1,998 research outputs found
Used-habitat calibration plots: a new procedure for validating species distribution, resource selection, and step-selection models
“Species distribution modeling” was recently ranked as one of the top five “research fronts” in ecology and the environmental sciences by ISI's Essential Science Indicators (Renner and Warton 2013), reflecting the importance of predicting how species distributions will respond to anthropogenic change. Unfortunately, species distribution models (SDMs) often perform poorly when applied to novel environments. Compounding on this problem is the shortage of methods for evaluating SDMs (hence, we may be getting our predictions wrong and not even know it). Traditional methods for validating SDMs quantify a model's ability to classify locations as used or unused. Instead, we propose to focus on how well SDMs can predict the characteristics of used locations. This subtle shift in viewpoint leads to a more natural and informative evaluation and validation of models across the entire spectrum of SDMs. Through a series of examples, we show how simple graphical methods can help with three fundamental challenges of habitat modeling: identifying missing covariates, non-linearity, and multicollinearity. Identifying habitat characteristics that are not well-predicted by the model can provide insights into variables affecting the distribution of species, suggest appropriate model modifications, and ultimately improve the reliability and generality of conservation and management recommendations
Atomistic Characterization of Stochastic Cavitation of a Binary Metallic Liquid under Negative Pressure
We demonstrate the stochastic nature of cavitation in a binary metallic liquid Cu_(46)Zr_(54) during hydrostatic expansion by employing molecular dynamics (MD) simulations using a quantum mechanics (QM)-derived potential. The activation volume is obtained from MD simulations and transition-state theory. Extrapolation of the pressure dependence of the activation volume from our MD simulations to low tensile pressure agrees remarkably with macroscale cavitation experiments. We find that classical nucleation theory can predict the cavitation rate if we incorporate the Tolman length derived from the MD simulations
Increasing incidence of sterile-site infections due to non-multidrug- resistant, oxacillin-resistant Staphylococcus aureus among hospitalized patients
Cremmer-Gervais r-matrices and the Cherednik Algebras of type GL2
We give an intepretation of the Cremmer-Gervais r-matrices for sl(n) in terms
of actions of elements in the rational and trigonometric Cherednik algebras of
type GL2 on certain subspaces of their polynomial representations. This is used
to compute the nilpotency index of the Jordanian r-matrices, thus answering a
question of Gerstenhaber and Giaquinto. We also give an interpretation of the
Cremmer-Gervais quantization in terms of the corresponding double affine Hecke
algebra.Comment: 6 page
Heisenberg Dimer Single Molecule Magnets in a Strong Magnetic Field
We calculate the static and dynamic properties of single crystal, single
molecule magnets consisting of equal spin or 5/2 dimers. The spins in
each dimer interact with each other via the Heisenberg exchange interaction and
with the magnetic induction via the Zeeman interaction, and
interdimer interactions are negligible. For antiferromagnetic couplings, the
static magnetization and specific heat exhibit interesting low temperature
and strong quantum effects. We calculate the frequency spectrum of
the Fourier transform of the real part of the time autocorrelation function
for arbitrary , and compare our results with
those obtained for classical spins. We also calculate the inelastic neutron
magnetic dynamical structure factor at arbitrary .Comment: 11 pages, 14 figures, submitted to Phys. Rev.
Predicted Optimum Composition for the Glass-Forming Ability of Bulk Amorphous Alloys: Application to Cu−Zr−Al
Metallic glasses have been established to have unique properties such as ductility, toughness, and soft magnetism with promising engineering applications. However, the glass-forming ability (GFA) has not been sufficient to synthesize the bulk metallic glasses (BMGs) required for many engineering applications. Attempts to develop the understanding of the GFA required to predict the optimum alloys have not yet been proven successful. We develop here a computational model based on molecular dynamics simulations that explains the dramatic change of GFA with alloying small amounts of Al into Cu−Zr. We find that the high GFA to form BMGs depends on a combination of three factors, (a) a low thermodynamic driving force for crystallization, (b) a high melt viscosity, and (c) large ratios of icosahedral structures in the liquid phase. These computational methods to predict these factors that suppress formation of crystal nuclei and slow the dynamic motions in the liquids are practical for in silico prediction of new alloys with optimal GFA
Excitation Spectrum and Superexchange Pathways in the Spin Dimer VODPO_4 . 1/2 D_2O
Magnetic excitations have been investigated in the spin dimer material
VODPO_4 \cdot 1/2 D_2O using inelastic neutron scattering. A dispersionless
magnetic mode was observed at an energy of 7.81(4) meV. The wavevector
dependence of the scattering intensityfrom this mode is consistent with the
excitation of isolated V^{4+} spin dimers with a V-V separation of 4.43(7) \AA.
This result is unexpected since the V-V pair previously thought to constitute
themagnetic dimer has a separation of 3.09 \AA. We identify an alternative V-V
pair as the likely magnetic dimer, which involves superexchange pathways
through a covalently bonded PO_4 group. This surprising result casts doubt on
the interpretation of (VO)_2P_2O_7 as a spin ladder.Comment: 4 pages, 4 postscript figures - identical to previous paper but
figure 2 and 3 hopefully more compatible .p
Evolution of level density step structures from 56,57-Fe to 96,97-Mo
Level densities have been extracted from primary gamma spectra for 56,57-Fe
and 96,97-Mo nuclei using (3-He,alpha gamma) and (3-He,3-He') reactions on
57-Fe and 97-Mo targets. The level density curves reveal step structures above
the pairing gap due to the breaking of nucleon Cooper pairs. The location of
the step structures in energy and their shapes arise from the interplay between
single-particle energies and seniority-conserving and seniority-non-conserving
interactions.Comment: 9 pages, including 5 figure
Using Large Databases of Groundwater Chemistry in the Northern Midwest USA: The Effects of Geologic and Anthropogenic Factors
Regional geochemical databases for the northern Midwest USA are being compiled to examine the various geogenic and anthropogenic factors that control the chemistry of groundwater. At the regional scale, variations are seen that are attributable to agricultural and urban effects, or to geologic factors. Examples of the former include enrichments of nitrate in groundwater, while examples of the latter mainly highlight geochemical differences between carbonate rocks and all other rock types in the region. This paper examines a few of these regional effects and the spatial scales at which they can be observe
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