16,187 research outputs found
Impact of Fano and Breit-Wigner resonances in the thermoelectric properties of nanoscale junctions
We show that the thermoelectric properties of nanoscale junctions featuring
states near the Fermi level strongly depend on the type of resonance generated
by such states, which can be either Fano or Breit-Wigner-like. We give general
expressions for the thermoelectric coefficients generated by the two types of
resonances and calculate the thermoelectric properties of these systems, which
encompass most nanoelectronics junctions. We include simulations of real
junctions where metalloporphyrin molecules bridge gold electrodes and prove
that for some metallic elements the thermoelectric properties show a large
variability. We find that the thermopower and figure of merit are largely
enhanced when the resonance gets close to the Fermi level and reach values much
higher than typical values found in other nanoscale junctions. The specific
value and temperature dependence are determined by a series of factors such as
the strength of the coupling between the state and other molecular states, the
symmetry of the state, the strength of the coupling between the molecule and
the leads and the spin filtering behavior of the junction.Comment: 9 pages, 11 figure
Dynamic Optimization of Nitrogen Use in Agriculture
Agricultural production is highly dependent on inorganic substances including fertilizers. High-yielding crop varieties, such as corn, require large amounts of primary nutrients including nitrogen, phosphorus and potassium. Farmers often add a surplus of nutrients to crops to maximize yields. Utilization of primary nutrients has increased by more than 300% while that of nitrogen alone has increased by more than 600% between 1960 and 2007 (USDA, 2009). From 1964 to 2007, the use of nitrogen in the corn sector alone increased from 1,623,000 to 5,714,000 nutrient tons (USDA, 2009). While increasing production, increased fertilizer use can potentially create negative externalities in the form of nitrate-nitrogen contamination in groundwater. Groundwater is the source of drinking water for about half the total U.S. population and nearly all of the rural population, and it provides over 50 billion gallons per day for agricultural needs (USGS, 2009). In the U.S. the main source of nitrate pollution in the groundwater results from the actions of farmers through the use of fertilizers and other chemicals (Haller, et al. 2009). Nitrogen-nitrate contamination can have adverse human affects including methemoglobinemia or ―blue-baby‖ syndrome (Majumdar, 2003). The potential for nitrate contamination in corn production is especially problematic as corn alone accounts for over 90% of feed grains produced in the U.S. (USDA, 2009). The USDA estimates that approximately 80 million acres of land is planted to corn, with the majority in the Heartland region (the Midwest) of the U.S. (2009). The Heartland region is primarily rural and much of the population there derives its drinking water from groundwater. Therefore, the potential for groundwater contamination is greatly increased in this region.Environmental Economics, Nitrogen/Nitrate Contamination, Dynamic Optimization, Agriculture, Agricultural and Food Policy, Demand and Price Analysis, Environmental Economics and Policy, C61, C63, Q10, Q51, Q53,
Scale-free Networks from Optimal Design
A large number of complex networks, both natural and artificial, share the
presence of highly heterogeneous, scale-free degree distributions. A few
mechanisms for the emergence of such patterns have been suggested, optimization
not being one of them. In this letter we present the first evidence for the
emergence of scaling (and smallworldness) in software architecture graphs from
a well-defined local optimization process. Although the rules that define the
strategies involved in software engineering should lead to a tree-like
structure, the final net is scale-free, perhaps reflecting the presence of
conflicting constraints unavoidable in a multidimensional optimization process.
The consequences for other complex networks are outlined.Comment: 6 pages, 2 figures. Submitted to Europhysics Letters. Additional
material is available at http://complex.upc.es/~sergi/software.ht
Choice-Nash Equilibria
We provide existence results for equilibria of games where players employ abstract (non binary) choice rules. Such results are shown to encompass as a relevant instance that of games where players have (non-transitive) SSB (Skew-Symmetric Bilinear) preferences, as will as other well-known transitive (e. g. Nash´s) and non-transitive (e. g. Shafer and Sonnenschein´s) models in the literature. Further, our general model contains games where players display procedural rationality.
Universality in the transport response of molecular wires physisorbed onto graphene electrodes
We analyze the low-voltage transport response of large molecular wires
bridging graphene electrodes, where the molecules are physisorbed onto the
graphene sheets by planar anchor groups. In our study, the sheets are pulled
away to vary the gap length and the relative atomic positions. The molecular
wires are also translated in directions parallel and perpendicular to the
sheets. We show that the energy position of the Breit-Wigner molecular
resonances is universal for a given molecule, in the sense that it is
independent of the details of the graphene edges, gaps lengths or of the
molecule positions. We discuss the need to converge carefully the k-sampling to
provide reasonable values of the conductance.Comment: 6 pages, 6 figure
Dynamic Optimization of Nitrogen Use in Agriculture
Agricultural and Food Policy, Crop Production/Industries,
Chromomagnetic Instability and Induced Magnetic Field in Neutral Two-Flavor Color Superconductivity
We find that the chromomagnetic instability existing in neutral two- flavor
color superconductivity at moderate densities is removed by the formation of an
inhomogeneous condensate of charged gluons and the corresponding induction of a
magnetic field. It is shown that this inhomogeneous ground state is
energetically favored over a homogeneous one. The spontaneous induction of a
magnetic field in a color superconductor at moderate densities can be of
interest for the astrophysics of compact stellar objects exhibiting strong
magnetic fields as magnetars.Comment: Version to appear in PR
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