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.

We introduce an electrically driven scheme to tune the polariton condensate energy in a high-finesse GaAs microcavity. In contrast to the conventional redshift observed in semiconductor quantum wells (QWs) under applied electrical bias arising from the quantum-confined Stark effect (QCSE), we report here the blueshift of a polariton condensate caused by controlled reduction of the Rabi splitting due to tunneling-induced charge buildup and fractional bleaching of QWs. At larger electrical bias, the QCSE becomes dominant, leading to a redshift in the linear regime, while in the nonlinear regime to the eventual quenching of the condensate emission. This ability to tune the polariton condensate energy brings within reach the realization of voltage-controlled polariton condensate devices and variable-wavelength sources of coherent light.

Titanium dioxide (${\mathrm{TiO}}_2$) plays a central role in the study of artificial photosynthesis, owing to its ability to perform photocatalytic water splitting. Despite over four decades of intense research efforts in this area, there is still some debate over the nature of the first water monolayer on the technologically relevant anatase ${\mathrm{TiO}}_2(101)$ surface. In this work, we use first-principles calculations to reverse engineer the experimental high-resolution x-ray photoelectron spectra measured for this surface by Walle et al. [J. Phys. Chem. C 115, 9545 (2011)] and find evidence supporting the existence of a mix of dissociated and molecular water in the first monolayer. Using both semilocal and hybrid functional calculations, we revise the current understanding of the adsorption energetics by showing that the energetic cost of water dissociation is reduced via the formation of a hydrogen-bonded hydroxyl-water complex. We also show that such a complex can provide an explanation of an unusual superstructure observed in high-resolution scanning tunneling microscopy experiments.

We have investigated the strain-dependent magnetic properties of epitaxial narrow-bandwidth manganite, ${\mathrm{Eu}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ (ESMO), thin films on various substrates traversing from compressive ($-2.4\%$) to tensile ($+1.5\%$) strain. The stoichiometry and crystalline quality of the films were confirmed with diffraction and spectroscopic-based characterization. Using field- and temperature-dependent magnetometry, we find that the epitaxial strain acts to suppress the ferromagnetic state, although films under compressive strain retain a greater propensity for ferromagnetic behavior. However, temperature-dependent resistivity reveals that ESMO thin films exhibit a ferromagnetic insulating state in contrast to wide-bandwidth ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ films under a comparable strain. The combination of a highly strain-dependent ferromagnetic state and robust insulating behavior may offer novel applications in spin filtering, sensing, or electronics.

A semiclassical simulation approach is presented for studying quantum noise in large-scale photonic circuits incorporating an ideal Kerr nonlinearity. A circuit solver is used to generate matrices defining a set of stochastic differential equations, in which the resonator field variables represent random samplings of the Wigner quasiprobability distributions. Although the semiclassical approach involves making a large-photon-number approximation, tests on one- and two-resonator circuits indicate satisfactory agreement between the semiclassical and full-quantum simulation results in the parameter regime of interest. The semiclassical model is used to simulate random errors in a large-scale circuit that contains 88 resonators and hundreds of components in total and functions as a four-bit ripple counter. The error rate as a function of on-state photon number is examined, and it is observed that the quantum fluctuation amplitudes do not increase as signals propagate through the circuit, an important property for scalability.

Multiphoton transitions in $\mathrm{GaSb}/\mathrm{GaAs}$ quantum-dot intermediate-band solar cells are investigated at variable temperature and excitation intensity. A transition temperature is observed that corresponds to the crossover between quantum-dot intraband transitions dominated by thermal escape due to infrared photogeneration. The transition temperature follows an Arrhenius relation with an activation energy of 220 meV that corresponds to the energy barrier observed by holes in the quantum dots. The transition temperature is in the range of 160–225 K for the temperature range studied, significantly higher than observed in previous type-I quantum-dot systems. These results illustrate the potential of type-II structures with deep confinement potentials and strong intraband absorption for future intermediate-band solar cells and quantum devices.

An analytic potential energy function is developed for simulating clusters and nanoparticles of aluminum and its hydride. An embedded-atom method is used which modulates the background electron density as a function of the number of nearest-neighbor atoms. The method is parametrized and tested using an extensive training set computed from first-principle density-functional theory. The potential energy function is found to be reliable for clusters of arbitrary size, shape, and composition ratio. The force field obtained from the analytic potential energy function is computationally efficient and well suited for simulating large systems of aluminum and aluminum hydride particles. A proposed molecular dynamics simulation related to hydrogen-storage technologies for onboard automotive applications is briefly discussed.

We report the existence of confined electronic states at the (110) and (111) surfaces of ${\mathrm{SrTiO}}_3$. Using angle-resolved photoemission spectroscopy, we find that the corresponding Fermi surfaces, subband masses, and orbital ordering are different from the ones at the (001) surface of ${\mathrm{SrTiO}}_3$. This occurs because the crystallographic symmetries of the surface and subsurface planes and the effective electron masses along the confinement direction influence the symmetry of the electronic structure and the orbital ordering of the $t_{2g}$ manifold. Remarkably, our analysis of the data also reveals that the carrier concentration and thickness are similar for all three surface orientations, despite their different polarities. The orientational tuning of the microscopic properties of two-dimensional electron states at the surface of ${\mathrm{SrTiO}}_3$ echoes the tailoring of macroscopic (e.g., transport) properties reported recently in ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ (110) and (111) interfaces, and is promising for searching new types of two-dimensional electronic states in correlated-electron oxides.

We explore the Fe(001)-$\mathit{p}(1\times 1)\mathrm{O}$ surface for use as a spin-polarizing mirror to implement spin sensitivity in display-type electron analyzers. We perform spin-dependent low-energy electron reflection measurements over a wide range of incident electron energies and angles. We derive maps of reflectivity, the Sherman function, and figure of merit. Several working points are identified. A maximum figure of merit of $2.8\times {10}^{-2}$ is found at an incidence angle of 58° and a kinetic energy of 2.8 eV.