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We computed the total water vapor outgassing rate for comet 67P/Churyumov-Gerasimenko for a time period spanning several months before and after the perihelion in 2015. The two-layer surface model includes time-dependent solar illumination, thermal emission, sublimation, heat transport into the nucleus, and gas diffusion through the dust layer. The model parameters include among others the thickness of the top dust layer and the ice content in the bottom layer. We fitted the model parameters so that the temporal evolution of the computed water outgassing rate matches published outgassing rates derived from observations with the Microwave Instrument for the Rosetta Orbiter (MIRO). We will discuss the evolution of the retrieved dust thickness and ice content before and after perihelion and the variations between the northern and southern hemispheres of the nucleus.

Summary Refractory rare-earth tellurides with the Th 3 P 4 structure type have attracted considerable interest as high-performance thermoelectric materials since the 1980s due to their high dimensionless figure of merit ( ZT ). Extensive work has been conducted on La 3−x Te 4 with peak ZT values greater than 1.1 at 1,273 K. The high ZT of La 3-x Te 4 is in part due to a large peak in the density of states near the Fermi level from the La 5d states. Here, we revisit Pr 3−x Te 4 , for which our electronic structure calculations predict a favorable modification of the density of states by the introduction of praseodymium's 4f electrons. This was experimentally verified by preparing Pr 3−x Te 4 samples with varying Pr vacancy concentrations using a mechanochemical synthesis approach. The thermoelectric properties were measured and a ZT of 1.7 at 1,200 K was achieved with Pr 2.74 Te 4 . The 50% improvement in peak ZT compared with La 3−x Te 4 resulted from an increased effective mass, improved Seebeck coefficient, and lower thermal conductivity.

Thermoelectric materials exhibit a voltage under an applied thermal gradient and are the heart of radioisotope thermoelectric generators (RTGs), which are the main power system for space missions such as Voyager I, Voyager II, and the Mars Curiosity rover. However, materials currently in use enable only modest thermal-to-electrical conversion efficiencies near 6.5% at the system level, warranting the development of material systems with improved thermoelectric performance. Previous work has demonstrated large thermoelectric figures of merit for lanthanum telluride (La_(3–x)Te_4), a high-temperature n-type material, achieving a peak zT value of 1.1 at 1275 K at an optimum cation vacancy concentration. Here, we present an investigation of the thermoelectric properties of neodymium telluride (Nd_(3–x)Te_4), another rare-earth telluride with a structure similar to La_(3–x)Te_4. Density functional theory (DFT) calculations predicted a significant increase in the Seebeck coefficient over La_(3–x)Te_4 at equivalent vacancy concentrations because of an increased density of states (DOS) near the Fermi level from the 4f electrons of Nd. The high-temperature electrical resistivity, Seebeck coefficient, and thermal conductivity were measured for Nd_(3–x)Te_4 at various carrier concentrations. These measurements were compared to La_(3–x)Te_4 in order to elucidate the impact of the four 4f electrons of Nd on the transport properties of Nd_(3–x)Te_4. A zT of 1.2 was achieved at 1273 K for Nd_(2.78)Te_4, which is a 10% improvement over that of La_(2.74)Te_4.

Remote submm-wave spectrometers have the capability of providing statistically significant numbers of isotopic composition measurements within the budget constraints of available planetary missions. This talk will present a mission and instrument concept that would enable an accurate measurement of the D/H ratio on not one but several dozens of comets in a four-year mission lifetime. The instrument would utilize advanced cryogenic detectors that would allow us to measure the abundance of the para and ortho spin states of water and its isotopologues. State of the art superconducting heterodyne receivers have been developed that provide detection sensitivities approaching the quantum limit in the 500 GHz frequency range enabling the measurement of D/H ratio on around 50 comets from an observatory stationed for example at the thermally benign Lagrange point L2

Lidar is one of few remote sensing methods available to researchers to sense below the oceanic air-surface. We present polarimetric lidar measurements of turbulence in a laboratory generated turbulent flow. We found that the nearforward light depolarization characterized by the depolarization rate γ(z), varies with the turbulent flow parameter: χ(z)∊(z)1/4, where χ(z) and ∊(z) are the respective depth dependent, temperature variance, and turbulent kinetic energy dissipation rates. The presence of particles in the flow modifies the values of γ in such a way that the ratio γ(z)/α(z) becomes independent of the particle concentration and depends only on χ(z)∊(z)1/4. We posit that the mechanism of light depolarization in turbulent flow with particles is forward scattered light interaction between turbulent refractive index inhomogeneities and flow particles. Such interactions result so that the observed depolarization rate, γ(z), is much larger than expected from ‘pure’ turbulent flow. Our observations open up the fascinating possibility of using lidar for turbulence measurements of aquatic flows.

We present an erratum to inform readers about the location of the data for our paper.

La3-xTe4 is a state-of-the-art high temperature n-type thermoelectric material with a previously reported maximum zT∼1.1 at 1273 K. Computational modeling suggests the La atoms play a crucial role in defining the density of states for La3-xTe4 in the conduction band. In addition to controlling charge carrier concentration, substitution with Ca2+ atoms on the La3+ site is explored as a potential means to tune the density of states and result in larger Seebeck coefficients. High purity, oxide-free samples are produced by ball milling of the elements and consolidated by spark plasma sintering. Powder XRD and electron microprobe analysis are used to characterize the material. High temperature thermoelectric properties are reported and compared with La3-xTe4 compositions. A maximum zT of 1.3 is reached at 1273 K for the composition La2.22Ca0.775Te4.

Currently, many satellite-based aerosol retrievals make use of lookup tables (LUTs) containing precomputed solutions to the radiative transfer (RT) equation. The benefit of this strategy is the avoidance of expensive runtime calculations, but its main drawback is that the LUTs discretize what is inherently a continuous, multivariate solution space. The operational retrieval algorithm for the Multi-angle Imaging SpectroRadiometer (MISR), for example, compares the observations to a set of 74 aerosol mixtures, each composed of particle models having prescribed optical properties and size distributions. In a recent “blind” study comparing the performance of several satellite retrieval algorithms on simulated data over a black surface, the MISR algorithm performed reasonably well in recovering the “true” spectral aerosol optical depths (AODs), but because the correct aerosol model was not contained within the MISR LUT, the retrieved AODs were biased low by ~ 14%. This motivated an investigation of whether an optimization approach, in which the aerosols are modeled by a set of continuously variable parameters recovered using nonlinear least-squares, could improve the results. In this paper, we demonstrate that such an approach using Levenberg–Marquardt optimization yields superior accuracy. Advances in computer speed, development of more efficient RT codes, and algorithm innovations will be necessary for this approach to satisfy the demands of a global, production-level satellite aerosol retrieval process, especially when used in conjunction with future instruments having enhanced sensitivity to diverse aerosol properties.

The European Space Agency (ESA) Rosetta spacecraft (Schulz, R., Alexander, C., Boehnhardt, H., Glassmeier, K.H. (Eds.) [2009]. “ROSETTA – ESA”) will encounter Comet 67P/Churyumov-Gerasimenko in 2014 and spend the next 18 months in the vicinity of the comet, permitting very high spatial and spectral resolution observations of the coma and nucleus. During this time, the heliocentric distance of the comet will change from ∼3.5 AU to ∼1.3 AU, accompanied by an increasing temperature of the nucleus and the development of the coma. The Microwave Instrument for the Rosetta Orbiter (MIRO) will observe the ground-state rotational transition (110–101) of H216O at 556.936 GHz, the two isotopologues H217O and H218O and other molecular transitions in the coma during this time (Gulkis, S. et al., [2007]. MIRO: Microwave Instrument for Rosetta Orbiter. Space Sci. Rev. 128, 561–597). The aim of this study is to simulate the water line spectra that could be obtained with the MIRO instrument and to understand how the observed line spectra with various viewing geometries can be used to study the physical conditions of the coma and the water excitation processes throughout the coma. We applied an accelerated Monte Carlo method to compute the excitations of the seven lowest rotational levels (101, 110, 212, 221, 303, 312, and 321) of ortho-water using a comet model with spherically symmetric water outgassing, density, temperature and expansion velocity at three different heliocentric distances 1.3 AU, 2.5 AU, and 3.5 AU. Mechanisms for the water excitation include water–water collisions, water–electron collisions, and infrared pumping by solar radiation. Synthetic line spectra are calculated at various observational locations and directions using the MIRO instrument parameters. We show that observations at varying viewing distances from the nucleus and directions have the potential to give diagnostic information on the continuum temperature and water outgassing rates at the surface of the nucleus, and the gas density, expansion velocity, and temperature of the coma as a function of distance from the nucleus. The gas expansion velocity and temperature affect the spectral line width and frequency shift of the line from the rest frequency, while the gas density (which is directly related to the outgassing rate) and the line excitation temperature determine the antenna temperature of the absorption and emission signal in the line profile.

Extensive regions of low-density cometary comae are characterized by important deviations from the Maxwell–Boltzmann velocity distribution, i.e. breakdown of thermodynamic equilibrium. The consequences of this on the shapes of emission and absorption lines, and for the acceleration of solid bodies due to gas drag, have rarely been investigated. These problems are studied here to aid in the development of future coma models, and in preparation for observations of Comet 67P/Churyumov–Gerasimenko from the ESA Rosetta spacecraft. Two topics in particular, related to Rosetta, are preparation for in situ observations of water, carbon monoxide, ammonia, and methanol emission lines by the mm/sub-mm spectrometer MIRO, as well as gas drag forces on dust grains and on the Rosetta spacecraft itself. Direct Simulation Monte Carlo (DSMC) modeling of H 2 O/CO mixtures in spherically symmetric geometries at various heliocentric distances are used to study the evolution of the (generally non-Maxwellian) velocity distribution function throughout the coma. Such distribution functions are then used to calculate Doppler broadening profiles and drag forces. It is found that deviation from thermodynamic equilibrium indeed is commonplace, and already at 2.5 AU from the Sun the entire comet coma displays manifestations of such breakdown, e.g., non-equal partitioning of energy between kinetic and rotational modes, causing substantial differences between translational and rotational temperatures. We exemplify how deviations from thermodynamic equilibrium affect the properties of Doppler broadened line profiles. Upper limits on the size of liftable dust grains as well as terminal grain velocities are presented. Furthermore, it is demonstrated that the drag-to-gravity force ratio is likely to decrease with decreasing cometocentric distance, which may be of relevance both for Rosetta and for the lander probe Philae.

The thermoelectric properties from 300–1275 K of calcium-doped La_(3−x)Te_4 are reported. La_(3−x)Te_4 is a high temperature n-type thermoelectric material with a previously reported zT_(max) 1.1 at 1273 K and x = 0.23. Computational modeling suggests the La atoms define the density of states of the conduction band for La_(3−x)Te_4. Doping with Ca^(2+) on the La^(3+) site is explored as a means of modifying the density of states to improve the power factor and to achieve a finer control over the carrier concentration. High purity, oxide-free samples are produced by ball milling of the elements and consolidation by spark plasma sintering. Calcium substitution upon the lanthanum site was confirmed by a combination of Rietveld refinements of powder X-ray diffraction data and wave dispersive spectroscopy. A zT_(max) 1.2 is reached at 1273 K for the composition La_(2.2)Ca_(0.78)Te_4 and the relative increase compared to La_(3−x)Te_4 is attributed to the finer carrier concentration.

Atomistic computations of electronic properties for nanostructures with strain (such as self-assembled quantum dots) typically consist of two components - a calculation of the individual atomic positions and the eigenstates of interest in the resulting Hamiltonian. Such simulations ultimately require artificial boundary conditions either through a truncation of the simulation domain or by the imposition of periodic boundary conditions, which necessarily introduce inaccuracies in both components of the computation. In simulations that include up to about 20 million atoms, it is demonstrated that the simulation domain truncation has little impact on the direct computation of the electronic energies but causes considerable inaccuracies in the calculation of the atomic positions unless the simulation domain is made much larger than the central quantum dot structure. The long-range nature of the lattice distortions induced by lattice mismatch is consequently expected to significantly alter the electronic structure of nearby quantum dots.

An accurate modeling of phonons, strain distributions, and Gruneisen coefficients is essential for the qualitative and quantitative design of modern nanoelectronic and nanooptoelectronic devices. The challenge is the development of a model that fits within an atomistic representation of the overall crystal yet remains computationally tractable. A simple model for introducing the anharmonicity of the interatomic potential into the Keating two-parameter valence-force-field model is developed. The new method is used for the calculation of acoustic phonon and strain effects in zinc-blende semiconductors. The model is fitted to the Gruneisen coefficients for long-wavelength acoustic phonons and reproduces the response to strain throughout the Brillouin zone in reasonable agreement with experiment.

Disorder-induced broadening of the conduction and valence band eigenenergies is calculated for an ensemble of dome-shaped InGaAs quantum dots of diameter 20nm using an sp3d5s* tight binding model.

Heat transport and ice sublimation in comets are interrelated processes reflecting properties acquired at the time of formation and during subsequent evolution. The Microwave Instrument on the Rosetta Orbiter (MIRO) acquired maps of the subsurface temperature of comet 67P/Churyumov-Gerasimenko, at 1.6 mm and 0.5 mm wavelengths, and spectra of water vapor. The total H2O production rate varied from 0.3 kg s(-1) in early June 2014 to 1.2 kg s(-1) in late August and showed periodic variations related to nucleus rotation and shape. Water outgassing was localized to the "neck" region of the comet. Subsurface temperatures showed seasonal and diurnal variations, which indicated that the submillimeter radiation originated at depths comparable to the diurnal thermal skin depth. A low thermal inertia (~10 to 50 J K(-1) m(-2) s(-0.5)), consistent with a thermally insulating powdered surface, is inferred.

Heat transport and ice sublimation in comets are interrelated processes reflecting properties acquired at the time of formation and during subsequent evolution. The Microwave Instrument on the Rosetta Orbiter (MIRO) acquired maps of the subsurface temperature of comet 67P/Churyumov-Gerasimenko, at 1.6 mm and 0.5 mm wavelengths, and spectra of water vapor. The total H 2 O production rate varied from 0.3 kg s –1 in early June 2014 to 1.2 kg s –1 in late August and showed periodic variations related to nucleus rotation and shape. Water outgassing was localized to the “neck” region of the comet. Subsurface temperatures showed seasonal and diurnal variations, which indicated that the submillimeter radiation originated at depths comparable to the diurnal thermal skin depth. A low thermal inertia (~10 to 50 J K –1 m –2 s –0.5 ), consistent with a thermally insulating powdered surface, is inferred.

We present sub-millimeter observations of the ground-state rotational transition (110–101) of water vapor from Comet C/2002 T7 (LINEAR) obtained with the MIRO instrument on the ESA Rosetta spacecraft (s/c) orbiter on April 30, 2004. At the time of the observations, the comet was at a distance of 0.63 AU from the Sun, 0.68 AU from the MIRO telescope, and about 7.5 days after its perihelion. The ground state rotation transition of ortho-water at 556.936 GHz was observed and integrated for ∼8 h using a frequency switched radiometer to provide short and long term stability. The MIRO beam size is 7.5 arcmin in terms of full width half maximum, corresponding to a radius of 1.1 × 105 km at the comet location. The observed signal line area of the water line spectrum is 4.3 ± 0.8 K km/s. Using a molecular excitation and radiation transfer model and assuming the spherically symmetric and constant radial expansion of gas in the coma, we estimate that the production rate of water is (1.0 ± 0.2) × 1030 molecules/s and the expansion velocity is 1.1 ± 0.2 km/s at the time of the MIRO observation. The present estimation of the water outgassing rate of the comet is in good agreement with other observation-based estimations when the outgassing rates with respect to the time after perihelion are compared. The Doppler-corrected center velocity of the observed line was red-shifted by 0.67 ± 0.13 km/s, of which only 0.18 km/s shift is explained by the model and attributed to a self-absorption effect. The potential sources of the additional red shift are discussed.

We have developed a method for calculating the field-emission current from a clean or adsorbate-covered metal surface using the transfer Hamiltonian method of Bardeen. The present formalism can be incorporated in accurate atomistic electronic structure methods, and so is capable of addressing system specific band structure effects, adsorbate-induced resonances, and is amenable to accurate treatments of the exchange-correlation potential close to as well as far from the metal surface. It therefore goes beyond the conventional Fowler-Nordheim treatment of field emission from a metal surface. We illustrate the utility of our method by calculating the field-emission current from a model jellium surface, using a local-density approximation exchange-correlation potential, modified to include the correct $\ensuremath{\sim}1/4x$ asymptotic behavior in the vacuum region. We find that the Fowler-Nordheim behavior can be recovered in the limit of low fields; in the limit of high fields, where the details of the self-consistent effective potential in the neighborhood of the surface become important, meaningful deviations from the Fowler-Nordheim current result.

Using a two-dimensional model, we have considered the effects of spatially changing fields and potentials, stochastic electron emission, and ballistic electron motion on the anode current and on the width of the electron beam in field emission displays. We have solved the electrostatic problem using the boundary element method. Our electron emission model evaluates the current density at the cathode surface from the tunneling transmission coefficient, which is calculated from the solution of the one-dimensional Schrodinger equation using a potential barrier which includes the effect of image charges and nonuniform electric field. The current density is used to calculate the rate of electron emission for each segment of the emitter’s surface. The emission time is assumed to follow a Poisson distribution. The electron’s velocity magnitude and angle with the normal to the surface are also stochastically generated following the probability distribution of field emitted electrons. Ballistic transport is used t...

In the present computational study, we focus on graphene ribbons with zigzag edge atoms with their unsaturated bonds either dangling or terminated by various adsorbates ~H, O, or Cs!. Using this system as a test case, we discuss the two important contributions to the work function—the first one being an anisotropic bulk property related to the electron affinity of the material, and the second one being directly related to the surface dipole moment caused by the spill over of electronic charge into the vacuum. The latter contribution, which tends to increase the work function, can to a large extent be minimized by a judicious choice of adsorbates ~typically, adsorbates that are more electropositive than the surface !. The former face-dependent contribution turns out to be the minimum possible work function achievable for a given surface. Our calculations are based on density-functional theory within the local density approximation using nonlocal pseudopotentials and a plane wave basis set. @S0163-1829~99!03632-2# tion of species at a surface alters the work function of the surface in an understandable manner, with adsorbates having higher electronegativities than the surface increasing the work function while those with lower electronegativities having the opposite effect. 5-7 In the present study, we quantify the above anticipated trends for the test case of graphene ribbon edges with various adsorbates. We also address an appealing—although not unanticipated—correlation between the edge dipole moment and the work function perpendicular to the edge ~and along the ribbon plane!. This correlation points to an interesting way of viewing the work function, viz., by partitioning it into an anisotropic ~face-dependent! bulk cohesive ~electron af- finitylike! part and a part entirely due to the surface or edge dipole moment. The latter, which is a positive contribution to the work function, is due to the spill over of the electron gas into the vacuum region, and can to a large extent be reduced by a judicious choice of adsorbates. In fact, this analysis indicates that there is a minimum possible work function associated with a particular surface or edge that can be at- tained when the surface or edge dipole moment can be made to vanish. Our choice of an allotrope of carbon as a test case in the present study is motivated by the fact that recent attention has focused on carbon-based materials due to their promise as potential candidates for cold-cathode field emission applications. 8 Among the allotropes of carbon, nanotubes seem to be active field emitters, although other forms—like fragments of graphene and diamond-like carbon—are also known to be active. 9 One possible reason the nanotubes are active may be because they display special electronic states localized at the tip atoms. 10,11 It has also been pointed out that the nanotubes may be quite defective, with one such defect being similar to a graphene edge. 12,13 While the car- bon atoms in a cylindrical defect-free nanotube are all three- fold coordinated ~just as in graphite!, those in defected tu- bules may be two-fold coordinated, with an entirely different p-electron network in its vicinity ~as in fragments of graphene!. A recent tight-binding study of graphene ribbons 14 with the edge carbon atoms passivated with H has demonstrated that graphene ribbons with zigzag edges dis- play special localized states near the Fermi level arising pri- marily due to the topology of the p electron networks at the edges. Here, we use density-functional methods and consider zigzag graphene edges with unsaturated bonds either dan- gling or passivated with H, O, or Cs, and assess the impor- tance of such terminations on the edge dipole moment and the work function perpendicular to the edge—quantities which are key to the electron emission properties of these confined systems. In the next section, we give details of the method and models used in this study. We comment about the specifics of the work function and dipole moment calculations and formally relate the former to the latter in Sec. III. In Sec. IV, we present electronic and geometric structure results, calcu- lated work functions and dipole moments for the unpassi- vated and H-, O-, and Cs-terminated zigzag ribbons. We fi- nally conclude with Sec. V.

The largest asteroid of the Solar System, (1) Ceres, has been thought to have an icy surface; here it is observed to be emitting water vapour. The presence of hydrated minerals on the surface of Ceres, the largest body in the Solar System's main asteroid belt, suggested that there may be water there too. Now infrared spectra obtained by ESA's Herschel Space Observatory provide unambiguous evidence that there is water ice at or near the surface of Ceres. Water vapour is issuing at a rate of at least 1026 molecules per second from sources on Ceres localized to mid-latitude regions. The water evaporation could be due to comet-like sublimation or to cryo-volcanism, in which volcanoes erupt volatiles such as water instead of molten rocks. This finding supports models that propose that the icy bodies such as comets may have migrated into the asteroid belt from beyond the notional 'snowline' dividing the early Solar System into a 'dry' inner and 'icy' outer regions. The ‘snowline’ conventionally divides Solar System objects into dry bodies, ranging out to the main asteroid belt, and icy bodies beyond the belt. Models suggest that some of the icy bodies may have migrated into the asteroid belt1. Recent observations indicate the presence of water ice on the surface of some asteroids2,3,4, with sublimation5 a potential reason for the dust activity observed on others. Hydrated minerals have been found6,7,8 on the surface of the largest object in the asteroid belt, the dwarf planet (1) Ceres, which is thought to be differentiated into a silicate core with an icy mantle9,10,11. The presence of water vapour around Ceres was suggested by a marginal detection of the photodissociation product of water, hydroxyl (ref. 12), but could not be confirmed by later, more sensitive observations13. Here we report the detection of water vapour around Ceres, with at least 1026 molecules being produced per second, originating from localized sources that seem to be linked to mid-latitude regions on the surface14,15. The water evaporation could be due to comet-like sublimation or to cryo-volcanism, in which volcanoes erupt volatiles such as water instead of molten rocks.

The traditional view of minor bodies in the (inner) Solar System is that they are split into icy comets and rocky asteroids. However this has been challenged by recent results, such as the discovery of comets on asteroidal orbits in the outer asteroid belt (between Mars and Jupiter) and the detection of water ice frost on the surface of asteroid (24) Themis. The discovery of water ice on the surface of asteroids has profound implications for how the Solar System formed, and challenges our ideas about the stability of ice in the inner Solar System. The study of volatiles in the asteroid belt places strong constraints on the temperature and composition distribution in the proto-planetary disk,and on possible sources of terrestrial water, and strongly constrains formation models of the early Solar System.

We derive the effective-mass equation for a superlattice structure by considering the barrier as a perturbation of the bulk material in the well. The renormalization constants as well as the momentum matrix elements are allowed to vary with space in the resulting equation. Terms additional to the usual effective-mass equation are found by using second-order perturbation. They are shown to have little effect on the subband separation and are small on the scale of the energy gap.

The full random-phase approximation (RPA) spectral function is calculated for an electron gas confined in a modulation-doped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum well. In the first subband we observe the typical structure of the two-dimensional plasmaron. The renormalization of the chemical potential is smaller than that of the edge of the density of states. In the second subband the single-state broadening and the band renormalization are found to be smaller than in the first subband. When the depolarization effect is neglected, the intersubband absorption line is narrower than the inhomogeneous broadening due to the different effective masses in the two subbands for the noninteracting electron system. This result is explained by the two-dimensional plasmaron structure in the first subband. The theoretical peak is narrower than the experimental one. If in the spectral function the self-energy is taken at the energy of the noninteracting electron, the corresponding absorption line is broader than the experimental one. If the depolarization effect is included, the approximate linewidth is found to be narrower and the full RPA linewidth broader than the experimental one.

The quality of the image recorded by the future James Webb Space Telescope or JWST will depend on the alignment of the multiple optical elements constituting the telescope. Several steps are necessary to align the optical system. In this paper, we describe a genetic algorithm (GA) that optimizes the last alignment stage or fine-phasing stage. We split the population of candidate solutions into several sub-populations in order to take into account the different parameter dependencies. Each optical element has several (seven) degrees of freedom. The worse ranked individuals undergo the GA variation operations on all their genes independently guarantying that solutions with independent genes will be searched. Conversely, since the symmetry of the problem imposes that most rotations, translations and curvatures must be related, we apply GA variation operations on only three (pseudo)-genes for better ranked solutions. Our method improves the current state of the art.

We have implemented and validated a model for the radiative transfer in molecular clouds and studied the water emission line for Comet 67P/Churyumov-Gerasimenko. Results are reported that show that the spectral profiles for water emission lines depend on the physical properties of the coma. This tool will be used to interpret observations by MIRO and will help build hydrodynamics models of comets.

We have developed manufacturable approaches to form single, vertically aligned carbon nanotubes, where the tubes are centered precisely, and placed within a few hundred nm of 1-1.5 m deep trenches. These wafer-scale approaches were enabled by chemically amplified resists and inductively coupled Cryo-etchers for forming the 3D nanoscale architectures. The tube growth was performed using dc plasma-enhanced chemical vapor deposition (PECVD), and the materials used for the pre-fabricated 3D architectures were chemically and structurally compatible with the high temperature (700 °C) PECVD synthesis of our tubes, in an ammonia and acetylene ambient. Tube characteristics were also engineered to some extent, by adjusting growth parameters, such as Ni catalyst thickness, pressure and plasma power during growth. Such scalable, high throughput top-down fabrication techniques, combined with bottom-up tube synthesis, should accelerate the development of PECVD tubes for applications such as interconnects, nano-electromechanical (NEMS), sensors or 3D electronics in general.

Multi-wall carbon nanotubes (MWNT) have stable elastic properties over a wide range of deformation amplitudes and their high polarizability is conducive to efficient bending in the inhomogeneous electric field close to patterned electrodes. We simulate the static switching properties of the MWNT using a custom NEMS simulation package. The MWNT is modeled as a solid cylinder with Young�s modulus taken in a range consistent with recent experimental data in the literature. The polarizability of the MWNT is obtained from empirical expressions in the literature. The static equilibrium equations for the bending of the MWNT in an electric field are solved for realistic geometry of the electrodes. The spatial distribution of electric field distribution in the vicinity of the tubes and the electrodes is obtained from a finite element solution of the Poisson equation. A Monte Carlo approach yields the switching voltage as a function of MWNT mechanical properties and electrode geometry, where a Gaussian distribution is assumed for the parametric variations.

We have demonstrated electrostatic switching in vertically oriented nanotubes or nanofibers, where a nanoprobe was used as the actuating electrode inside an SEM. When the nanoprobe was manipulated to be in close proximity to a single tube, switching voltages between 10 V – 40 V were observed, depending on the geometrical parameters. The turn-on transitions appeared to be much sharper than the turn-off transitions which were limited by the tube-to-probe contact resistances. In many cases, stiction forces at these dimensions were dominant, since the tube appeared stuck to the probe even after the voltage returned to 0 V, suggesting that such structures are promising for nonvolatile memory applications. The stiction effects, to some extent, can be adjusted by engineering the switch geometry appropriately. Nanoscale mechanical measurements were also conducted on the tubes using a custom-built nanoindentor inside an SEM, from which preliminary material parameters, such as the elastic modulus, were extracted. The mechanical measurements also revealed that the tubes appear to be well adhered to the substrate. The material parameters gathered from the mechanical measurements were then used in developing an electrostatic model of the switch using a commercially available finite-element simulator. The calculated pull-in voltages appeared to be in agreement to the experimentally obtained switching voltages to first order.

Vertically aligned carbon nanotube (CNT) nano-electro-mechanical (NEM) switches are currently being investigated for their application in radiation-hard, high temperature space electronics. Carbon nanotubes are attractive for switching applications since electrostatically-actuated CNT switches have low actuation voltages and power requirements, while allowing GHz switching speeds that stem from the inherently high elastic modulus and low mass of the CNT. Our NEM structure consists of CNTs that are grown using dc plasma-enhanced (PE) chemical-vapor-deposition (CVD) for forming vertically aligned, rigid tubes. A gas mixture of acetylene and ammonia were used for tube growth at a total pressure of a few Torr and temperatures up to 700 °C. We have formed arrays of single, vertically aligned tubes directly on Si, which was enabled by this first report of an optical lithography approach used to generate isolated tubes compared to e-beam lithography that is conventionally used. Vertical NEM switch devices were formed where single, vertically aligned tubes were seen within deep trenches, in close proximity to conducting electrodes.

The electronic structure for a strained silicon quantum well grown on a tilted SiGe substrate is calculated using an empirical tight-binding method. For a zero substrate tilt angle the two lowest minima of the conduction band define a non-zero valley splitting at the center of the Brillouin zone. A finite tilt angle for the substrate results in displacing the two lowest conduction band minima to finite k0 and -k0 in the Brillouin zone with equal energy. The vanishing of the valley splitting for quantum wells grown on tilted substrates is found to be a direct consequence of the periodicity of the steps at the interfaces between the quantum well and the buffer materials.

International audience; Observations of the nucleus of 67P/Churyumov-Gerasimenko in the millimeter-wave continuum have been obtained by the Microwave Instrument for the Rosetta Orbiter (MIRO). We present data obtained at wavelengths of 0.5 mm and 1.6 mm during September 2014 when the nucleus was at heliocentric distances between 3.45 and 3.27 AU. The data are fit to simple models of the nucleus thermal emission in order to characterize the observed behavior and make quantitative estimates of important physical parameters, including thermal inertia and absorption properties at the MIRO wavelengths. MIRO brightness temperatures on the irregular surface of 67P are strongly affected by the local solar illumination conditions, and there is a strong latitudinal dependence of the mean brightness temperature as a result of the seasonal orientation of the comet's rotation axis with respect to the Sun. The MIRO emission exhibits strong diurnal variations, which indicate that it arises from within the thermally varying layer in the upper centimeters of the surface. The data are quantitatively consistent with very low thermal inertia values, between 10-30 JK(-1) m(-2) s(-1/2), with the 0.5 mm emission arising from 1 cm beneath the surface and the 1.6 mm emission from a depth of 4 cm. Although the data are generally consistent with simple, homogeneous models, it is difficult to match all of its features, suggesting that there may be some vertical structure within the upper few centimeters of the surface. The MIRO brightness temperatures at high northern latitudes are consistent with sublimation of ice playing an important role in setting the temperatures of these regions where, based on observations of gas and dust production, ice is known to be sublimating.

Substituting group 14 and 16 elements on the pnictogen site in the skutterudite structure yields a class of valence-precise ternary AX_(1.5)Y_(1.5) compounds (A = Co, Rh, Ir, X = Sn, Ge, and Y = S, Se, Te), in which X and Y form an ordered sub-structure. Compared with unfilled binary skutterudites, pnictogen-substituted phases exhibit extremely low lattice thermal conductivity due to increased structural complexity. Here, we investigate the role of the transition metal species in determining the electronic structure and transport properties of Asn_(1.5)Te_(1.5) compounds with A = Co, Rh, Ir. Density functional calculations using fully ordered structures reveal semiconducting behavior in all three compounds, with the band gap varying from 0.2 to 0.45 eV. In CoSn_(1.5)Te_(1.5), the electronic density of states near the gap is significantly higher than for A = Ir or Rh, leading to higher effective masses and higher Seebeck coefficients. Experimentally, Ir and Rh samples exhibit relatively large p-type carrier concentrations and degenerate semiconducting behavior. In contrast, CoSn_(1.5)Te_(1.5) shows mixed conduction, with n-type carriers dominating the Seebeck coefficient and light, high mobility holes dominating the Hall coefficient. zT values of up to 0.35 were obtained, and further improvement is expected upon optimization of the carrier concentration or with n-type doping.

The valley splitting of the first few Landau levels is calculated as a function of the magnetic field for electrons confined in a strained silicon quantum well grown on a tilted SiGe substrate, using a parameterized tight-binding method. For a zero substrate tilt angle, the valley splitting slightly decreases with increasing magnetic field. In contrast, the valley splitting for a finite substrate tilt angle exhibits a strong and non-monotonous dependence on the magnetic field strength. The valley splitting of the first Landau level shows an exponential increase followed by a slow saturation as the magnetic field strength increases. The valley splitting of the second and third Landau levels shows an oscillatory behavior. The non-monotonous dependence is explained by the phase variation of the Landau level wave function along the washboard-like interface between the tilted quantum well and the buffer material. The phase variation is the direct consequence of the misorientation between the crystal axis and the confinement direction of the quantum well. This result suggests that the magnitude of the valley splitting can be tuned by controlling the Landau-level filling factor through the magnetic field and the doping concentration.

*† ‡ The development of new mission concepts requires efficient methodologies to analyze, design, and simulate the concepts before implementation. New mission concepts are increasingly considering the use of ion thrusters for fuel-efficient navigation in deep space. This paper presents parallel, evolutionary computing methods to design trajectories of spacecraft propelled by ion thrusters and assesses the trade-off between delivered payload mass and required flight time. The developed methods utilize a distributed computing environment in order to speed up computation, and use evolutionary algorithms to find globally Paretooptimal solutions. The methods are coupled with two main traditional trajectory design approaches, which are called direct and indirect. In the direct approach, thrust control is discretized in either arc time or arc length, and the resulting discrete thrust vectors are optimized. In the indirect approach, the thrust control problem is transformed into a co-state control problem and the initial values of the co-state vector are optimized. The developed methods are applied to two problems: 1) an orbit transfer around the Earth and 2) a transfer between two distance retrograde orbits around Europa, the icy Galilean moon closest to Jupiter. The optimal solutions found with the present methods are comparable to other state-of-the-art trajectory optimizers, while the required computation time is often several orders of magnitude less thanks to an intelligent design of control vector discretization, advanced algorithmic parameterization, and parallel computing.

The valley splitting 􏰀energy difference between the states of the lowest doublet􏰁 in strained silicon quantum wells with a V-shaped potential is calculated variationally using a two-band tight-binding model. The approximation is valid for a moderately long 􏰀approximately 5.5 – 13.5 nm􏰁 quantum well with a V-shaped potential which can be produced by a realistic delta-doping on the order of nd 􏰈 1012 cm−2. The splitting versus applied field 􏰀steepness of the V-shaped potential􏰁 curves show interesting behavior: a single minimum and for some doublets, a parity reversal as the field is increased. These characteristics are explained through an analysis of the variational wave function and energy functional.

A detailed study of reduced-basis tight-binding models of electrons in semiconducting quantum wells is presented. The focus is on systems with degenerate valleys, such as silicon in silicon germanium heterostructures, in the low-density limit, relevant to proposed quantum computing architectures. Analytic results for the bound states of systems with hard-wall boundaries are presented and used to characterize the valley splitting in silicon quantum wells. The analytic solution in a no-spin-orbit model agrees well with larger tight-binding calculations that do include spin-orbit coupling. Numerical investigations of the valley splitting for finite band offsets are presented that indicate that the hard-wall results are a good guide to the behavior in real quantum wells.

The effect of wetting layers on the strain and electronic structure of InAs self-assembled quantum dots grown on GaAs is investigated with an atomistic valence-force-field model and an empirical tight-binding model. By comparing a dot with and without a wetting layer, we find that the inclusion of the wetting layer weakens the strain inside the dot by only 1% relative change, while it reduces the energy gap between a confined electron and hole level by as much as 10%. The small change in the strain distribution indicates that strain relaxes only little through the thin wetting layer. The large reduction of the energy gap is attributed to the increase of the confining-potential width rather than the change of the potential height. First-order perturbation calculations or, alternatively, the addition of an InAs disk below the quantum dot confirm this conclusion. The effect of the wetting layer on the wave function is qualitatively different for the weakly confined electron state and the strongly confined hole state. The electron wave function shifts from the buffer to the wetting layer, while the hole shifts from the dot to the wetting layer., 14 pages, 3 figures, and 3 tables

The modeling of finite-extent semiconductor nanostructures that are embedded in a host material requires the numerical treatment of the boundary in a finite simulation domain. For the study of a self-assembled InAs dot embedded in GaAs, three kinds of boundary conditions are examined within the empirical tight-binding model: (i) the periodic boundary condition, (ii) raising the orbital energies of surface atoms, and (iii) raising the energies of dangling bonds at the surface. The periodic boundary condition requires a smooth boundary and consequently a larger GaAs buffer than the two nonperiodic boundary conditions. Between the nonperiodic conditions, the dangling-bond energy shift is more efficient than the orbital-energy shift, in terms of the elimination of nonphysical surface states in the middle of the gap. A dangling-bond energy shift bigger than 5 eV efficiently eliminates all of the mid-gap surface states and leads to interior states that are highly insensitive to the change of the energy shift., 20 pages, 9 figures

Self-assembled InAsSb/InGaAs quantum dots are candidates for optical detectors and emitters in the 2–5 micron band with a wide range of applications for atmospheric chemistry studies. It is known that while the energy band gap of unstrained bulk InAs1−xSbx is smallest for x=0.62, the biaxial strain for bulk InAs1−xSbx grown on In0.53Ga0.47As shifts the energy gap to higher energies and the smallest band gap is reached for x=0.51. The aim of the present study is to examine how the electronic confinement in the quantum dots modifies these simple considerations. We have calculated the electronic structure of lens shaped InAs1−xSbx quantum dots with diameter 37 nm and height 4 nm embedded in a In0.53Ga0.47As matrix of thickness 7 nm and lattice matched to an InP buffer. The relaxed atomic positions were determined by minimizing the elastic energy obtained from a valence force field description of the inter-atomic interaction. The electronic structure was calculated with an empirical tight binding approach. For Sb concentrations larger than x=0.5, it is found that the InSb/ In0.53Ga0.47As heterostructure becomes type II leading to no electron confined in the dot. It is also found that the energy gap decreases with increasing Sb content in contradiction with previous experimental results. A possible explanation is a significant variation is quantum dot size with Sb content.

A theory based on localized-orbital approaches is developed to describe the valley splitting observed in silicon quantum wells. The theory is appropriate in the limit of low electron density and relevant for proposed quantum computing architectures. The valley splitting is computed for realistic devices using the quantitative nanoelectronic modeling tool NEMO. A simple, analytically solvable tight-binding model is developed, it yields much physical insight, and it reproduces the behavior of the splitting in the NEMO results. The splitting is in general nonzero even in the absence of electric field in contrast to previous works. The splitting in a square well oscillates as a function of S, the number of layers in the quantum well, with a period that is determined by the location of the valley minimum in the Brillouin zone. The envelope of the splitting decays as $S^3$. Finally the feasibility of observing such oscillations experimentally in modern Si/SiGe heterostructures is discussed., Comment: 19 pages, including 4 figures

The electronic properties and Seebeck coefficients of Ce3Te4 and La3Te4 are computed using Density Functional Theory with on-site Coulomb interaction correction. We found that the Seebeck coefficients of Ce3Te4 and La3Te4 are almost equal at temperatures larger than the Curie temperature of Ce3Te4, and in good agreement with the measurements reported by May et al. [Phys. Rev. B 86, 035135 (2012)]. At temperatures below the Curie temperature, the Seebeck coefficient of Ce3Te4 increases due to the ferromagnetic ordering, which leads the f-electron of Ce to contribute to the Seebeck coefficient in the relevant range of electron concentration.