10,966 research outputs found
Sex Ratio and Sexual Dimorphism in \u3ci\u3eFormica Exsectoides\u3c/i\u3e, the Allegheny Mound Ant (Hymenoptera: Formicidae)
We excavated 66 mounds from 6 populations of Formica exsectoides in Michigan jack pine, collecting sexual caste pupae for sex ratio estimates and measurement of dimorphism. Reproductive caste brood was present in only 37 ofthe 66 mounds, and presence of reproductive caste brood was associated with larger mound surface area. Females were heavier than males, but did not differ from males in energy density. Sexes did not differ in timing or rate of development. Sex ratio estimates based on individual mounds ranged from 1.0 (all male) to 0.08 (female·biased). Four of the six study populations were strongly male·biased, while sex ratio estimates for the remaining populations did not differ from equal investment. While this interpopulation variation may be caused by genetic factors, the equal investment populations were 10· cated in or near patches of clear·cut forest, suggesting that environmental impacts should be investigated
On giant piezoresistance effects in silicon nanowires and microwires
The giant piezoresistance (PZR) previously reported in silicon nanowires is
experimentally investigated in a large number of surface depleted silicon nano-
and micro-structures. The resistance is shown to vary strongly with time due to
electron and hole trapping at the sample surfaces. Importantly, this time
varying resistance manifests itself as an apparent giant PZR identical to that
reported elsewhere. By modulating the applied stress in time, the true PZR of
the structures is found to be comparable with that of bulk silicon
An exactly solvable model of a superconducting to rotational phase transition
We consider a many-fermion model which exhibits a transition from a
superconducting to a rotational phase with variation of a parameter in its
Hamiltonian. The model has analytical solutions in its two limits due to the
presence of dynamical symmetries. However, the symmetries are basically
incompatible with one another; no simple solution exists in intermediate
situations. Exact (numerical) solutions are possible and enable one to study
the behavior of competing but incompatible symmetries and the phase transitions
that result in a semirealistic situation. The results are remarkably simple and
shed light on the nature of phase transitions.Comment: 11 pages including 1 figur
Investigation of nonlinear interaction phenomena in the ionosphere
Ionospheric phenomena as thermal radiation noise, propagation of naturally occurring radio noise through ionosphere, and generation of very low frequency emission
Representations of the Weyl group and Wigner functions for SU(3)
Bases for SU(3) irreps are constructed on a space of three-particle tensor
products of two-dimensional harmonic oscillator wave functions. The Weyl group
is represented as the symmetric group of permutations of the particle
coordinates of these space. Wigner functions for SU(3) are expressed as
products of SU(2) Wigner functions and matrix elements of Weyl transformations.
The constructions make explicit use of dual reductive pairs which are shown to
be particularly relevant to problems in optics and quantum interferometry.Comment: : RevTex file, 11 pages with 2 figure
Geometrical dependence of decoherence by electronic interactions in a GaAs/GaAlAs square network
We investigate weak localization in metallic networks etched in a two
dimensional electron gas between mK and mK when electron-electron
(e-e) interaction is the dominant phase breaking mechanism. We show that, at
the highest temperatures, the contributions arising from trajectories that wind
around the rings and trajectories that do not are governed by two different
length scales. This is achieved by analyzing separately the envelope and the
oscillating part of the magnetoconductance. For K we find
\Lphi^\mathrm{env}\propto{T}^{-1/3} for the envelope, and
\Lphi^\mathrm{osc}\propto{T}^{-1/2} for the oscillations, in agreement with
the prediction for a single ring \cite{LudMir04,TexMon05}. This is the first
experimental confirmation of the geometry dependence of decoherence due to e-e
interaction.Comment: LaTeX, 5 pages, 4 eps figure
Effect of the Pauli principle on photoelectron spin transport in GaAs
In p+ GaAs thin films, the effect of photoelectron degeneracy on spin
transport is investigated theoretically and experimentally by imaging the spin
polarization profile as a function of distance from a tightly-focussed light
excitation spot. Under degeneracy of the electron gas (high concentration, low
temperature), a dip at the center of the polarization profile appears with a
polarization maximum at a distance of about from the center. This
counterintuitive result reveals that photoelectron diffusion depends on spin,
as a direct consequence of the Pauli principle. This causes a concentration
dependence of the spin stiffness while the spin dependence of the mobility is
found to be weak in doped material. The various effects which can modify spin
transport in a degenerate electron gas under local laser excitation are
considered. A comparison of the data with a numerical solution of the coupled
diffusion equations reveals that ambipolar coupling with holes increases the
steady-state photo-electron density at the excitation spot and therefore the
amplitude of the degeneracy-induced polarization dip. Thermoelectric currrents
are predicted to depend on spin under degeneracy (spin Soret currents), but
these currents are negligible except at very high excitation power where they
play a relatively small role. Coulomb spin drag and bandgap renormalization are
negligible due to electrostatic screening by the hole gas
Multi-Agent Simulation of Human Behavior in Naval Air Defense
This paper appeared in the Naval Engineers' Journal, 116 (4), Fall 2004, 53-64.The AEGIS Cruiser Air-Defense Simulation is a program that models the operations of a Combat Information Center team performing
air defense for a U.S. Navy battle group. It uses multi-agent system technology and is implemented in Java. Conceived primarily to
assist personnel in air-defense training and doctrine formulation, the simulation provides insight into the factors (skills, experience,
fatigue, aircraft numbers, weather, etc.) that influence performance, especially under intense or stressful situations, and the task
bottlenecks. It simulates air contacts (aircraft and missiles) as well as the actions and mental processes of the watchstanders. All
simulated events are logged, to permit performance analysis and reconstruction for post-scenario training. Validation of the simulation
was done with the help of expert practitioners of air defense at the AEGIS Training & Readiness Center (ATRC), San Diego,
California, USA.Approved for public release; distribution is unlimited
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