Growth dynamics of C60 thin films: effect of molecular structure

The surface morphology and growth behavior of fullerene thin films have been studied by atomic force microscopy and height difference correlation function analysis. In contrast to the large growth exponents (beta) previously reported for other organic semiconductor thin-film materials, a relatively small beta value of 0.39±0.10 was determined. Simulations of (1+1)-dimensional surface lateral diffusion models indicate that the evolution of deep grain boundaries leads to a rapid increase in beta. We suggest that the commonly observed large beta values for organic thin films are due to their intrinsically anisotropic molecular structures and hence different stacking directions between crystallite domains

Optimal Timer Based Selection Schemes

Timer-based mechanisms are often used to help a given (sink) node select the best helper node among many available nodes. Specifically, a node transmits a packet when its timer expires, and the timer value is a monotone non-increasing function of its local suitability metric. The best node is selected successfully if no other node's timer expires within a 'vulnerability' window after its timer expiry, and so long as the sink can hear the available nodes. In this paper, we show that the optimal metric-to-timer mapping that (i) maximizes the probability of success or (ii) minimizes the average selection time subject to a minimum constraint on the probability of success, maps the metric into a set of discrete timer values. We specify, in closed-form, the optimal scheme as a function of the maximum selection duration, the vulnerability window, and the number of nodes. An asymptotic characterization of the optimal scheme turns out to be elegant and insightful. For any probability distribution function of the metric, the optimal scheme is scalable, distributed, and performs much better than the popular inverse metric timer mapping. It even compares favorably with splitting-based selection, when the latter's feedback overhead is accounted for.Comment: 21 pages, 6 figures, 1 table, submitted to IEEE Transactions on Communications, uses stackrel.st

Genetic Modifiers of Systemic Lupus Erythematosus in FcγRIIB−/− Mice

FcγRIIB is a potent lupus susceptibility gene as demonstrated by the observation that mice deficient in this molecule develop spontaneous antinuclear antibodies (ANA) and fatal glomerulonephritis when on the C57BL/6 background. To determine the mechanisms underlying the epistasis displayed by this gene we have constructed hybrids between FcγRIIB−/− and the systemic lupus erythematosus (SLE) modifiers yaa and lpr and the susceptibility locus Sle1. Sle1 and B6.RIIB−/− are both physically and functionally coupled; compound heterozygotes of Sle1 and B6.RIIB−/− develop significant disease, while single heterozygotes display no evidence of autoimmunity or disease, indicating that these genes lie on the same genetic pathway resulting in the loss of tolerance to nuclear antigens. However, the generation of ANA in itself is insufficient to account for the severity of autoimmune disease in this model, as demonstrated by analysis of yaa and lpr hybrids. Thus, B6.RIIB−/−/lpr mice are protected from disease progression, despite equivalent titers of ANA. In contrast, B6.RIIB−/−/yaa mice have significantly enhanced disease despite reduced ANA titers. Yaa modifies the specificity and thus the pathogenicity of the B6. RIIB−/− ANA, by converting them to antinucleolar antibodies. In addition to these known modifier pathways, we have discovered two novel, recessive loci contributed by the C57BL/6 genome that are required for the ANA phenotype, further indicating the epistatic properties of this SLE model

Dielectric properties of condensed systems composed of fragments

The dielectric properties of molecules or nanostructures are usually modified in a complex manner, when assembled into a condensed phase. We propose a first-principles method to compute polarizabilities of sub-entities of solids and liquids, which accounts for multipolar interactions at all orders, and is applicable to any semiconductor or insulator. The method only requires the evaluation of induced fields in the condensed phase, with no need of multiple calculations for each constituent. As an example, we present results for the molecular polarizabilities of water in a wide pressure and temperature range. We found that at ambient conditions, the dipole-induced-dipole approximation is sufficiently accurate and the Clausius-Mossotti relation may be used, e.g. to obtain molecular polarizabilities from experimental refractive indexes. However with increasing pressure this approximation becomes unreliable and in the case of ice X the Clausius-Mossotti relation is not valid.Comment: 16 pages, 4 figures, 1 tabl

The band structure of BeTe - a combined experimental and theoretical study

Using angle-resolved synchrotron-radiation photoemission spectroscopy we have determined the dispersion of the valence bands of BeTe(100) along $\Gamma X$, i.e. the [100] direction. The measurements are analyzed with the aid of a first-principles calculation of the BeTe bulk band structure as well as of the photoemission peaks as given by the momentum conserving bulk transitions. Taking the calculated unoccupied bands as final states of the photoemission process, we obtain an excellent agreement between experimental and calculated spectra and a clear interpretation of almost all measured bands. In contrast, the free electron approximation for the final states fails to describe the BeTe bulk band structure along $\Gamma X$ properly.Comment: 21 pages plus 4 figure

Methionine restriction extends lifespan of \u3ci\u3eDrosophila melanogaster\u3c/i\u3e under conditions of low amino-acid status

Reduced methionine (Met) intake can extend lifespan of rodents; however, whether this regimen represents a general strategy for regulating aging has been controversial. Here we report that Met restriction extends lifespan in both fruit flies and yeast, and that this effect requires low amino-acid status. Met restriction in Drosophila mimicks the effect of dietary restriction and is associated with decreased reproduction. However, under conditions of high amino-acid status, Met restriction is ineffective and the trade-off between longevity and reproduction is not observed. Overexpression of InRDN or Tsc2 inhibits lifespan extension by Met restriction, suggesting the role of TOR signalling in the Met control of longevity. Overall, this study defines the specific roles of Met and amino-acid imbalance in aging and suggests that Met restiction is a general strategy for lifespan extension

Evidence of Counter-Streaming Ions near the Inner Pole of the HERMeS Hall Thruster

NASA is continuing the development of a 12.5-kW Hall thruster system to support a phased exploration concept to expand human presence to cis-lunar space and eventually to Mars. The development team is transitioning knowledge gained from the testing of the government-built Technology Development Unit (TDU) to the contractor-built Engineering Test Unit (ETU). A new laser-induced fluorescence diagnostic was developed to obtain data for validating the Hall thruster models and for comparing the behavior of the ETU and TDU. Analysis of TDU LIF data obtained during initial deployment of the diagnostics revealed evidence of two streams of ions moving in opposite directions near the inner front pole. These two streams of ions were found to intersect the downstream surface of the front pole at large oblique angles. This data points to a possible explanation for why the erosion rate of polished pole covers were observed to decrease over the course of several hundred hours of thruster operation

Design and Calibration of a Lightweight Physics-Based Model for Fluid-Mediated Self-Assembly of Robotic Modules

In this paper, we consider a system consisting of multiple floating robotic modules performing self-assembly. Faithfully modeling such a system and its inter-module interactions typically involves capturing the hydrodynamic forces acting on the modules using computationally expensive fluid dynamic modeling tools. This poses restrictions on the usability of the resulting models. Here, we present a new approach towards modeling such systems. First, we show how the hardware and firmware of the robotic modules can be faithfully modeled in a high-fidelity robotic simulator. Second, we develop a physics plugin to recreate the hydrodynamic forces acting on the modules and propose a trajectory-based method for calibrating the plugin model parameters. Our calibration method employs a Particle Swarm Optimization (PSO) algorithm, and consists of minimizing the difference between Mean Squared Displacement (MSD) data extracted from real and simulated trajectories of multiple robotic modules

Magnetic surface reconstruction in the van der Waals antiferromagnet Fe1+xTe

We acknowledge financial support from the EPSRC (EP/R031924/1 and EP/R032130/1) and NIST Center for Neutron Research. C.H. acknowledges support by the Austrian Science Fund (FWF) Project No. P32144-N36 and the VSC4 of the Vienna University of TechnologyFe1+xTe is a two-dimensional van der Waals antiferromagnet that becomes superconducting on anion substitution on the Te site. The properties of the parent phase of Fe1+xTe are sensitive to the amount of interstitial iron situated between the iron-tellurium layers. Fe1+xTe displays collinear magnetic order coexisting with low-temperature metallic resistivity for small concentrations of interstitial iron x and helical magnetic order for large values of x. While this phase diagram has been established through scattering [see, for example, E. E. Rodriguez et al., Phys. Rev. B 84, 064403 (2011); S. Rossler et al., ibid. 84, 174506 (2011)], recent scanning tunneling microscopy measurements [C. Trainer et al., Sci. Adv. 5, eaav3478 (2019)] have observed a different magnetic structure for small interstitial iron concentrations x with a significant canting of the magnetic moments along the crystallographic c axis of θ = 28° ± 3°. In this paper, we revisit themagnetic structure of Fe1.09Te using spherical neutron polarimetry and scanning tunneling microscopy to search for this canting in the bulk phase, and we compare surface and bulk magnetism. The results show that the bulk magnetic structure of Fe1.09Te is consistent with collinear in-plane order (θ= 0 with an error of ∼ 5°). Comparison with scanning tunneling microscopy on a series of Fe1+xTe samples reveals that the surface exhibits a magnetic surface reconstruction with a canting angle of the spins of θ = 29.8°. We suggest that this is a consequence of structural relaxation of the surface layer resulting in an out-of-plane magnetocrystalline anisotropy. The magnetism in Fe1+xTe displays different properties at the surface when the symmetry constraints of the bulk are removed.Publisher PDFPeer reviewe