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A precision measurement by the Alpha Magnetic Spectrometer on the International Space Station of the positron fraction in primary cosmic rays in the energy range from 0.5 to 350 GeV based on $6.8\times {10}^6$ positron and electron events is presented. The very accurate data show that the positron fraction is steadily increasing from 10 to $\sim 250\mathrm{GeV}$, but, from 20 to 250 GeV, the slope decreases by an order of magnitude. The positron fraction spectrum shows no fine structure, and the positron to electron ratio shows no observable anisotropy. Together, these features show the existence of new physical phenomena.

Water diffuses into the conduction plane of lithium $\beta$-alumina. Infrared absorption measurements of single crystals treated with ${\mathrm{H}}_2$${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ show that, although the water concentration was homogeneous throughout the sample, most of the ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ remained near the surface. This result suggests that the oxygen in a water molecule is frequently replaced by other oxygen ions in the crystal. A diffusion mechanism which involves the dissociation of the protons from the water molecule and the jump of the oxygen ion by an interstitialcy movement into an O(5) oxygen site in the conduction plane is proposed.

Cross-beam energy transfer (CBET) is a significant energy-loss mechanism in directly driven inertial-confinement-fusion (ICF) targets. One strategy for mitigating CBET is to increase the bandwidth of the laser light, thereby disrupting the resonant three-wave interactions that underlie this nonlinear scattering process. Here, we report on numerical simulations performed with the wave-based code lpse that show a significant reduction in CBET for bandwidths of $2–5$ THz (corresponding to a normalized bandwidth of $0.2\%–0.6\%$ at a laser wavelength of $351\mathrm{nm}$) under realistic plasma conditions. Such bandwidths are beyond those available with current high-energy lasers used for ICF, but could be achieved using stimulated rotation Raman scattering in diatomic gases like nitrogen.

The transverse bulk polariton dispersion curves in the region above 400 ${\mathrm{cm}}^{-1\mathrm{}}$ in uniaxial ${\mathrm{Al}}_2$${\mathrm{O}}_3$ were determined from the inflection points on the smooth edges of the high-reflectivity bands in infrared spectra measured at various angles of incidence for TM polarization.

A theoretical model based on the method of super transition arrays (STA) is used to compute the emissivities, opacities, and average ionization states of carbon (C) and polystyrene (CH) plasmas in the warm-dense matter regime in which the coupling constant varies between 0.02 to 2.0. The accuracy of results of STA calculations is assessed by benchmarking against the available experimental data and results obtained using other theoretical methods, assuming that a state of local thermodynamic equilibrium exists in the plasma. In the case of a carbon plasma, the STA method yields spectral features that are in reasonably good agreement with Dirac-Fock and Hartree-Fock-Slater theories; in the case of CH, we find that STA-derived opacities are very similar to those derived using quantum-molecular-dynamics density-functional theory and Hartree-Fock method down to plasma temperature of about 20 eV. Our calculations also compare favorably with available experimental measurements of Gamboa et al. [High Energy Density Phys. 11, 75 (2014)] of the plasma temperature and average ionization state behind a blast wave in a pure carbon foam. Although the STA-computed average-ionization charge state in the rarefaction region appears to be lower than the experimental data, it is within the experimental uncertainty and the discrepancy is nevertheless consistent with results reported using an atomic kinetic model. In addition, we further predict the temperature dependence of average ionization states of CH plasma in the same temperature range as for the carbon plasma.

Planar laser-plasma interaction (LPI) experiments at the National Ignition Facility (NIF) have allowed access for the first time to regimes of electron density scale length ($\sim 500$ to $700\mu \mathrm{m}$), electron temperature ($\sim 3$ to 5 keV), and laser intensity (6 to $16\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$) that are relevant to direct-drive inertial confinement fusion ignition. Unlike in shorter-scale-length plasmas on OMEGA, scattered-light data on the NIF show that the near-quarter-critical LPI physics is dominated by stimulated Raman scattering (SRS) rather than by two-plasmon decay (TPD). This difference in regime is explained based on absolute SRS and TPD threshold considerations. SRS sidescatter tangential to density contours and other SRS mechanisms are observed. The fraction of laser energy converted to hot electrons is $\sim 0.7\%$ to 2.9%, consistent with observed levels of SRS. The intensity threshold for hot-electron production is assessed, and the use of a Si ablator slightly increases this threshold from $\sim {4\times 10}^{14}$ to $\sim 6\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. These results have significant implications for mitigation of LPI hot-electron preheat in direct-drive ignition designs.

Infrared absorption of O${\mathrm{H}}^{-}$, O${\mathrm{D}}^{-}$, and O${\mathrm{T}}^{-}$ in $\alpha$-${\mathrm{Al}}_2$${\mathrm{O}}_3$ at 77 K occurs at energies of 3282, 2439, and 2069 ${\mathrm{cm}}^{-1\mathrm{}}$, respectively. A small band at 3266 ${\mathrm{cm}}^{-1\mathrm{}}$ disappeared after high-temperature treatment of the crystal and was attributed to an O${\mathrm{H}}^{-}$ ion which is perturbed by an unidentified defect. Calculations of the electrostatic potential combined with measurements of the polarization of the absorbed light indicated that the O${\mathrm{H}}^{-}$ ion lies in the basal plane of the crystal. No evidence was found from measurements of the EPR spectrum of a $\gamma$-irradiated crystal for the presence of $V_{\mathrm{OH}}^{-}$ centers in $\alpha$-${\mathrm{Al}}_2$${\mathrm{O}}_3$.

The dispersion of ordinary and extraordinary surface polaritons in uniaxial ${\mathrm{Al}}_2$${\mathrm{O}}_3$ crystals has been measured with use of the attenuated-total-reflection method. Some additional reflectivity minima are observed, which are not associated with the excitation of surface polaritons, but are due to the low-reflectivity regions bounded by the optical-phonon-mode frequencies and the bulk-polariton dispersion curve. The discrepancies among the theoretical surface-polariton dispersion curves calculated with the use of various methods are discussed.

The small-signal impedance of chemical cells with ‘‘blocking’’ electrodes is dominated at low frequencies (f<∼100 Hz) by the impedance of the metal-electrolyte interfaces which is accurately represented by ${\mathrm{Z}}_{\mathrm{i}}$=A(jω${)}^{\mathrm{-}\mathrm{n}}$. The frequency exponent n, n<1, is sensitive to the texture of the interface, and several recent theories have been proposed that relate n with the fractal dimension d¯ of the rough interface. We find no correlation between n and d¯ from experiments using aqueous electrolytes and metal and semiconductor electrodes.

The strongly constrained and appropriately normed (SCAN) semilocal density functional [J. Sun, A. Ruzsinszky, and J. P. Perdew, Phys. Rev. Lett. 115, 036402 (2015)] obeys all 17 known exact constraints for meta-generalized-gradient approximations (meta-GGAs), and it includes some medium-range correlation effects. Long-range London dispersion interactions are still missing, but they can be accounted for via an appropriate correction scheme. In this study, we combine SCAN with an efficient London dispersion correction and show that lattice energies of simple organic crystals can be improved with the applied correction by 50%. The London-dispersion corrected SCAN meta-GGA outperforms all other tested London-dispersion corrected meta-GGAs for molecular geometries. Our method yields mean absolute deviations (MADs) for main group bond lengths that are consistently below 1 pm, rotational constants with MADs of 0.2%, and noncovalent distances with MADs below 1%. For a large database of general main group thermochemistry and kinetics ($\sim 800$ chemical species), one of the lowest weighted mean absolute deviations for long-range corrected meta-GGA functionals is achieved. Noncovalent interactions are of average quality, and hydrogen bonded systems in particular seem to suffer from overestimated polarization related to the self-interaction error of SCAN. We also discuss some consequences of numerical sensitivity encountered for meta-GGAs.

We performed integrated experiments on impact ignition, in which a portion of a deuterated polystyrene (CD) shell was accelerated to about $600\mathrm{km}/\mathrm{s}$ and was collided with precompressed CD fuel. The kinetic energy of the impactor was efficiently converted into thermal energy generating a temperature of about 1.6 keV. We achieved a two-order-of-magnitude increase in the neutron yield by optimizing the timing of the impact collision, demonstrating the high potential of impact ignition for fusion energy production.

Infrared spectra of Ti${\mathrm{O}}_2$ single crystals containing either hydrogen, deuterium, or tritium were measured at 300, 77, and 8 K. Bands observed at 3276, 2437, and 2065 ${\mathrm{cm}}^{-1\mathrm{}}$ near 300 K are, respectively, assigned to O${\mathrm{H}}^{-}$, O${\mathrm{D}}^{-}$, and O${\mathrm{T}}^{-}$ based on a two-term anharmonic-oscillator model of the hydrogenic species. The frequencies were also analyzed by using a linear hydrogen-bonded model with harmonic forces, but the correlation between frequencies and bandwidths was at variance with that of known H-bonded systems. The absorption strength per ion of O${\mathrm{T}}^{-}$ was calculated at several levels of approximation, and the best value determined was $a_{\mathrm{T}}=9.24\mathrm{}\times {10}^{-18\mathrm{}}$ cm. The harmonic and anharmonic mean-square displacements of O${\mathrm{H}}^{-}$, O${\mathrm{D}}^{-}$, and O${\mathrm{T}}^{-}$ were calculated, and the differences between these quantities reflect the changes in the infrared bandwidths with reduced mass.

We analyze the two-dimensional stability of a shock wave driven by a steadily moving corrugated piston in an inviscid fluid with an arbitrary equation of state. For $h\le -1$ or $h>h_c$, where $h$ is the D'yakov parameter and $h_c$ is the Kontorovich limit, we find that small perturbations on the shock front are unstable and grow—at first quadratically and later linearly—with time. Such instabilities are associated with nonequilibrium fluid states and imply a nonunique solution to the hydrodynamic equations. The above criteria are consistent with instability limits observed in shock-tube experiments involving ionizing and dissociating gases and may have important implications for driven shocks in laser-fusion, astrophysical, and/or detonation studies.

First-order induced Raman scattering has been observed from single crystals of LiCl containing ${\mathrm{Ni}}^{2+}$ ions. Sharp bands at 247, 216, 161, and 122 ${\mathrm{cm}}^{-1\mathrm{}}$ are assigned to $A_{1g}$, $T_{2g}$, $E_g$, and $T_{2g}$ modes, respectively, on the basis of polarization measurements with <100> and <110> crystals. No differences were observed in the experimental peak positions, bandwidths, or relative intensities for crystals in which the ${\mathrm{Ni}}^{2+}$ concentration differed by a factor of 2. Theoretical calculations suggest that the $A_{1g}$ and the higher frequency $T_{2g}$ bands are due to vibrations about the ${\mathrm{Ni}}^{2+}$ ions, while the $E_g$ and possibly the lower-frequency $T_{2g}$ bands can be assigned to the local modes about the cation vacancies.

The predominant hydrogen-containing species in Ti${\mathrm{O}}_2$ (rutile) are O${\mathrm{H}}^{-}$ (hydroxyl) ions, in which the oxygen occupies a regular oxygen ion site, and the O-H bond is perpendicular to the $c$ axis. It is proposed that diffusion of hydrogen parallel to the $c$ axis proceeds by a proton jump from one ${\mathrm{O}}^{2-}$ ion to another along the channel as represented by O${\mathrm{H}}^{-}$···${\mathrm{O}}^{2-}$ → ${\mathrm{O}}^{2-}$···${\mathrm{H}}^{+}$···${\mathrm{O}}^{2-}$ → ${\mathrm{O}}^{2-}$···H${\mathrm{O}}^{-}$. It is also proposed that diffusion perpendicular to the $c$ axis proceeds by a rotation of the O${\mathrm{H}}^{-}$ bond to move the proton from one channel to an adjacent channel, followed by a proton jump to another ${\mathrm{O}}^{2-}$ ion in the same channel. From a potential-energy model, which includes a Morse function to represent the O${\mathrm{H}}^{-}$ bond, as well as electrostatic and repulsive terms, the activation energies for hydrogen and tritium diffusion parallel to the $c$ axis were calculated to be (including a zero-point energy correction) 0.60 and 0.69 eV, respectively, in good agreement with the respective experimental values of 0.59 and 0.75 eV. The calculated activation energy for diffusion perpendicular to the $c$ axis was 1.23 eV (no zero-point energy correction), as compared to the experimental values of 1.28 and 1.11 eV, respectively, for hydrogen and tritium. The calculated equilibrium orientation of the O${\mathrm{H}}^{-}$ ion in Ti${\mathrm{O}}_2$ and the calculated stretching frequency of this species were also in good agreement with the respective experimental results.

Effects of quantized free scalar fields in cosmological spacetimes with big rip singularities are investigated. The energy densities for these fields are computed at late times when the expansion is very rapid. For the massless minimally coupled field it is shown that an attractor state exists in the sense that, for a large class of states, the energy density of the field asymptotically approaches the energy density it would have if it was in the attractor state. Results of numerical computations of the energy density for the massless minimally coupled field and for massive fields with minimal and conformal couplings to the scalar curvature are presented. For the massive fields the energy density is seen to always asymptotically approach that of the corresponding massless field. The question of whether the energy densities of quantized fields can be large enough for backreaction effects to remove the big rip singularity is addressed.

According to the Franck-Condon theory for the dissociation of molecules by light, we should also expect that two colliding atoms may recombine with the emission of light. This should occur if conditions are arranged so that there are appreciable quantities of the free atoms present. Experiments were carried out with flames of chlorine and hydrogen and hydrogen-oxygen flames containing the halogens. Spectra of these flames are found to be continuous extending into the ultra-violet. It is shown that the violet limits of these spectra are in agreement with the hypothesis that they are due to the recombination of the normal and excited halogen atoms. Circulations show that there are enough collisions of this type in our flames to account for the intensity of the light emitted.

Based on the random phase approximation (RPA), RPA renormalization [J. E. Bates and F. Furche, J. Chem. Phys. 139, 171103 (2013)] is a robust many-body perturbation theory that works for molecules and materials because it does not diverge as the Kohn-Sham gap approaches zero. Additionally, RPA renormalization enables the simultaneous calculation of RPA and beyond-RPA correlation energies since the total correlation energy is the sum of a series of independent contributions. The first-order approximation (RPAr1) yields the dominant beyond-RPA contribution to the correlation energy for a given exchange-correlation kernel, but systematically underestimates the total beyond-RPA correction. For both the homogeneous electron gas model and real systems, we demonstrate numerically that RPA renormalization beyond first order converges monotonically to the infinite-order beyond-RPA correlation energy for several model exchange-correlation kernels and that the rate of convergence is principally determined by the choice of the kernel and spin polarization of the ground state. The monotonic convergence is rationalized from an analysis of the RPA renormalized correlation energy corrections, assuming the exchange-correlation kernel and response functions satisfy some reasonable conditions. For spin-unpolarized atoms, molecules, and bulk solids, we find that RPA renormalization is typically converged to 1 meV error or less by fourth order regardless of the band gap or dimensionality. Most spin-polarized systems converge at a slightly slower rate, with errors on the order of 10 meV at fourth order and typically requiring up to sixth order to reach 1 meV error or less. Slowest to converge, however, open-shell atoms present the most challenging case and require many higher orders to converge.

The ground-state equilibrium properties of copper-gold alloys have been explored with the state-of-the-art random phase approximation (RPA). Our estimated lattice constants agree with the experiment within a mean absolute percentage error of 1.4 percent. Semilocal functionals, such as the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) and the strongly constrained and appropriately normed (SCAN) functional, fail to provide accurate bulk moduli, which indicates their inability to describe the system in a stretched or compressed state with respect to the equilibrium geometry. The PBE, PBE revised for solids, and revised Tao-Perdew-Staroverov-Scuseria functionals by Perdew et al. predict too-low formation energies, while the SCAN slightly overestimates it. The inclusion of thermal correction or the long-range dispersion provides a negligible contribution to the formation energies estimated with semilocal density functional theory. The spin-orbit coupling improves the formation energies of PBE only by 7–8 meV, while it intensifies the overestimation of SCAN. We found that the nonlocality present in RPA is able to describe the transition between two delocalized electron densities (bulk elemental constituents to crystallized alloys), as required to provide accurate formation energies without any further corrections. Based on our results, we conclude that it is difficult to find a universal density functional which can give accurate results for a wide range of properties of intermetallic alloys. However, RPA can capture different bonding situations, often better than other density functionals. It gives accurate results for a wide range of ground-state properties for the alloys, generated from metals with completely filled $d$ shells.