Recent attempts have been made to convert the broadband solar spectrum into filtered light, for the purpose of boosting the efficiency of a planar solar cell. By developing a thermodynamic model, the rigorous efficiency limits of the optically boosted solar cells are presented, which are achievable in principle with a piecewise function of the front emissivity or the energy-redistribution lifetime.

To control the time-response performance of widely used cerium-activated scintillators in cutting-edge medical-imaging devices, such as time-of-flight positron-emission tomography, a comprehensive understanding of the role of Ce valence states, especially stable ${\mathrm{Ce}}^{4+}$, in the scintillation mechanism is essential. However, despite some progress made recently, an understanding of the physical processes involving ${\mathrm{Ce}}^{4+}$ is still lacking. The aim of this work is to clarify the role of ${\mathrm{Ce}}^{4+}$ in scintillators by studying ${\mathrm{Ca}}^{2+}$ codoped ${\mathrm{Gd}}_3{\mathrm{Ga}}_3{\mathrm{Al}}_2{\mathrm{O}}_{12}:\mathrm{Ce}(\mathrm{GGAG}:\mathrm{Ce})$. By using a combination of optical absorption spectra and x-ray absorption near-edge spectroscopies, the correlation between ${\mathrm{Ca}}^{2+}$ codoping content and the ${\mathrm{Ce}}^{4+}$ fraction is seen. The energy-level diagrams of ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Ce}}^{4+}$ in the ${\mathrm{Gd}}_3{\mathrm{Ga}}_3{\mathrm{Al}}_2{\mathrm{O}}_{12}$ host are established by using theoretical and experimental methods, which indicate a higher position of the $5d_1$ state of ${\mathrm{Ce}}^{4+}$ in the forbidden gap in comparison to that of ${\mathrm{Ce}}^{3+}$. Underlying reasons for the decay-time acceleration resulting from ${\mathrm{Ca}}^{2+}$ codoping are revealed, and the physical processes of the ${\mathrm{Ce}}^{4+}$-emission model are proposed and further demonstrated by temperature-dependent radioluminescence spectra under x-ray excitation.

Employing first-principles calculations, we investigate the efficiency of spin injection from a ferromagnetic electrode (Ni) into graphene and a possible enhancement by using a barrier between the electrode and graphene. Three types of barriers, $h$-BN, Cu(111), and graphite, of various thickness (0–3 layers) are considered, and the electrically biased conductance of the $\mathrm{Ni}/\text{barrier}/\text{graphene}$ junction is calculated. It is found that the minority-spin-transport channel of graphene can be strongly suppressed by the insulating $h$-BN barrier, resulting in a high spin-injection efficiency. On the other hand, the calculated spin-injection efficiencies of $\mathrm{Ni}/\mathrm{Cu}/\text{graphene}$ and $\mathrm{Ni}/\text{graphite}/\text{graphene}$ junctions are low, due to the spin-conductance mismatch. Further examination of the electronic structure of the system reveals that the high spin-injection efficiency in the presence of a tunnel barrier is due to its asymmetric effects on the two spin states of graphene.

The resistance of copper grain boundaries (GBs) is calculated systematically through a full atomistic quantum approach. A set of twin GBs, including the coherent twin GB, is generated by density functional theory (DFT) total energy relaxation starting from the coincidence site lattice (CSL) model. The atomic structure of the GBs is used to construct two-probe transport junctions for quantum-transport analysis by carrying out DFT within the Green’s function formalism. The specific resistivity calculated for the coherent twin GB is found to be quantitatively consistent with the available experimental and theoretical data. The specific resistivity and reflection coefficient of other more complex GBs are predicted. The interfacial energy density and specific resistivity are both found to inversely relate with the planar density of coincidence sites. Comparison of our calculated specific resistivities and reflection coefficients with the corresponding GB-averaged experimental quantities shines light on the microstructure of the samples.

A solid-state picosecond laser is used to ablate semiconductor thin films in spatially localized areas, providing an alternative to device isolation strategies based on chemical or ion etching techniques. Field-effect transistors (FETs) of emerging organic and inorganic materials often utilize a continuous semiconductor film and an array of top-contact electrodes. Electrically isolating individual FET components from other circuit elements is essential in order to reduce parasitic capacitances and unwanted current pathways, both to improve device performance and to enable the observation of new or enhanced physical phenomena. We pattern FET arrays with ultrafast-pulse-duration (1.5 ps) and low-fluence ($0.09\mathrm{J}{\mathrm{cm}}^{-2}$) optical pulses using the fundamental wavelength (1030 nm) of an Yb-YAG laser. We investigate two representative semiconductor materials. First, zinc oxide (ZnO) is deposited onto $\mathrm{Si}/{\mathrm{SiO}}_2$ substrates by sol-gel methods and used to create $n$-channel FETs with aluminum top electrodes. Isolation of individual FETs enables the clear observation of photomodulation of the FET device parameters via photoinduced electron donation from an adsorbed chromophore. The second system comprises thin-film bilayers of tellurium and organic semiconductor molecules sequentially vapor-deposited onto $\mathrm{Si}/{\mathrm{SiO}}_2$ substrates, with gold electrodes deposited last. Charge carrier mobility is maintained for devices isolated by picosecond lasers, but leakage currents through the FET dielectric are drastically reduced.

Tuning the optoelectronic properties through alloying is essential for semiconductor technology. Currently, mostly isovalent and isostructural alloys are used (e.g., groups IV and III-V), but a vast and unexplored space of novel functional materials is conceivable when considering more complex alloys by mixing aliovalent and heterostructural constituents. The real challenge lies in the quantitative property prediction for such complex alloys to guide their experimental exploration. We develop an approach to predict compositional dependence of both band-structure and electrical properties from ab initio calculations by extending the conventional dilute-defect model to higher (alloy) concentrations. Considering alloying of aliovalent (Mg, Zn, Cd) cations and isovalent anions (S, Se) into ${\mathrm{Cu}}_2\mathrm{O}$, we predict tunability of band-gap energies and doping levels over a wide range, including the type conversion from $p$ to $n$ type. Initial synthesis and characterization of Zn-and Se-substituted ${\mathrm{Cu}}_2\mathrm{O}$ support the defect model, suggesting these alloys as promising novel oxide-semiconductor materials.

We visualize the formation of fingered flow in dry model sandy soils under different rain conditions using a quasi-2D experimental setup and systematically determine the impact of the soil grain diameter and surface wetting properties on the water channeling phenomenon. The model sandy soils we use are random closely packed glass beads with varied diameters and surface treatments. For hydrophilic sandy soils, our experiments show that rain water infiltrates a shallow top layer of soil and creates a horizontal water wetting front that grows downward homogeneously until instabilities occur to form fingered flows. For hydrophobic sandy soils, in contrast, we observe that rain water ponds on the top of the soil surface until the hydraulic pressure is strong enough to overcome the capillary repellency of soil and create narrow water channels that penetrate the soil packing. Varying the raindrop impinging speed has little influence on water channel formation. However, varying the rain rate causes significant changes in the water infiltration depth, water channel width, and water channel separation. At a fixed rain condition, we combine the effects of the grain diameter and surface hydrophobicity into a single parameter and determine its influence on the water infiltration depth, water channel width, and water channel separation. We also demonstrate the efficiency of several soil water improvement methods that relate to the rain water channeling phenomenon, including prewetting sandy soils at different levels before rainfall, modifying soil surface flatness, and applying superabsorbent hydrogel particles as soil modifiers.

A hybrid structure combining the advantages of a topological insulator (TI), dielectric ferromagnet (FM), and graphene is investigated to realize the electrically controlled correlation between electronic and magnetic subsystems for low-power, high-functional applications. Two-dimensional Dirac-fermion states provide an ideal environment to facilitate strong coupling through the surface interactions with proximate materials. The unique properties of FM-TI and FM-graphene interfaces make it possible for active “manipulation” and “propagation,” respectively, of the information state variable based solely on the spin logic platform through electrical gate biases. Our theoretical analysis verifies the feasibility of the concept for logic application with both current-driven and currentless interconnect approaches. The device and circuit characteristics are also examined in realistic conditions, suggesting the desired low-power performance with the estimated energy consumption for copy or not operations as low as the attojoule level.

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.