We explore the nonlinear optical response of three-dimensional gyroidal metamaterials, which show greater than tenfold enhancements compared to all other metallic nanomaterials as well as bulk gold. A simple analytical model for this metamaterial response shows how the reflectivity spectrum scales with the metal fill fraction and the refractive index of the material that the metallic nanostructure is embedded in. The ultrafast response arising from the interconnected three-dimensional nanostructure can be separated into electronic and lattice contributions with strong spectral dependences on the dielectric filling of the gyroids, which invert the sign of the nonlinear transient reflectivity changes. These metamaterials thus provide a wide variety of tunable nonlinear optical properties, which can be utilized for frequency mixing, optical switching, phase modulators, novel emitters, and enhanced sensing.

We use micromagnetic simulations to demonstrate the feasibility of creating magnetic logic gates that process binary data encoded within the internal magnetization structure of domain walls in ferromagnetic nanowires. In the simulated nanowires, domain walls take the form of magnetic vortices, where the magnetization circulates either clockwise or anticlockwise. By exploiting differences in how these two domain-wall states interact with both notch-shaped defects and junctions in the nanowires, we design nanowire segments that act as not, fan-out, nand, and, or, and nor logic gates. Potentially, these gates could be cascaded to perform any desired logical operation. Our simulations demonstrate the possibility of a class of magnetic devices in which domain walls carry digital information rather than merely delineate it.

Mass spectrometry is used in a wide range of scientific disciplines including proteomics, pharmaceutics, forensics, and fundamental physics and chemistry. Given this ubiquity, there is a worldwide effort to improve the efficiency and resolution of mass spectrometers. However, the performance of all techniques is ultimately limited by the initial phase-space distribution of the molecules being analyzed. Here, we dramatically reduce the width of this initial phase-space distribution by sympathetically cooling the input molecules with laser-cooled, cotrapped atomic ions, improving both the mass resolution and detection efficiency of a time-of-flight mass spectrometer by over an order of magnitude. Detailed molecular-dynamics simulations verify the technique and aid with evaluating its effectiveness. This technique appears to be applicable to other types of mass spectrometers.

The external quantum efficiency (EQE) spectra of high-efficiency ${\mathrm{CuInGaSe}}_2$ (CIGS)-based solar cells fabricated by a standard three-stage process are simulated by incorporating the effects of (i) the $\mathsf{V}$-shaped Ga-compositional profile within a $1.8\text{−}\mu \mathrm{m}$-thick CIGS layer and (ii) the light scattering caused by the submicron-textured structure. For the EQE calculation of the CIGS solar cells, we develop a simulation scheme in which the total light absorption in the solar cell is assessed using the experimental reflectance spectrum, whereas the absorptance of each solar-cell layer is deduced by assuming a flat optical model. The optical effect of the double-graded Ga-compositional profile in the CIGS layers is calculated explicitly based on a complete CIGS optical database established recently. A highly accurate EQE simulation for CIGS solar cells is made possible by the developed simple calculation method. In particular, our technique allows the determination of the partial EQE contributions for different thicknesses and wavelengths in the CIGS-based solar cells. The EQE analysis reveals that the carrier collection efficiency in the CIGS light-absorber layers is almost unity, but the light absorption in the $1\text{−}\mu \mathrm{m}$-thick CIGS bottom region is negligible, confirming that the bottom layer with higher Ga content plays a dominant role as a back-surface field with the conduction-band grading. We find that the major optical loss occurs in the ZnO:Al, CdS, and Mo component layers with a corresponding current loss of approximately $3\mathrm{mA}/{\mathrm{cm}}^2$ in each layer. Furthermore, an EQE simulation method for arbitrary CIGS solar-cell structures is developed by imposing the antireflection condition in the calculation of the reflectance spectra. By applying this technique, the effects of the Ga-compositional profiles and thicknesses of various solar-cell component layers on the EQE spectrum are determined.

In his original work, Josephson predicted that a phase-dependent conductance should be present in superconducting tunnel junctions, an effect difficult to detect, mainly because it is hard to single it out from the usual nondissipative Josephson current. We propose a solution for this problem that consists of using different superconducting materials to realize the two junctions of a superconducting interferometer. According to the Ambegaokar-Baratoff relation the two junctions have different conductances if the critical currents are equal, thus the Josephson current can be suppressed by fixing the magnetic flux in the loop at half of a flux quantum without canceling the phase-dependent conductance. Our proposal can be used to study the phase-dependent conductance, an effect present in principle in all superconducting weak links. From the standpoint of nonlinear circuit theory, such a device is in fact an ideal memristor with possible applications to memories and neuromorphic computing in the framework of ultrafast and low-energy-consumption superconducting digital circuits.

The ability to spatially resolve the degree to which extended defects impact carrier diffusion lengths and lifetimes is important for determining upper limits for defect densities in semiconductor devices. We show that a new spatially and temporally resolved photoluminescence (PL) imaging technique can be used to accurately extract carrier lifetime values in the immediate vicinity of dark-line defects in $\mathrm{CdTe}/\mathrm{MgCdTe}$ double heterostructures. A series of PL images captured during the decay process show that extended defects with a density of $1.4\times {10}^5{\mathrm{cm}}^{-2}$ deplete photogenerated charge carriers from the surrounding semiconductor material on a nanosecond time scale. The technique makes it possible to elucidate the interplay between nonradiative carrier recombination and carrier diffusion and reveals that they both combine to degrade the PL intensity over a fractional area that is much larger than the physical size of the defects. Carrier lifetimes are correctly determined from numerical simulations of the decay behavior by taking these two effects into account. Our study demonstrates that it is crucial to measure and account for the influence of local defects in the measurement of carrier lifetime and diffusion, which are key transport parameters for the design and modeling of advanced solar-cell and light-emitting devices.

The statistical properties of fracture strength of brittle and quasibrittle materials are often described in terms of the Weibull distribution. However, the weakest-link hypothesis, commonly used to justify it, is expected to fail when fracture occurs after significant damage accumulation. Here we show that this implies that the Weibull distribution is unstable in a renormalization-group sense for a large class of quasibrittle materials. Our theoretical arguments are supported by numerical simulations of disordered fuse networks. We also find that for brittle materials such as ceramics, the common assumption that the strength distribution can be derived from the distribution of preexisting microcracks by using Griffith’s criteria is invalid. We attribute this discrepancy to crack bridging. Our findings raise questions about the applicability of Weibull statistics to most practical cases.

We present an experimental study of phase locking in a stochastic magnetic oscillator. The system comprises a magnetic tunnel junction with a superparamagnetic free layer, whose magnetization dynamics is driven with spin torques through an external periodic driving current. We show that synchronization of this stochastic oscillator to the input current is possible for current densities below $3\times 10^6\mathrm{A}/{\mathrm{cm}}^2$, and occurs for input frequencies lower than the natural mean frequency of the stochastic oscillator. We show that such injection locking is robust and leads to a drastic reduction in the phase diffusion of the stochastic oscillator, despite the presence of a frequency mismatch between the oscillator and the excitation.

Organic-inorganic perovskites are attracting increasing attention for their use in high-performance solar cells. Nevertheless, a detailed understanding of charge generation, interplay of excitons and free charge carriers, and recombination pathways, crucial for further device improvement, remains incomplete. Here, we present an analytical model describing both equilibrium properties of free charge carriers and excitons in the presence of electronic subgap trap states and their time evolution after photoexcitation in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_{3-x}{\mathrm{Cl}}_x$. At low fluences the charge-trapping pathways limit the photoluminescence quantum efficiency, whereas at high fluences the traps are predominantly filled and recombination of the photogenerated species is dominated by efficient radiative processes. We show experimentally that the photoluminescence quantum efficiency approaches 100% at low temperatures and at high fluences, as predicted by our model. Our approach provides a theoretical framework to understand the fundamental physics of perovskite semiconductors and to help in designing and enhancing the material for improved optoelectronic device operation.

A graphene layer on top of a dielectric can dramatically influence the ability of the material for radiative heat transfer. This property of graphene is used to improve the performance and reduce costs of near-field thermophotovoltaic cells. Instead of low-band-gap semiconductors it is proposed to use graphene-on-silicon Schottky photovoltaic cells. One layer of graphene absorbs around 90% of incoming radiation and increases the heat transfer. This strong absorption is due to the excitation of plasmons in graphene, which are automatically tuned in resonance with the emitted light in the midinfrared range. The absorbed radiation excites electron-hole pairs in graphene, which are separated by the surface field induced by the Schottky barrier. For a quasimonochromatic source the generated power is one order of magnitude larger and the efficiency is on the same level as for semiconductor photovoltaic cells.

Spin transport in nondegenerate semiconductors is expected to pave the way to the creation of spin transistors, spin logic devices, and reconfigurable logic circuits, because room-temperature (RT) spin transport in Si has already been achieved. However, RT spin transport has been limited to degenerate Si, which makes it difficult to produce spin-based signals because a gate electric field cannot be used to manipulate such signals. Here, we report the experimental demonstration of spin transport in nondegenerate Si with a spin metal-oxide-semiconductor field-effect transistor (MOSFET) structure. We successfully observe the modulation of the Hanle-type spin-precession signals, which is a characteristic spin dynamics in nondegenerate semiconductors. We obtain long spin transport of more than $20\mathit{\mu }\mathrm{m}$ and spin rotation greater than $4\pi$ at RT. We also observe gate-induced modulation of spin-transport signals at RT. The modulation of the spin diffusion length as a function of a gate voltage is successfully observed, which we attribute to the Elliott-Yafet spin relaxation mechanism. These achievements are expected to lead to the creation of practical Si-based spin MOSFETs.

Sulfur cathodes have much larger capacities than transition-metal-oxide cathodes used in commercial lithium-ion batteries but suffer from unsatisfactory capacity retention and long-term cyclability. Capacity degradation originates from soluble lithium polysulfides gradually diffusing into the electrolyte. Understanding of the formation and dynamics of soluble polysulfides during the discharging process at the atomic level remains elusive, which limits further development of lithium-sulfur ($\mathrm{Li}\text{−}\mathrm{S}$) batteries. Here we report first-principles molecular dynamics simulations and density functional calculations, through which the discharging products of $\mathrm{Li}\text{−}\mathrm{S}$ batteries are studied. We find that, in addition to simple ${\mathrm{Li}}_2{\mathrm{S}}_n$ ($1\le n\le 8$) clusters generated from single cyclooctasulfur (${\mathrm{S}}_8$) rings, large $\mathrm{Li}\text{−}\mathrm{S}$ clusters form by collectively coupling several different rings to minimize the total energy. At high lithium concentration, a Li-S network forms at the sulfur surfaces. The results can explain the formation of the soluble Li-S complex, such as ${\mathrm{Li}}_2{\mathrm{S}}_8$, ${\mathrm{Li}}_2{\mathrm{S}}_6$, and ${\mathrm{Li}}_2{\mathrm{S}}_4$, and the insoluble ${\mathrm{Li}}_2{\mathrm{S}}_2$ and ${\mathrm{Li}}_2\mathrm{S}$ structures. In addition, we show that the presence of oxygen impurities in graphene, particularly oxygen atoms bonded to vacancies and edges, may stabilize the lithium polysulfides that may otherwise diffuse into the electrolyte.

We formulate a theory of the ac spin Hall magnetoresistance in a bilayer system consisting of a magnetic insulator such as yttrium iron garnet and a heavy metal such as platinum. We derive expressions for the dc voltage generation based on the drift-diffusion spin model and quantum mechanical boundary condition at the interface that reveal a spin-torque ferromagnetic resonance (ST-FMR). We predict that ST-FMR experiments will reveal valuable information on the current-induced magnetization dynamics of magnetic insulators and the ac spin Hall effect.

Pressure-controlled displacement of an oil-water interface is studied in dense packings of functionalized glass beads with well-defined spatial wettability correlations. An enhanced dissipation is observed if the typical extension $\xi$ of the same-type wetting domains is smaller than the average bead diameter $d$. Three-dimensional imaging using x-ray microtomography shows that the frequencies $n(s)$ of residual droplet volumes $s$ for different $\xi$ collapse onto the same curve. This indicates that the additional dissipation for small $\xi$ is due to contact line pinning rather than an increase of capillary break-up and coalescence events.

Controlling the interaction of an ultrafast laser pulse with a thin film remains a difficult task, especially when aiming to confine material modifications to subwavelength scales. We introduce a method to achieve reproducible submicron ablation of thin films from a dielectric surface, in a back-irradiation geometry. First, the pulse of 45-fs duration and 800-nm central wavelength nonlinearly interacts with the dielectric and undergoes strong but reproducible modifications of its intensity profile. Then, the pulse ablates a thin polymer film [four bilayers of poly(allylamine hydrochloride) and poly(sodium 4-styrene-sulfonate), 8 nm thick] from the back surface. We measure the hole with atomic force microscopy and study the influence of laser energy and focal plane position. The radius of the resulting hole is determined by a threshold intensity for ablation. Therefore, we also demonstrate how measuring the radius as a function of focal plane position provides a new approach to profiling a tightly focused laser beam under nonlinear propagation conditions. We compare the beam profile with that predicted by a widely used propagation model and show that the latter can semiquantitatively be applied to estimate the size of achievable holes.

We study polycrystalline $B2$-type ${\mathrm{Co}}_2\mathrm{FeAl}$ (CFA) full-Heusler-alloy-based magnetic tunnel junctions (MTJs) fabricated on a $\mathrm{Si}/{\mathrm{SiO}}_2$ amorphous substrate. Polycrystalline CFA films with a (001) orientation, a high $B2$ ordering, and a flat surface are achieved by using a MgO buffer layer. A tunnel magnetoresistance ratio up to 175% is obtained for a MTJ with a $\mathrm{CFA}/\mathrm{MgO}/\mathrm{CoFe}$ structure on a 7.5-nm-thick MgO buffer. Spin-transfer-torque-induced magnetization switching is achieved in the MTJs with a 2-nm-thick polycrystalline CFA film as a switching layer. By using a thermal activation model, the intrinsic critical current density ($J_{\mathit{c}0}$) is determined to be $8.2\times {10}^6\mathrm{A}/{\mathrm{cm}}^2$, which is lower than $2.9\times {10}^7\mathrm{A}/{\mathrm{cm}}^2$, the value for epitaxial CFA MTJs [Appl. Phys. Lett. 100, 182403 (2012)]. We find that the Gilbert damping constant ($\alpha$) evaluated by using ferromagnetic resonance measurements for the polycrystalline CFA film is approximately 0.015 and is almost independent of the CFA thickness (2–18 nm). The low $J_{c0}$ for the polycrystalline MTJ is mainly attributed to the low $\alpha$ of the CFA layer compared with the value in the epitaxial one (approximately 0.04).

We demonstrate a general non–Derjaguin-Landau-Verwey-Overbeek method to stabilize colloids in liquids. By this method, colloidal particles that initially form unstable suspension and sediment from the liquid are stabilized by the addition of salt to the suspending liquid. Yet, the salt is not expected to adsorb or directly interact with the surface of the colloids. For the method to work, the liquid should be a mixture, and the salt needs to be antagonistic such that each ion is preferentially solvated by a different component of the mixture. The stabilization may depend on the salt content, mixture composition, or distance from the mixture’s coexistence line.

We study the controlled generation of single microbubbles (approximately $100\mu \mathrm{m}$) at solid surfaces in water. By focusing a nanosecond pressure pulse ($<100\mathrm{ns}$), the single microbubble is deterministically generated over the focal area, which is rather immune to surface heterogeneity that is typically responsible for uncontrolled bubble nucleation. Such controlled bubble nucleation is realized by strong negative pressure generated by pulsed optical excitation of a carbon-nanotube–polymer composite and its tight focusing (approximately $100\mu \mathrm{m}$). The negative-pressure amplitude ($>50\mathrm{MPa}$), which is further increased by acoustic interference at an impedance-mismatched boundary, even exceeds the cavitation threshold of distilled water in a free-field (boundary-free) condition. This strong pressure can activate ubiquitous submicron nuclei in both water and surfaces, making the nucleation process controllable but irrelevant to a pit array ($<10\mu \mathrm{m}$ in diameter). The ability to control bubble formation can lead to new applications in cell-level therapy, e.g., ablation of individual cancerous cells (microhistotripsy).

The aluminum–zinc-vacancy (${\mathrm{Al}}_{\mathrm{Zn}}\text{−}{\mathit{V}}_{\mathrm{Zn}}$) complex is identified as one of the dominant defects in Al-containing $n$-type ZnO after electron irradiation at room temperature with energies above 0.8 MeV. The complex is energetically favorable over the isolated ${\mathit{V}}_{\mathrm{Zn}}$, binding more than 90% of the stable ${\mathit{V}}_{\mathrm{Zn}}$’s generated by the irradiation. It acts as a deep acceptor with the ($0/-$) energy level located at approximately 1 eV above the valence band. Such a complex is concluded to be a defect of crucial and general importance that limits the $n$-type doping efficiency by complex formation with donors, thereby literally removing the donors, as well as by charge compensation.