8,901 research outputs found
Localized Electron States Near a Metal-Semiconductor Nanocontact
The electronic structure of nanowires in contact with metallic electrodes of
experimentally relevant sizes is calculated by incorporating the electrostatic
polarization potential into the atomistic single particle Schr\"odinger
equation. We show that the presence of an electrode produces localized
electron/hole states near the electrode, a phenomenon only exhibited in
nanostructures and overlooked in the past. This phenomenon will have profound
implications on electron transport in such nanosystems. We calculate several
electrode/nanowire geometries, with varying contact depths and nanowire radii.
We demonstrate the change in the band gap of up to 0.5 eV in 3 nm diameter CdSe
nanowires and calculate the magnitude of the applied electric field necessary
to overcome the localization.Comment: 11 pages 4 figure
Transferable atomic multipole machine learning models for small organic molecules
Accurate representation of the molecular electrostatic potential, which is
often expanded in distributed multipole moments, is crucial for an efficient
evaluation of intermolecular interactions. Here we introduce a machine learning
model for multipole coefficients of atom types H, C, O, N, S, F, and Cl in any
molecular conformation. The model is trained on quantum chemical results for
atoms in varying chemical environments drawn from thousands of organic
molecules. Multipoles in systems with neutral, cationic, and anionic molecular
charge states are treated with individual models. The models' predictive
accuracy and applicability are illustrated by evaluating intermolecular
interaction energies of nearly 1,000 dimers and the cohesive energy of the
benzene crystal.Comment: 11 pages, 6 figure
Heat transfer characteristics of hypersonic waveriders with an emphasis on the leading edge effects
The heat transfer characteristics in surface radiative equilibrium and the aerodynamic performance of blunted hypersonic waveriders are studied along two constant dynamic pressure trajectories for four different Mach numbers. The inviscid leading edge drag was found to be a small (4 to 8 percent) but not negligible fraction of the inviscid drag of the vehicle. Although the viscous drag at the leading edge can be neglected, the presence of the leading edge will influence the transition pattern of the upper and the lower surfaces and therefore affect the viscous drag of the entire vehicle. For an application similar to the National Aerospace Plane (NASP), the present study demonstrates that the waverider remains a valuable concept at high Mach numbers if a state-of-the-art active cooling device is used along the leading edge. At low Mach number (less than 5), the study shows the surface radiative cooling might be sufficient. In all cases, radiative cooling is sufficient for the upper and lower surfaces of the vehicle if ceramic composites are used as thermal protection
Coarse-grained interaction potentials for polyaromatic hydrocarbons
Using Kohn-Sham density functional theory (KS-DFT), we have studied the
interaction between various polyaromatic hydrocarbon molecules. The systems
range from mono-cyclic benzene up to hexabenzocoronene (hbc). For several
conventional exchange-correlation functionals potential energy curves of
interaction of the - stacking hbc dimer are reported. It is found
that all pure local density or generalized gradient approximated functionals
yield qualitatively incorrect predictions regarding structure and interaction.
Inclusion of a non-local, atom-centered correction to the KS-Hamiltonian
enables quantitative predictions. The computed potential energy surfaces of
interaction yield parameters for a coarse-grained potential, which can be
employed to study discotic liquid-crystalline mesophases of derived
polyaromatic macromolecules
The Loving of the Game: A Study of Basketry in the Mammoth Cave Area
The production and marketing of baskets in the Mammoth Cave area of Kentucky from 1880 to the present is observed in light of the cultural, technical, aesthetic, ad traditional aspects involved. The process of making a white oak ribbed basket is documented, as well as the technical and aesthetic variables from which the basket maker renders his product. The changing role of social organization and communication in the production and marketing of a traditional craft objects is also considered
Unsolved Problems in Virtual Knot Theory and Combinatorial Knot Theory
This paper is a concise introduction to virtual knot theory, coupled with a
list of research problems in this field.Comment: 65 pages, 24 figures. arXiv admin note: text overlap with
arXiv:math/040542
Identifying ENSO Phase Impacts on Area Yield Insurance Rates: An Application of Non-Parametric Analysis
The paper reports results of non-parametric analysis of peanut, corn, and cotton yield distributions by the ElNino Southern Oscillation (ENSO) phases in the Southeastern U.S. For validation purposes, the historical yield data is complemented by a set of simulated peanut yields generated using daily weather data. The hypothesis, justified by the observed South-Eastern climate differences and research on ENSO cycles and planting dates, is that different climate conditions during ENSO cycles translate into different yield distributions and, therefore, insurance premiums (loss to coverage ratios). Kernel density estimates of historical county yield data show consistent patterns in the actuarially fair rate schedules grouped by ENSO phases and geographical areas. In particular, corn and cotton yield insurance premiums appear to be the most dependent on the ENSO phases and are the highest, regardless of coverage, during ElNino and the lowest during LaNina. Peanut premiums are higher during Neutral years and lowest during LaNina. The results appear to be robust to the transformations used to make the yield series stationary. While these dependencies do not necessarily correspond to the precipitation and solar radiation characteristics of the corresponding ENSO cycles in the Southeastern US, drawing direct analogies with yield variability is premature as many less documented factors, like the spacing of sunny and rainy days, may be just as important. The comparisons of the empirical and simulated peanut yield distributions show that they are similar in many ways and that the dissimilarities can be explained by known factors. These findings should be more relevant for the area yield insurance as opposed to the APH arrangements as the yield data used in designing contracts for the former reflects the systemic risk more influenced by climate than by the farm-level, basis risk factors accommodated in the APH plans.Risk and Uncertainty, Q140, C220, G220,
Scattering rates and lifetime of exact and boson excitons
Although excitons are not exact bosons, they are commonly treated as such
provided that their composite nature is included in effective scatterings
dressed by exchange. We here \emph{prove} that, \emph{whatever these
scatterings are}, they cannot give both the scattering rates and
the exciton lifetime , correctly: A striking factor 1/2 exists between
and the sum of 's, which originates from the
composite nature of excitons, irretrievably lost when they are bosonized. This
result, which appears as very disturbing at first, casts major doubts on
bosonization for problems dealing with \emph{interacting} excitons
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