Molecular dynamics study of the thermopower of Ag, Au, and Pt nanocontacts

Using molecular dynamics simulations of many junction stretching processes we analyze the thermopower of silver (Ag), gold (Au), and platinum (Pt) atomic contacts. In all cases we observe that the thermopower vanishes on average within the standard deviation and that its fluctuations increase for decreasing minimum cross-section of the junctions. However, we find a suppression of the fluctuations of the thermopower for the s-valent metals Ag and Au, when the conductance originates from a single, perfectly transmitting channel. Essential features of the experimental results for Au, Ag, and copper (Cu) of Ludoph and van Ruitenbeek [Phys. Rev. B 59, 12290 (1999)], as yet unaddressed by atomistic studies, can hence be explained by considering the atomic and electronic structure at the disordered narrowest constriction of the contacts. For the multivalent metal Pt our calculations predict the fluctuations of the thermopower to be larger by one order of magnitude as compared to Ag and Au, and suppressions of the fluctuations as a function of the conductance are absent.Comment: 13 pages, 10 figure

Ab-initio study of the thermopower of biphenyl-based single-molecule junctions

Employing ab-initio electronic structure calculations combined with the non-equilibrium Green's function technique, we study the dependence of the thermopower Q on the conformation in biphenyl-based single-molecule junctions. For the series of experimentally available biphenyl molecules, alkyl side chains allow us to gradually adjust the torsion angle \phi\ between the two phenyl rings from 0 to 90{\deg} and to control in this way the degree of \pi-electron conjugation. Studying different anchoring groups and binding positions, our theory predicts that the absolute values of the thermopower decrease slightly towards larger torsion angles, following an a+b*cos^{2}\phi\ dependence. The anchoring group determines the sign of Q and a,b, simultaneously. Sulfur and amine groups give rise to Q,a,b>0, while for cyano Q,a,b<0. The different binding positions can lead to substantial variations of the thermopower mostly due to changes in the alignment of the frontier molecular orbital levels and the Fermi energy. We explain our ab-initio results in terms of a \pi-orbital tight-binding model and a minimal two-level model, which describes the pair of hybridizing frontier orbital states on the two phenyl rings. The variations of the thermopower with \phi\ seem to be within experimental resolution.Comment: 8 pages, 4 figues, 3 table

Understanding the Electronic Transport Through Single Noble Gas Atoms

We present a theoretical study of the conductance of atomic junctions comprising single noble gas atoms (He, Ne, Ar, Kr, and Xe) coupled to gold electrodes. The aim is to elucidate how the presence of noble gas atoms affects the electronic transport through metallic atomic-size contacts. Our analysis, based on density functional theory and including van der Waals interactions, shows that for the lightest elements (He and Ne) no significant current flows through the noble gas atoms and their effect is to reduce the conductance of the junctions by screening the interaction between the gold electrodes. This explains the observations reported in metallic atomic-size contacts with adsorbed He atoms. Conversely, the heaviest atoms (Kr and Xe) increase the conductance due to the additional current path provided by their valence p states.Comment: 3 figure

Identification of the current path for a conductive molecular wire on a tripodal platform

We present the chemical synthesis as well as charge transport measurements and calculations for a new tripodal platform based on a rigid 9,9′-spirobifluorene equipped with a phenylene-ethynylene wire. The transport experiments are performed with the help of the low-temperature mechanically controlled break junction technique with gold electrodes. By combining experimental and theoretical investigations of elastic and inelastic charge transport, we show that the current proceeds through the designated molecular wire and identify a binding geometry that is compatible with the experimental observations. The conductive molecular wire on the platform features a well-defined and relatively high conductance of the order of 10 −3 G 0 despite the length of the current path of more than 1.7 nm, demonstrating that this platform is suitable to incorporate functional units like molecular switches or sensors

Team-level programming of drone sensor networks

Autonomous drones are a powerful new breed of mobile sensing platform that can greatly extend the capabilities of traditional sensing systems. Unfortunately, it is still non-trivial to coordinate multiple drones to perform a task collaboratively. We present a novel programming model called team-level programming that can express collaborative sensing tasks without exposing the complexity of managing multiple drones, such as concurrent programming, parallel execution, scaling, and failure recovering. We create the Voltron programming system to explore the concept of team-level programming in active sensing applications. Voltron offers programming constructs to create the illusion of a simple sequential execution model while still maximizing opportunities to dynamically re-task the drones as needed. We implement Voltron by targeting a popular aerial drone platform, and evaluate the resulting system using a combination of real deployments, user studies, and emulation. Our results indicate that Voltron enables simpler code and produces marginal overhead in terms of CPU, memory, and network utilization. In addition, it greatly facilitates implementing correct and complete collaborative drone applications, compared to existing drone programming systems

Cluster-based density-functional approach to quantum transport through molecular and atomic contacts

We present a cluster-based density-functional approach to model charge transport through molecular and atomic contacts. The electronic structure of the contacts is determined in the framework of density functional theory, and the parameters needed to describe transport are extracted from finite clusters. A similar procedure, restricted to nearest-neighbor interactions in the electrodes, has been presented by Damle et al. [Chem. Phys. 281, 171 (2002)]. Here, we show how to systematically improve the description of the electrodes by extracting bulk parameters from sufficiently large metal clusters. In this way we avoid problems arising from the use of nonorthogonal basis functions. For demonstration we apply our method to electron transport through Au contacts with various atomic-chain configurations and to a single-atom contact of Al.Comment: 18 pages, 13 figure

Nucleon-nucleon elastic scattering analysis to 2.5 GeV

A partial-wave analysis of NN elastic scattering data has been completed. This analysis covers an expanded energy range, from threshold to a laboratory kinetic energy of 2.5 GeV, in order to include recent elastic pp scattering data from the EDDA collaboration. The results of both single-energy and energy-dependent analyses are described.Comment: 23 pages of text. Postscript files for the figures are available from ftp://clsaid.phys.vt.edu/pub/said/n

Nutritional Factors Modulating Alu Methylation inan Italian Sample from The Mark-Age StudyIncluding Offspring of Healthy Nonagenarians

Alu hypomethylation promotes genomic instability and is associated with aging and age-related diseases. Dietary factors affect global DNA methylation, leading to changes in genomic stability and gene expression with an impact on longevity and the risk of disease. This preliminary study aims to investigate the relationship between nutritional factors, such as circulating trace elements, lipids and antioxidants, and Alu methylation in elderly subjects and offspring of healthy nonagenarians. Alu DNA methylation was analyzed in sixty RASIG (randomly recruited age-stratified individuals from the general population) and thirty-two GO (GeHA offspring) enrolled in Italy in the framework of the MARK-AGE project. Factor analysis revealed a different clustering between Alu CpG1 and the other CpG sites. RASIG over 65 years showed lower Alu CpG1 methylation than those of GO subjects in the same age class. Moreover, Alu CpG1 methylation was associated with fruit and whole-grain bread consumption, LDL2-Cholesterol and plasma copper. The preserved Alu methylation status in GO, suggests Alu epigenetic changes as a potential marker of aging. Our preliminary investigation shows that Alu methylation may be affected by food rich in fibers and antioxidants, or circulating LDL subfractions and plasma copper