3,983 research outputs found
Community-level Response to Habitat Structure Manipulations: An Experimental Case Study in a Tropical Ecosystem
Across the globe, environmental change is resulting in novel ecosystems that have altered habitat structure and functioning. Research is needed to understand how changes in habitat structure in these new ecosystems impact community interactions, especially when these manipulations are being proposed to reduce invasive species. We conducted an experiment in Hawaii to determine how changes in habitat structure, represented by leaf litter and understory vegetation, affect the abundance of an invasive generalist predator, the coqui frog (Eleutherodactylus coqui) and its potential prey (invertebrates). This study consisted of four treatments: two vegetation treatments (50% and 100% removal of vegetation with diameter at breast heightcm) and two leaf litter treatments (50% and 100% removal). Removal of 50% of habitat structure, either vegetation or leaf litter, was not sufficient to produce long-term changes in coqui or invertebrate densities. Only full removal of habitat structure resulted in reduced densities of coqui after four months. The abundance of leaf litter invertebrates and invertebrates flying close to the forest floor was higher in the 100% vegetation removal treatment compared to leaf litter removal treatments, and the abundance of foliage invertebrates was higher in the 100% leaf litter removal treatment compared to vegetation removal treatments. Invertebrate responses were complicated because they not only responded to the loss of habitat but also the reduction of coquis in treatments. Coquis in treatments moved to microhabitats that contained increased prey. Treatments appeared to impact coquis by removing structure needed for diurnal retreats, breeding and foraging. In summary, both the 100% removal of leaf litter or vegetation can reduce coqui densities in relatively small (20 m × 20 m) areas, even when surrounded by intact, invaded forest. This study provides greater understanding of the impact of habitat structure manipulation, a typical management employed to control an invasive frog, in a novel ecosystem
Air Conditions Close to the Ground and the Effect on Airplane Landings
This report presents the results of an investigation undertaken to determine the feasibility of making glide landings in gusty air. Wind velocities were measured at several stations between the ground and a height of 51 feet, and flight tests were made to determine the actual influence of gusts on an airplane gliding close to the ground
Critical exponents of the quantum phase transition in a planar antiferromagnet
We have performed a large scale quantum Monte Carlo study of the quantum
phase transition in a planar spin-1/2 Heisenberg antiferromagnet with CaV4O9
structure. We obtain a dynamical exponent z=1.018+/-0.02. The critical
exponents beta, nu and eta agree within our errors with the classical 3D O(3)
exponents, expected from a mapping to the nonlinear sigma model. This confirms
the conjecture of Chubukov, Sachdev and Ye [Phys. Rev. B 49, 11919 (1994)] that
the Berry phase terms in the planar Heisenberg antiferromagnet are dangerously
irrelevant.Comment: 5 pages including 4 figures; revised version: some minor changes and
added reference
Astrophysical S-factors for fusion reactions involving C, O, Ne and Mg isotopes
Using the Sao Paulo potential and the barrier penetration formalism we have
calculated the astrophysical factor S(E) for 946 fusion reactions involving
stable and neutron-rich isotopes of C, O, Ne, and Mg for center-of-mass
energies E varying from 2 MeV to 18-30 MeV (covering the range below and above
the Coulomb barrier). We have parameterized the energy dependence S(E) by an
accurate universal 9-parameter analytic expression and present tables of fit
parameters for all the reactions. We also discuss the reduced 3-parameter
version of our fit which is highly accurate at energies below the Coulomb
barrier, and outline the procedure for calculating the reaction rates. The
results can be easily converted to thermonuclear or pycnonuclear reaction rates
to simulate various nuclear burning phenomena, in particular, stellar burning
at high temperatures and nucleosynthesis in high density environments.Comment: 30 pages including 11 tables, 4 figures, ADNDT, accepte
Stochastic series expansion method with operator-loop update
A cluster update (the ``operator-loop'') is developed within the framework of
a numerically exact quantum Monte Carlo method based on the power series
expansion of exp(-BH) (stochastic series expansion). The method is generally
applicable to a wide class of lattice Hamiltonians for which the expansion is
positive definite. For some important models the operator-loop algorithm is
more efficient than loop updates previously developed for ``worldline''
simulations. The method is here tested on a two-dimensional anisotropic
Heisenberg antiferromagnet in a magnetic field.Comment: 5 pages, 4 figure
Ultrafast supercontinuum spectroscopy of carrier multiplication and biexcitonic effects in excited states of PbS quantum dots
We examine the multiple exciton population dynamics in PbS quantum dots by
ultrafast spectrally-resolved supercontinuum transient absorption (SC-TA). We
simultaneously probe the first three excitonic transitions over a broad
spectral range. Transient spectra show the presence of first order bleach of
absorption for the 1S_h-1S_e transition and second order bleach along with
photoinduced absorption band for 1P_h-1P_e transition. We also report evidence
of the one-photon forbidden 1S_{h,e}-1P_{h,e} transition. We examine signatures
of carrier multiplication (multiexcitons for the single absorbed photon) from
analysis of the first and second order bleaches, in the limit of low absorbed
photon numbers (~ 10^-2), at pump energies from two to four times the
semiconductor band gap. The multiexciton generation efficiency is discussed
both in terms of a broadband global fit and the ratio between early- to
long-time transient absorption signals.. Analysis of population dynamics shows
that the bleach peak due to the biexciton population is red-shifted respect the
single exciton one, indicating a positive binding energy.Comment: 16 pages, 5 figure
Recommended from our members
Investigations in Gallium Removal
Gallium present in weapons plutonium must be removed before it can be used for the production of mixed-oxide (MOX) nuclear reactor fuel. The main goal of the preliminary studies conducted at Texas A and M University was to assist in the development of a thermal process to remove gallium from a gallium oxide/plutonium oxide matrix. This effort is being conducted in close consultation with the Los Alamos National Laboratory (LANL) personnel involved in the development of this process for the US Department of Energy (DOE). Simple experiments were performed on gallium oxide, and cerium-oxide/gallium-oxide mixtures, heated to temperatures ranging from 700--900 C in a reducing environment, and a method for collecting the gallium vapors under these conditions was demonstrated
Duality, thermodynamics, and the linear programming problem in constraint-based models of metabolism
It is shown that the dual to the linear programming problem that arises in
constraint-based models of metabolism can be given a thermodynamic
interpretation in which the shadow prices are chemical potential analogues, and
the objective is to minimise free energy consumption given a free energy drain
corresponding to growth. The interpretation is distinct from conventional
non-equilibrium thermodynamics, although it does satisfy a minimum entropy
production principle. It can be used to motivate extensions of constraint-based
modelling, for example to microbial ecosystems.Comment: 4 pages, 2 figures, 1 table, RevTeX 4, final accepted versio
The Physical Origins of Entropy Production, Free Energy Dissipation and their Mathematical Representations
A complete mathematical theory of nonequilibrium thermodynamics of stochastic
systems in terms of master equations is presented. As generalizations of
isothermal entropy and free energy, two functions of states play central roles:
the Gibbs entropy and the relative entropy , which are related via the
stationary distribution of the stochastic dynamics. satisfies the
fundamental entropy balance equation with entropy production
rate and heat dissipation rate , while . For
closed systems that satisfy detailed balance: . For open system
one has where the housekeeping heat
was first introduced in the phenomenological nonequilibrium steady state
thermodynamics. Entropy production consists of free energy dissipation
associated with spontaneous relaxation, , and active energy pumping that
sustains the open system . The amount of excess heat involved in the
relaxation .Comment: 4 pages, no figure
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