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.

Hot isostatic compression is an interesting method for modifying the structure and properties of bulk inorganic glasses. However, the structural and topological origins of the pressure-induced changes in macroscopic properties are not yet well understood. In this study, we report on the pressure and composition dependences of density and micromechanical properties (hardness, crack resistance, and brittleness) of five soda-lime borosilicate glasses with constant modifier content, covering the extremes from Na-Ca borate to Na-Ca silicate end members. Compression experiments are performed at pressures $\le 1.0\mathrm{GPa}$ at the glass transition temperature in order to allow processing of large samples with relevance for industrial applications. In line with previous reports, we find an increasing fraction of tetrahedral boron, density, and hardness but a decreasing crack resistance and brittleness upon isostatic compression. Interestingly, a strong linear correlation between plastic (irreversible) compressibility and initial trigonal boron content is demonstrated, as the trigonal boron units are the ones most disposed for structural and topological rearrangements upon network compaction. A linear correlation is also found between plastic compressibility and the relative change in hardness with pressure, which could indicate that the overall network densification is responsible for the increase in hardness. Finally, we find that the micromechanical properties exhibit significantly different composition dependences before and after pressurization. The findings have important implications for tailoring microscopic and macroscopic structures of glassy materials and thus their properties through the hot isostatic compression method.

We report the design and implementation of a high-performance superconducting quantum-interference proximity transistor based on aluminum-copper technology. With the adoption of a thin and short copper nanowire, we demonstrate full phase-driven modulation of the proximity-induced minigap in the normal-metal density of states. Under optimal bias, we record unprecedentedly high flux-to-voltage (up to $3\mathrm{mV}/{\mathrm{\Phi }}_0$) and flux-to-current (exceeding $100\mathrm{nA}/{\mathrm{\Phi }}_0$) transfer function values at subkelvin temperatures, where ${\mathrm{\Phi }}_0$ is the flux quantum. The best magnetic-flux resolution (as low as $500\mathrm{n}{\mathrm{\Phi }}_0/\sqrt{\mathrm{Hz}}$ at 240 mK being limited by the room-temperature preamplification stage) is reached under fixed current bias. These figures of merit combined with ultralow power dissipation and micrometer-size dimensions make this mesoscopic interferometer attractive for low-temperature applications such as the investigation of the magnetization of small spin populations.

Based on a nonequilibrium Green’s-function approach to the calculation of emission currents from first principles, we show that cathodes consisting of ${\mathrm{LaB}}_6/{\mathrm{BaB}}_6$ superlattices can yield an order-of-magnitude higher thermionic current densities than pure ${\mathrm{LaB}}_6$ cathodes. Because of a 0.46-eV lowering of the work function, such a heterostructure cathode could thus be operated at significantly lower temperatures. Neither the stability nor the magnitude of electronic tunneling coefficients is compromised in the superlattice system as compared to pure ${\mathrm{LaB}}_6$, which is in contrast to the generally reduced stability and large dipole barriers in the case of adsorbate-induced lowering of the work function. The heterostructure could thus be used as a cathode material that at the same time is stable and has emission properties superior to those of pure ${\mathrm{LaB}}_6$.

Pushing the limits in temporal resolution for transmission electron microscopy (TEM) requires a revolutionary change in the electron source technology. In this paper, we study the possibility of employing a radio-frequency photoinjector as the electron source for a time-resolved TEM. By raising the beam energy to the relativistic regime, we minimize the space-charge effects which otherwise limit the spatiotemporal resolution of the instrument. Analysis and optimization of the system taking into account the achievable beam brightness, electron flux on the sample, chromatic and spherical aberration of the electron optic system, and space-charge effects in image formation are presented and supported by detailed numerical modeling. The results demonstrate the feasibility of 10-nm–10-ps spatiotemporal resolution single-shot MeV TEM.

We show how resonant laser spectroscopy of the trion optical transitions in a self-assembled quantum dot can be used to determine the temperature of a nearby electron reservoir. At finite magnetic field, the spin-state occupation of the Zeeman-split quantum-dot electron ground states is governed by thermalization with the electron reservoir via cotunneling. With resonant spectroscopy of the corresponding excited trion states, we map out the spin occupation as a function of magnetic field to establish optical thermometry for the electron reservoir. We demonstrate the implementation of the technique in the subkelvin temperature range where it is most sensitive and where the electron temperature is not necessarily given by the cryostat base temperature.

Resonant laser spectroscopy of a negatively charged self-assembled quantum dot is utilized to measure the temperature of a three-dimensional fermionic reservoir down to 100 mK. With a magnetic field applied to the quantum dot, the single-charged ground state is split by the Zeeman energy. As the quantum dot is in tunnel contact with a thermal electron reservoir, a thermal occupation of the quantum-dot spin states is enforced by cotunneling processes. Resonant laser-induced fluorescence is used in order to measure the thermal quantum-dot spin-state population.

Kinetic Monte Carlo (KMC) simulations have been previously used to model and understand a wide range of behaviors in bulk heterojunction (BHJ) organic photovoltaic devices, from fundamental mechanisms to full device performance. One particularly unique and valuable aspect of this type of modeling technique is the ability to explicitly implement models for the bicontinuous nanostructured morphology present in these devices. For this purpose, an Ising-based method for creating model BHJ morphologies has become prevalent. However, this technique can be computationally expensive, and a detailed characterization of this method has not yet been published. Here, we perform a thorough characterization of this method and describe how to efficiently generate controlled model BHJ morphologies. We show how the interaction energy affects the tortuosity of the interconnected domains and the resulting charge-transport behavior in KMC simulations. We also demonstrate how to dramatically reduce calculation time by several orders of magnitude without detrimentally affecting the resulting morphologies. In the end, we propose standard conditions for generating model morphologies and introduce an open-source software tool. These developments to the Ising method provide a strong foundation for future simulation and modeling of BHJ organic photovoltaic devices that will lead to a more detailed understanding of the important link between morphological features and device performance.

We describe the coherent manipulation of harmonic oscillator and qubit modes using resonant trains of single flux quantum pulses in place of microwaves. We show that coherent rotations are obtained for pulse-to-pulse spacing equal to the period of the oscillator. We consider a protocol for preparing bright and dark harmonic oscillator pointer states. Next, we analyze rotations of a two-state qubit system. We calculate gate errors due to timing jitter of the single flux quantum pulses and due to weak anharmonicity of the qubit. We show that gate fidelities in excess of 99.9% are achievable for sequence lengths of order 20 ns.

In this work, we use an extended tight-binding approach for calculating the Fermi-energy dependence of the structural deformation of chiral single-wall carbon nanotubes (SWNTs). We show that, in general, nanotube strains occur in such a way as to avoid a net charge from being accumulated on the nanotube. We also investigate the effect of the Fermi-energy-induced strains on the electronic structure of SWNTs, showing that the optical transition energies change by up to 0.5 eV due to the induced strains and that this change is nearly independent of how the nanotube is deformed. Finally, we also consider the contribution of the electron-electron Coulomb repulsion to the total energy by using an effective regularized potential energy model. We show that the inclusion of the Coulomb repulsion leads to larger strains and smaller net charges transferred to the nanotube.

Metamaterial concepts have opened the door to high-optical anisotropy beyond levels of naturally occurring materials but usually with limited spectral bandwidth. Here, we report on the design, realization, and experimental and numerical characterization of broadband chiral metafoils exhibiting at the same time exceptionally high-contrast circular polarization in the THz range. We demonstrate that the observed chirality is achieved by a simple modification of the structure of well-established metafoils, namely, through a shift of interconnecting lines by half a unit cell length between each row. The bandwidth of the observed circular-polarization-selective response is demonstrated to be up to about one octave at a center frequency of 2.4 THz and the ratio of the transmission of oppositely circularly polarized radiation is shown to reach about 700%, with the maximum transmittance being approximately 70%. Although implemented here at THz frequencies, broadband chiral metafoils may be extended to higher infrared and optical frequencies by hot embossing or nanoimprinting. They may be used as circular polarizers and beam stops in THz optical and infrared systems.

In this work, the on-chip quantum interference of two single surface plasmons is achieved using dielectric-loaded surface-plasmon-polariton waveguides. The high visibility (greater than 90%) proves the bosonic nature of single plasmons and emphasizes the feasibility of achieving basic quantum logic gates for linear optical quantum computation. The effect of intrinsic losses in plasmonic waveguides with regard to quantum-information processing is also discussed. Although the influence of this effect is negligible in the current experiment, our studies reveal that such losses can dramatically reduce quantum-interference visibility in certain cases; thus, quantum coherence must be carefully considered when designing quantum photonic integrated circuit devices.

We address the tunneling current in a graphene/$h$-BN/graphene heterostructure as a function of the twisting between the crystals. The twisting induces a modulation of the hopping amplitude between the graphene layers that provides the extra momentum necessary to satisfy momentum and energy conservation and to activate coherent tunneling between the graphene electrodes. Conservation rules limit the tunneling to states with wave vectors lying at the conic curves defined by the intersection of two Dirac cones shifted in momentum and energy. There is a critical voltage where the intersection is a straight line, and the joint density of states presents a maximum. This peak in the number of states where both the wave vectors and the energies are matched reflects in a peak in the tunneling current and in a negative differential conductivity.