Search

We describe an experimental concept at the National Ignition Facility for specifically tailored spherical implosions to compress hydrogen to extreme densities (up to $\sim$800$\times$ solid density, electron number density n$_e$$\sim$4$\times$10$^{25}$ cm$^{-3}$ ) at moderate temperatures (T$\sim$200 eV), i.e., to conditions, which are relevant to the interiors of red dwarf stars. The dense plasma will be probed by laser-generated x-ray radiation of different photon energy to determine the plasma opacity due to collisional (free-free) absorption and Thomson scattering. The obtained results will benchmark radiation transport models, which in the case for free-free absorption show strong deviations at conditions relevant to red dwarfs. This very first experimental test of free-free opacity models at these extreme states will help to constrain where inside those celestial objects energy transport is dominated by radiation or convection. Moreover, our study will inform models for other important processes in dense plasmas, which are based on electron-ion collisions, e.g., stopping of swift ions or electron-ion temperature relaxation.

Ammonia is predicted to be one of the major components in the depths of the ice giant planets Uranus and Neptune. Their dynamics, evolution, and interior structure are insufficiently understood and models rely imperatively on data for equation of state and transport properties. Despite its great significance, the experimentally accessed region of the ammonia phase diagram today is still very limited in pressure and temperature. Here we push the probed regime to unprecedented conditions, up to $\sim$350 GPa and $\sim$40000 K. Along the Hugoniot, the temperature measured as a function of pressure shows a subtle change in slope at $\sim$7000 K and $\sim$90 GPa, in agreement with ab initio simulations we have performed. This feature coincides with the gradual transition from a molecular liquid to a plasma state. Additionally, we performed reflectivity measurements, providing the first experimental evidence of electronic conduction in high-pressure ammonia. Shock reflectance continuously rises with pressure above 50 GPa and reaches saturation values above 120 GPa. Corresponding electrical conductivity values are up to 1 order of magnitude higher than in water in the 100 GPa regime, with possible significant contributions of the predicted ammonia-rich layers to the generation of magnetic dynamos in ice giant interiors., Comment: Please visit publisher's website for supplementary information

Recent dynamic compression experiments [M. D. Knudson et al., Science 348, 1455 (2015); P. M. Celliers et al., Science 361, 677 (2018)] have observed the insulator-metal transition in dense liquid deuterium, but with an approximately 95 GPa difference in the quoted pressures for the transition at comparable estimated temperatures. It was claimed in the latter of these two papers that a very large latent heat effect on the temperature was overlooked in the first, requiring correction of those temperatures downward by a factor of two, thereby putting both experiments on the same theoretical phase boundary and reconciling the pressure discrepancy. We have performed extensive path-integral molecular dynamics calculations with density functional theory to directly calculate the isentropic temperature drop due to latent heat in the insulator-metal transition for dense liquid deuterium and show that this large temperature drop is not consistent with the underlying thermodynamics., Comment: 7 pages, 5 figures

The structure of the fluid carbon phase in the pressure region of the graphite, diamond, and BC8 solid phase is investigated. We find increasing coordination numbers with an increase in density. From zero to $30$ GPa, the liquid shows a decrease of packing efficiency with increasing temperature. However, for higher pressures, the coordination number increases with increasing temperature. Up to $1.5$ eV and independent of the pressure up to $1000$ GPa, a double-peak structure in the ion structure factors exists, indicating persisting covalent bonds. Over the whole pressure range from zero to $1000$ GPa, the fluid structure and properties are strongly determined by such covalent bonds., Comment: 11 figures

The extreme ultraviolet plasma emission from liquid microsized argon droplets exposed to intense near-infrared laser pulses has been investigated. Emission from the warm dense matter targets is recorded in a spectral range in between 16 and 30 nm at laser intensities of 10^14 W/cm^2. Above the emission threshold, soft x-ray radiation exponentially increases with the pulse energy whereby a strong dependence of the yields on the pulse duration is observed, which points at an effective electron collisional heating of the microplasma by inverse bremsstrahlung. Accompanying hydrodynamic simulations reveal the temporal and spatial development of the microplasma conditions. The good agreement in between the measured and calculated emission spectra as well as the extracted electron temperatures confirm that hydrodynamic simulations can be applied in the analysis of strongly excited droplets., Comment: 13 pages, 10 figures

We study the interaction of intense ultrashort laser pulses with cryogenic He jets using 2d/3v relativistic Particle-in-Cell simulations (XOOPIC). Of particular interest are laser intensities $(10^{15}-10^{20})$ W/cm$^2$, pulse lengths $\le 100$ fs, and the frequency regime $\sim 800$ nm for which the jets are initially transparent and subsequently not homogeneously ionized. Pulses $\ge 10^{16}$ W/cm$^2$ are found to drive ionization along the jet and outside the laser spot, the ionization-front propagates along the jet at a fraction of the speed of light. Within the ionized region, there is a highly transient field, which may be interpreted as two-surface wave decay and as a result of the charge-neutralizing disturbance at the jet-vacuum interface. The ionized region has solid-like densities and temperatures of few to hundreds of eV, i.e., warm and hot dense matter regimes. Such extreme conditions are relevant for high-energy densities as found, e.g., in shock-wave experiments and inertial confinement fusion studies. The temporal evolution of the ionization is studied considering theoretically a pump-probe x-ray Thomson scattering (XRTS) scheme. We observe plasmon and non-collective modes that are generated in the jet, and their amplitude is proportional to the ionized volume. Our theoretical findings could be tested at free-electron laser facilities such as FLASH and the European XFEL (Hamburg) and the LCLS (Stanford).

The strikingly low luminosity of Uranus (Teff ~ Teq) constitutes a long-standing challenge to our understanding of Ice Giant planets. Here we present the first Uranus structure and evolution models that are constructed to agree with both the observed low luminosity and the gravity field data. Our models make use of modern ab initio equations of state at high pressures for the icy components water, methane, and ammonia. Proceeding step by step, we confirm that adiabatic models yield cooling times that are too long, even when uncertainties in the ice:rock ratio (I:R) are taken into account. We then argue that the transition between the ice/rock-rich interior and the H/He-rich outer envelope should be stably stratified. Therefore, we introduce a simple thermal boundary and adjust it to reproduce the low luminosity. Due to this thermal boundary, the deep interior of the Uranus models are up to 2--3 warmer than adiabatic models, necessitating the presence of rocks in the deep interior with a possible I:R of $1\times$ solar. Finally, we allow for an equilibrium evolution (Teff ~ Teq) that begun prior to the present day, which would therefore no longer require the current era to be a "special time" in Uranus' evolution. In this scenario, the thermal boundary leads to more rapid cooling of the outer envelope. When Teff ~ Teq is reached, a shallow, subadiabatic zone in the atmosphere begins to develop. Its depth is adjusted to meet the luminosity constraint. This work provides a simple foundation for future Ice Giant structure and evolution models, that can be improved by properly treating the heat and particle fluxes in the diffusive zones., Comment: 13 pages, Accepted to Icarus

The controversy with respect to the role of electron-electron collisions in the dynamic conductivity of dense plasmas is resolved. In particular, the dc conductivity is analyzed in the low-density, non-degenerate limit where the Spitzer theory is valid and electron-electron collisions lead to the well-known reduction of the result for a Lorentz plasma. With increasing degeneracy, the contribution of electron-electron collisions to the dc conductivity is decreasing and can be neglected for the liquid metal domain where the Ziman-Faber theory is applicable. We show how electron-electron collisions have to be implemented in calculations based on the Kubo-Greenwood formula which is prevalently applied in simulations for the frequency-dependent conductivity in the warm dense matter region, i.e. for arbitrary degeneracy., Comment: 21 pages, 4 figures

PLATO 2.0 has recently been selected for ESA's M3 launch opportunity (2022/24). Providing accurate key planet parameters (radius, mass, density and age) in statistical numbers, it addresses fundamental questions such as: How do planetary systems form and evolve? Are there other systems with planets like ours, including potentially habitable planets? The PLATO 2.0 instrument consists of 34 small aperture telescopes (32 with 25 sec readout cadence and 2 with 2.5 sec candence) providing a wide field-of-view (2232 deg2) and a large photometric magnitude range (4-16 mag). It focusses on bright (4-11 mag) stars in wide fields to detect and characterize planets down to Earth-size by photometric transits, whose masses can then be determined by ground-based radial-velocity follow-up measurements. Asteroseismology will be performed for these bright stars to obtain highly accurate stellar parameters, including masses and ages. The combination of bright targets and asteroseismology results in high accuracy for the bulk planet parameters: 2%, 4-10% and 10% for planet radii, masses and ages, respectively. The planned baseline observing strategy includes two long pointings (2-3 years) to detect and bulk characterize planets reaching into the habitable zone (HZ) of solar-like stars and an additional step-and-stare phase to cover in total about 50% of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect and characterize hundreds of small planets, and thousands of planets in the Neptune to gas giant regime out to the HZ. It will therefore provide the first large-scale catalogue of bulk characterized planets with accurate radii, masses, mean densities and ages. This catalogue will include terrestrial planets at intermediate orbital distances, where surface temperatures are moderate. Coverage of this parameter range with statistical numbers of bulk characterized planets is unique to PLATO 2.0., Comment: 63 pages, 17 figures, submitted to Experimental Astronomy (ExA)

The core mass of Saturn is commonly assumed to be 10-25 ME as predicted by interior models with various equations of state (EOSs) and the Voyager gravity data, and hence larger than that of Jupiter (0-10 ME). We here re-analyze Saturn's internal structure and evolution by using more recent gravity data from the Cassini mission and different physical equations of state: the ab initio LM-REOS which is rather soft in Saturn's outer regions but stiff at high pressures, the standard Sesame-EOS which shows the opposite behavior, and the commonly used SCvH-i EOS. For all three EOS we find similar core mass ranges, i.e. of 0-20 ME for SCvH-i and Sesame EOS and of 0-17 ME for LM-REOS. Assuming an atmospheric helium mass abundance of 18%, we find maximum atmospheric metallicities, Zatm of 7x solar for SCvH-i and Sesame-based models and a total mass of heavy elements, MZ of 25-30 ME. Some models are Jupiter-like. With LM-REOS, we find MZ=16-20 ME, less than for Jupiter, and Zatm less than 3x solar. For Saturn, we compute moment of inertia values lambda=0.2355(5). Furthermore, we confirm that homogeneous evolution leads to cooling times of only about 2.5 Gyr, independent on the applied EOS. Our results demonstrate the need for accurately measured atmospheric helium and oxygen abundances, and of the moment of inertia for a better understanding of Saturn's structure and evolution., Comment: 13 pages, accepted to Icarus, figure 6 modified after acceptence

Since the Voyager fly-bys of Uranus and Neptune, improved gravity field data have been derived from long-term observations of the planets' satellite motions, and modified shape and solid-body rotation periods were suggested. A faster rotation period (-40 min) for Uranus and a slower rotation period (+1h20) of Neptune compared to the Voyager data were found to minimize the dynamical heights and wind speeds. We apply the improved gravity data, the modified shape and rotation data, and the physical LM-R equation of state to compute adiabatic three-layer structure models, where rocks are confined to the core, and homogeneous thermal evolution models of Uranus and Neptune. We present the full range of structure models for both the Voyager and the modified shape and rotation data. In contrast to previous studies based solely on the Voyager data or on empirical EOS, we find that Uranus and Neptune may differ to an observationally significant level in their atmospheric heavy element mass fraction Z1 and nondimensional moment of inertia, nI. For Uranus, we find Z1 < 8% and nI=0.2224(1), while for Neptune Z1 < 65% and nI=0.2555(2) when applying the modified shape and rotation data, while for the unmodified data we compute Z1 < 17% and nI=0.230(1) for Uranus and Z1 < 54% and nI=0.2410(8) for Neptune. In each of these cases, solar metallicity models (Z1=0.015) are still possible. The cooling times obtained for each planet are similar to recent calculations with the Voyager rotation periods: Neptune's luminosity can be explained by assuming an adiabatic interior while Uranus cools far too slowly. More accurate determinations of these planets' gravity fields, shapes, rotation periods, atmospheric heavy element abundances, and intrinsic luminosities are essential for improving our understanding of the internal structure and evolution of icy planets., Comment: accepted to Planet. Space Sci., special edition

Recently there has been tremendous increase in the number of identified extra-solar planetary systems. Our understanding of their formation is tied to exoplanet internal structure models, which rely upon equations of state of light elements and compounds like water. Here we present shock compression data for water with unprecedented accuracy that shows water equations of state commonly used in planetary modeling significantly overestimate the compressibility at conditions relevant to planetary interiors. Furthermore, we show its behavior at these conditions, including reflectivity and isentropic response, is well described by a recent first-principles based equation of state. These findings advocate this water model be used as the standard for modeling Neptune, Uranus, and "hot Neptune" exoplanets, and should improve our understanding of these types of planets., Comment: Accepted to Phys. Rev. Lett.; supplementary material attached including 2 figures and 2 tables; to view attachments, please download and extract the gzipped tar source file listed under "Other formats"

Transit and radial velocity observations continuously discover an increasing number of exoplanets. However, when it comes to the composition of the observed planets the data are compatible with several interior structure models. Thus, a planetary parameter sensitive to the planet's density distribution could help constrain this large number of possible models even further. We aim to investigate to what extent an exoplanet's interior can be constrained in terms of core mass and envelope metallicity by taking the tidal Love number k_2 into account as an additional possibly observable parameter. Because it is the only planet with an observationally determined k_2, we constructed interior models for the Hot Jupiter exoplanet HAT-P-13b by solving the equations of hydrostatic equilibrium and mass conservation for different boundary conditions. In particular, we varied the surface temperature and the outer temperature profile, as well as the envelope metallicity within the widest possible parameter range. We also considered atmospheric conditions that are consistent with nongray atmosphere models. For all these models we calculated the Love number k_2 and compared it to the allowed range of k_2 values that could be obtained from eccentricity measurements of HAT-P-13b. We use the example of HAT-P-13b to show the general relationships between the quantities temperature, envelope metallicity, core mass, and Love number of a planet. For any given k_2 value a maximum possible core mass can be determined. For HAT-P-13b we find Mcore < 27 ME, based on the latest eccentricity measurement. We are able to constrain both the envelope and bulk metallicity of HAT-P-13b to 1 -- 11 times stellar metallicity and the extension of the isothermal layer in the planet's atmosphere to 3 -- 44 bar. Assuming equilibrium tidal theory, we find lower limits on the tidal Q consistent with 10^3 - 10^5., Comment: 8 pages, 5 figures A&A, accepted

We present the Young Exoplanet Transit Initiative (YETI), in which we use several 0.2 to 2.6m telescopes around the world to monitor continuously young (< 100 Myr), nearby (< 1 kpc) stellar clusters mainly to detect young transiting planets (and to study other variability phenomena on time-scales from minutes to years). The telescope network enables us to observe the targets continuously for several days in order not to miss any transit. The runs are typically one to two weeks long, about three runs per year per cluster in two or three subsequent years for about ten clusters. There are thousands of stars detectable in each field with several hundred known cluster members, e.g. in the first cluster observed, Tr-37, a typical cluster for the YETI survey, there are at least 469 known young stars detected in YETI data down to R=16.5 mag with sufficient precision of 50 milli-mag rms (5 mmag rms down to R=14.5 mag) to detect transits, so that we can expect at least about one young transiting object in this cluster. If we observe 10 similar clusters, we can expect to detect approximately 10 young transiting planets with radius determinations. The precision given above is for a typical telescope of the YETI network, namely the 60/90-cm Jena telescope (similar brightness limit, namely within +/-1 mag, for the others) so that planetary transits can be detected. For planets with mass and radius determinations, we can calculate the mean density and probe the internal structure. We aim to constrain planet formation models and their time-scales by discovering planets younger than 100 Myr and determining not only their orbital parameters, but also measuring their true masses and radii, which is possible so far only by the transit method. Here, we present an overview and first results. (Abstract shortened), Comment: 15 pages, 10 figures, AN accepted 2011 June 15

In order to accurately model giant planets, a whole set of observational constraints is needed. As the conventional constraints for extrasolar planets like mass, radius, and temperature allow for a large number of acceptable models, a new planetary parameter is desirable in order to further constrain planetary models. Such a parameter may be the tidal Love number k2. In this paper we aim to study the capability of k2 to reveal further information about the interior structure of a planet. With theoretical planetary models we investigate how the tidal Love number k2 responds to the internal density distribution of a planet. In particular, we demonstrate the effect of the degeneracy of k2 due to a density discontinuity in the envelope of a three-layer planetary model. The effect of a possible outer density discontinuity masks the effect of the core mass on the Love number k2. Hence, there is no unique relationship between the Love number k2 and the core mass of a planet. We show that the degeneracy of k2 with respect to a layer boundary in the envelope also occurs in existing planets, e.g. Saturn and the Hot Neptune GJ436b. As a result of the degeneracy, the planetary parameter k2 cannot be used to further constrain Saturnian models and for GJ436b only a maximum possible core mass can be derived from a given k2. To significantly narrow the uncertainty about the core mass of GJ436b the combined knowledge of k2 and atmospheric metallicity and temperature profile is necessary., Comment: 7 pages, 4 figures, accepted

The planet GJ 1214b is the second known super-Earth with a measured mass and radius. Orbiting a quiet M-star, it receives considerably less mass-loss driving X-ray and UV radiation than CoRoT-7b, so that the interior may be quite dissimilar in composition, including the possibility of a large fraction of water. We model the interior of GJ 1214b assuming a two-layer (envelope+rock core) structure where the envelope material is either H/He, pure water, or a mixture of H/He and H2O. Within this framework we perform models of the thermal evolution and contraction of the planet. We discuss possible compositions that are consistent with Mp=6.55 ME, Rp=2.678 RE, an age tau=3-10 Gyr, and the irradiation level of the atmosphere. These conditions require that if water exists in the interior, it must remain in a fluid state, with important consequences for magnetic field generation. These conditions also require the atmosphere to have a deep isothermal region extending down to 80-800 bar, depending on composition. Our results bolster the suggestion of a metal-enriched H/He atmosphere for the planet, as we find water-world models that lack an H/He atmosphere to require an implausibly large water-to-rock ratio of more than 6:1. We instead favor a H/He/H2O envelope with high water mass fraction (~0.5-0.85), similar to recent models of the deep envelope of Uranus and Neptune. Even with these high water mass fractions in the H/He envelope, generally the bulk composition of the planet can have subsolar water:rock ratios. Dry, water-enriched, and pure water envelope models differ to an observationally significant level in their tidal Love numbers k2 of respectively ~0.018, 0.15, and 0.7., Comment: 11 pages, 6 figures, 1 table, accepted to ApJ

GJ 436b is the first extrasolar planet discovered that resembles Neptune in mass and radius. The particularly interesting property of Neptune-sized planets is that their mass Mp and radius Rp are close to theoretical M-R relations of water planets. Given Mp, Rp, and equilibrium temperature, however, various internal compositions are possible. A broad set of interior structure models is presented here that illustrates the dependence of internal composition and possible phases of water occurring in presumably water-rich planets, such as GJ 436b on the uncertainty in atmospheric temperature profile and mean density. We show how the set of solutions can be narrowed down if theoretical constraints from formation and model atmospheres are applied or potentially observational constraints for the atmospheric metallicity Z1 and the tidal Love number k2. We model the interior by assuming either three layers (hydrogen-helium envelope, water layer, rock core) or two layers (H/He/H2O envelope, rocky core). For water, we use the equation of state H2O-REOS based on FT-DFT-MD simulations. Some admixture of H/He appears mandatory for explaining the measured radius. For the warmest considered models, the H/He mass fraction can reduce to 10^-3, still extending over ~0.7 REarth. If water occurs, it will be essentially in the plasma phase or in the superionic phase, but not in an ice phase. Metal-free envelope models have 0.020.3, and further to 0.2 Mp, if k2>0.5, and core mass and Z1 become sensitive functions of k2. To further narrow the set of solutions, a proper treatment of the atmosphere and the evolution is necessary., Comment: 9 pages, accepted to A&A

Collective X-ray Thomson scattering has become a versatile tool for the diagnostics of dense plasmas. Assuming homogeneous density and temperature throughout the target sample, these parameters can be determined directly from the plasmon dispersion and the ratio of plasmon amplitudes via detailed balance. In inhomogeneous media, the scattering signal is an average of the density and temperature dependent scattering cross-section weighted with the density and temperature profiles. We analyse Thomson scattering spectra in the XUV range from near solid density hydrogen targets generated by free electron laser radiation.The influence of plasma inhomogeneities on the scattering spectrum is investigated by comparing density and temperature averaged scattering signals to calculations assuming homogeneous targets. We found discrepancies larger than 10% between the mean electron density and the effective density as well as between the mean temperature and the effective temperature., Comment: 6 pages, 5 figures

We investigate the effects of Pauli blocking on the properties of hydrogen at high pressures, where recent experiments have shown a transition from insulating behavior to metal-like conductivity. Since the Pauli principle prevents multiple occupation of electron states (Pauli blocking), atomic states disintegrate subsequently at high densities (Mott effect). We calculate the energy shifts due to Pauli blocking and discuss the Mott effect solving an effective Schroedinger equation for strongly correlated systems. The ionization equilibrium is treated on the basis of a chemical approach. Results for the ionization equilibrium and the pressure in the region 4.000 K < T < 20.000 K are presented. We show that the transition to a highly conducting state is softer than found in earlier work. A first order phase transition is observed at T < 6.450 K, but a diffuse transition appears still up to 20.000 K., Comment: 8 pages, 4 figures, version accepted for publication in Journal of Physics A: Mathematical and Theoretical, special issue

The equation of state of hydrogen, helium, and water effects interior structure models of giant planets significantly. We present a new equation of state data table, LM-REOS, generated by large scale quantum molecular dynamics simulations for hydrogen, helium, and water in the warm dense matter regime, i.e.for megabar pressures and temperatures of several thousand Kelvin, and by advanced chemical methods in the complementary regions. The influence of LM-REOS on the structure of Jupiter is investigated and compared with state-of-the-art results within a standard three-layer model consistent with astrophysical observations of Jupiter. Our new Jupiter models predict an important impact of mixing effects of helium in hydrogen with respect to an altered compressibility and immiscibility., Comment: to appear in ApJ in August 2008, 11 figures

We have performed HNC calculations for dense beryllium plasma as studied experimentally using x-ray Thomson scattering, recently. We treated non-equilibrium situations with different electron and ion temperatures which are relevant in pump-probe experiments on ultra-short time scales. To consider quantum effects adequately, we used effective pair potentials to describe the interactions. Results are compared with classical as well as quantum corrected Debye model calculations., Comment: 7 pages, 4 figures. Contribution to the 12th International Workshop on the Physics of Non-Ideal Plasmas PNP 12, Sept. 4 - 8, 2006, Darmstadt, Germany. To appear in Contrib. Plasma Phys

We determine the spectral photon yield from a hot dense plasma irradiated by VUV-FEL light in a Thomson scattering experiment. The Thomson signal is compared to the emission background mainly caused by bremsstrahlung photons. We determine experimental conditions that allow for a signal-to-background ratio larger than unity. By derivation of the Thomson and the bremsstrahlung spectrum from linear response theory we present a consistent quantum statistical approach to both processes. This allows for a systematic treatment of medium and quantum effects such as dynamical screening and strong collisions. Results are presented for the threshold FEL-intensity as a function of density and temperature. We show that the account for quantum effects leads to larger thresholds as compared to previous work., Comment: 13 pages, 10 figures, and 1 table v2: minor corrections in figures and appendix

On the basis of a quantum-statistical approach to the electrical conductivity of nonideal plasmas we derive analytical results in the classical low-density regime, in the degenerate Born limit, and for the contribution of the Debye-Onsager relaxation effect. These explicit results are used to construct an improved interpolation formula of the electrical conductivity valid in a wide range of temperature and density which allows to compare with available experimental data of nonideal plasmas., Comment: 7 pages, 1 table

Quantum Monte Carlo methods have proven to predict atomic and bulk properties of light and non-light elements with high accuracy. Here we report on the first variational quantum Monte Carlo (VMC) calculations for solid surfaces. Taking the boundary condition for the simulation from a finite layer geometry, the Hamiltonian, including a nonlocal pseudopotential, is cast in a layer resolved form and evaluated with a two-dimensional Ewald summation technique. The exact cancellation of all Jellium contributions to the Hamiltonian is ensured. The many-body trial wave function consists of a Slater determinant with parameterized localized orbitals and a Jastrow factor with a common two-body term plus a new confinement term representing further variational freedom to take into account the existence of the surface. We present results for the ideal (110) surface of Galliumarsenide for different system sizes. With the optimized trial wave function, we determine some properties related to a solid surface to illustrate that VMC techniques provide standard results under full inclusion of many-body effects at solid surfaces., Comment: 9 pages with 2 figures (eps) included, Latex 2.09, uses REVTEX style, submitted to Phys. Rev. B

Variational quantum Monte Carlo calculations are reported for the bulk GaAs semiconductor in order to present values for the ground-state energy, the lattice constant, the bulk modulus, and some derived properties. The statistical accuracy is significantly higher than the remaining differences to the experimental values, especially for the total-energy upper bound. The agreement with experiment is satisfactory. The results are also compared with those of density functional calculations. The accuracy of our results is comparable to the best of those calculations., Comment: 5 pages RevteX, 1 Postscript figure in press Phys. Rev. B (15 Aug 96)

The impact ionization rate and its orientation dependence in k space is calculated for ZnS. The numerical results indicate a strong correlation to the band structure. The use of a q-dependent screening function for the Coulomb interaction between conduction and valence electrons is found to be essential. A simple fit formula is presented for easy calculation of the energy dependent transition rate., Comment: 9 pages LaTeX file, 3 EPS-figures (use psfig.sty), accepted for publication in PRB as brief Report (LaTeX source replaces raw-postscript file)