Our findings significantly expand the capabilities of metasurfaces in controlling EM waves, and can stimulate high-performance multifunctional metadevices facing more challenging and diversified application demands.

We study the radiative decay, or fluorescence, of excitons in organic solar cells as a function of its geometrical parameters. Contrary to their nonradiative counterpart, fluorescence losses strongly depend on the environment. By properly tuning the thicknesses of the buffer layers between the active regions of the cell and the electrodes, the exciton lifetime and, hence, the exciton diffusion length can be increased. The importance of this phenomenon depends on the radiative quantum efficiency, which is the fraction of the exciton decay that is intrinsically due to fluorescence. Besides this effect, interferences within the cell control the efficiency of sunlight injection into the active layers. The optimal cell design must rely on a consideration of these two aspects. By properly managing fluorescence losses, one can significantly improve the cell performance. To demonstrate this fact, we use realistic material parameters inspired from literature data and obtain an increase of power-conversion efficiency from 11.3% to 12.7%. Conversely, not to take into account the strong dependence of fluorescence on the environment may lead to a suboptimal cell design and a degradation of cell performance. The presence of radiative losses, however small, significantly changes the optimal set of thicknesses. We illustrate the latter situation with experimental material data.

We present a scalable scheme for executing the error-correction cycle of a monolithic surface-code fabric composed of fast-flux-tunable transmon qubits with nearest-neighbor coupling. An eight-qubit unit cell forms the basis for repeating both the quantum hardware and coherent control, enabling spatial multiplexing. This control uses three fixed frequencies for all single-qubit gates and a unique frequency-detuning pattern for each qubit in the cell. By pipelining the interaction and readout steps of ancilla-based $X$- and $Z$-type stabilizer measurements, we can engineer detuning patterns that avoid all second-order transmon-transmon interactions except those exploited in controlled-phase gates, regardless of fabric size. Our scheme is applicable to defect-based and planar logical qubits, including lattice surgery.

The discovery of the unprecedented figure of merit $ZT$ of SnSe has sparked a large number of studies on the fundamental physics of this material and further improvement through guided materials design and optimization. Motivated by its rich chemical-bonding characters, unusual multivalley electronic structure, and the sensitivity of the band-edge states to lattice strains, we carry out accurate quasiparticle calculations for the low-temperature phase SnSe under strains. We illustrate how the band-edge states can be engineered by lattice strains, including the size and the nature of the band gap, the positions of the band extrema in the Brillouin zone, and the control of the number of electron and/or hole valleys. The distinct atomic origin and orientation of the wave functions of the different band-edge states dictates the relative shift in their band energy, enabling active control of the near-edge electronic structure of this material. Our work demonstrates that strain engineering is a promising way to manipulate the low-energy electronic structure of SnSe, which can have profound influences on the optical and transport properties of this material.

Color centers in diamond provide a promising platform for quantum optics in the solid state, with coherent optical transitions and long-lived electron and nuclear spins. Building upon recent demonstrations of nanophotonic waveguides and optical cavities in single-crystal diamond, we now demonstrate on-chip diamond nanophotonics with a high-efficiency fiber-optical interface achieving $>90\%$ power coupling at visible wavelengths. We use this approach to demonstrate a bright source of narrow-band single photons based on a silicon-vacancy color center embedded within a waveguide-coupled diamond photonic crystal cavity. Our fiber-coupled diamond quantum nanophotonic interface results in a high flux (approximately 38 kHz) of coherent single photons (near Fourier limited at $<1\text{−}\mathrm{GHz}$ bandwidth) into a single-mode fiber, enabling possibilities for realizing quantum networks that interface multiple emitters, both on chip and separated by long distances.

We propose to use graphene-based Josephson junctions (GJJs) to detect single photons in a wide electromagnetic spectrum from visible to radio frequencies. Our approach takes advantage of the exceptionally low electronic heat capacity of monolayer graphene and its constricted thermal conductance to its phonon degrees of freedom. Such a system could provide high-sensitivity photon detection required for research areas including quantum information processing and radio astronomy. As an example, we present our device concepts for GJJ single-photon detectors in both the microwave and infrared regimes. The dark count rate and intrinsic quantum efficiency are computed based on parameters from a measured GJJ, demonstrating feasibility within existing technologies.

Reliable generation of single photons is of key importance for fundamental physical experiments and to demonstrate quantum protocols. Waveguide-based photon-pair sources have shown great promise in this regard due to their large spectral tunability, high generation rates, and long temporal coherence of the photon wave packet. However, integrating such sources with fiber-optic networks often results in a strong degradation of performance. We answer this challenge by presenting an alignment-free source of photon pairs in the telecommunications band that maintains heralding efficiency $>50\%$ even after fiber pigtailing, photon separation, and pump suppression. The source combines this outstanding performance in heralding efficiency with a compact, stable, and easy-to-use “plug-and-play” package: one simply connects a laser to the input and detectors to the output, and the source is ready to use. This high performance can be achieved even outside the lab without the need for alignment which makes the source extremely useful for any experiment or demonstration needing heralded single photons.

We study the key domain-wall properties in segmented nanowire loop-based structures used in domain-wall-based sensors. The two reasons for device failure, namely, distribution of the domain-wall propagation field (depinning) and the nucleation field are determined with magneto-optical Kerr effect and giant-magnetoresistance (GMR) measurements for thousands of elements to obtain significant statistics. Single layers of ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$, a complete GMR stack with ${\mathrm{Co}}_{90}{\mathrm{Fe}}_{10}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ as a free layer, and a single layer of ${\mathrm{Co}}_{90}{\mathrm{Fe}}_{10}$ are deposited and industrially patterned to determine the influence of the shape anisotropy, the magnetocrystalline anisotropy, and the fabrication processes. We show that the propagation field is influenced only slightly by the geometry but significantly by material parameters. Simulations for a realistic wire shape yield a curling-mode type of magnetization configuration close to the nucleation field. Nonetheless, we find that the domain-wall nucleation fields can be described by a typical Stoner-Wohlfarth model related to the measured geometrical parameters of the wires and fitted by considering the process parameters. The GMR effect is subsequently measured in a substantial number of devices (3000) in order to accurately gauge the variation between devices. This measurement scheme reveals a corrected upper limit to the nucleation fields of the sensors that can be exploited for fast characterization of the working elements.

We have directly imaged reversible electrical switching of the cycloidal rotation direction (magnetic polarity) in a $(111{)}_{\mathrm{pc}}\text{−}{\mathrm{BiFeO}}_3$ epitaxial-film device at room temperature by nonresonant x-ray magnetic scattering. Consistent with previous reports, fully relaxed $(111{)}_{\mathrm{pc}}\text{−}{\mathrm{BiFeO}}_3$ epitaxial films consisting of a single ferroelectric domain are found to comprise a submicron-scale mosaic of magnetoelastic domains, all sharing a common direction of the magnetic polarity, which is found to switch reversibly upon reversal of the ferroelectric polarization without any measurable change of the magnetoelastic domain population. A real-space polarimetry map of our device clearly distinguishes between regions of the sample electrically addressed into the two magnetic states with a resolution of a few tens of micron. Contrary to the general belief that the magneto-electric coupling in ${\mathrm{BiFeO}}_3$ is weak, we find that electrical switching has a dramatic effect on the magnetic structure, with the magnetic moments rotating on average by 90° at every cycle.

We report the design, analysis, and characterization of a three-inductor radio-frequency identification (RFID) and transceiver system for potential applications in individual cell tracking and monitoring. The RFID diameter is $22\mu \mathrm{m}$ and can be naturally internalized by living cells. Using magnetic resonance coupling, the system shows resonance shifts when the RFID is present and also when the RFID loading capacitance changes. It operates at 60 GHz with a high signal magnitude up to $-50\mathrm{dB}$ and a sensitivity of 0.2. This miniaturized RFID with a high signal magnitude is a promising step toward continuous, real-time monitoring of activities at cellular levels.

A thermodynamic model for light-emitting diodes (LEDs) is developed by considering energy and entropy flows in the system. Thermodynamic constraints have previously been considered separately for the reversible process of electroluminescence in LEDs and for light extraction and collimation in other optical systems. By considering both processes in the LED model, an irreversible upper bound for the conversion of electrical energy to optical energy is derived and shown to be higher than unity, but tighter and more realistic than the reversible case. We also model a LED as an endoreversible heat engine where the carrier-transport processes can be directly connected to the elements of a thermodynamic cycle.

The zinc nitride ${\mathrm{Zn}}_3{\mathrm{N}}_2$ is composed of inexpensive and earth-abundant Zn and N elements and shows high electron mobility exceeding $100{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Although various technological applications of ${\mathrm{Zn}}_3{\mathrm{N}}_2$ have been suggested so far, the synthesis of high-quality ${\mathrm{Zn}}_3{\mathrm{N}}_2$ samples, especially single crystals, is still challenging, and therefore its basic properties are not yet well understood. Indeed, the reported band gaps of as-grown ${\mathrm{Zn}}_3{\mathrm{N}}_2$ widely scatter from 0.85 to 3.2 eV. In this study, we investigate the large gap variation of ${\mathrm{Zn}}_3{\mathrm{N}}_2$ in terms of the Burstein-Moss (BM) effect and point-defect energetics using first-principles calculations. First, we discuss the relation between electron carrier concentration and optical gaps based on the electronic structure obtained using the Heyd-Scuseria-Ernzerhof hybrid functional. The calculated fundamental band gap is 0.84 eV in a direct-type band structure. Second, thermodynamic stability of ${\mathrm{Zn}}_3{\mathrm{N}}_2$ is assessed using the ideal-gas model in conjunction with the rigid-rotor model for gas phases and first-principles phonon calculations for solid phases. Third, carrier generation and compensation by native point defects and unintentionally introduced oxygen and hydrogen impurities are discussed. The results suggest that a significant BM shift occurs mainly due to oxygen substitutions on nitrogen sites and hydrogen interstitials. However, gaps larger than 2.0 eV would not be due to the BM shift because of the Fermi-level pinning caused by acceptorlike zinc vacancies and hydrogen-on-zinc impurities. Furthermore, we discuss details of peculiar defects such as a nitrogen-on-zinc antisite with azidelike atomic and electronic structures.

Background forces are linear long-range interactions of the cantilever body with its surroundings that must be compensated for in order to reveal tip-surface force, the quantity of interest for determining material properties in atomic force microscopy. We provide a mathematical derivation of a method to compensate for background forces, apply it to experimental data, and discuss how to include background forces in simulation. Our method, based on linear-response theory in the frequency domain, provides a general way of measuring and compensating for any background force and it can be readily applied to different force reconstruction methods in dynamic AFM.

We measure the electroluminescence of light-emitting diodes (LEDs) on substrates with low dislocation densities (LDD) at ${10}^6{\mathrm{cm}}^{-2}$ and low ${10}^8{\mathrm{cm}}^{-2}$, and compare them to LEDs on substrates with high dislocation densities (HDD) closer to ${10}^{10}{\mathrm{cm}}^{-2}$. The external quantum efficiencies (EQEs) are fitted using the $ABC$ model with and without localization. The nonradiative-recombination (NR) coefficient $A$ is constant for HDD LEDs, indicating that the NR is dominated by dislocations at all wavelengths. However, $A$ strongly increases for LDD LEDs by a factor of 20 when increasing the emission wavelength from 440 to 540 nm. We attribute this to an increased density of point defects due to the lower growth temperatures used for longer wavelengths. The radiative recombination coefficient $B$ follows the squared wave-function overlap for all samples. Using the observed coefficients, we calculate the peak efficiency as a function of the wavelength. For HDD LEDs the change of wave-function overlap (i.e., $B$) is sufficient to reduce the EQE as observed, while for LDD LEDs also the NR coefficient $A$ must increase to explain the observed EQEs. Thus, reducing NR is important to improving the EQEs of green LEDs, but this cannot be achieved solely by reducing the dislocation density: point defects must also be addressed.