36,753 research outputs found
Towards a historical ecology of intertidal foraging in the Mafia Archipelago: archaeomalacology and implications for marine resource management
Understanding the timing and nature of human influence on coastal and island ecosystems is becoming a central concern in archaeological research, particularly when investigated within a historical ecology framework. Unfortunately, the coast and islands of eastern Africa have not figured significantly within this growing body of literature, but are important given their historically contingent environmental, social, and political contexts, as well as the considerable threats now posed to marine ecosystems. Here, we begin developing a longer-term understanding of past marine resource use in the Mafia Archipelago (eastern Africa), an area of high ecological importance containing the Mafia Island Marine Park. Focusing on the comparatively less researched marine invertebrates provides a means for initiating discussion on potential past marine ecosystem structure, human foraging and environmental shifts, and the implications for contemporary marine resource management. The available evidence suggests that human-environment interactions over the last 2000 years were complex and dynamic; however, these data raise more questions than answers regarding the specific drivers of changes observed in the archaeomalacological record. This is encouraging as a baseline investigation and emphasizes the need for further engagement with historical ecology by a range of cognate disciplines to enhance our understanding of these complex issues
Interacting Dark Resonances: Interference Effects Induced by Coherently Altered Quantum Superpositions
We predict the possibility of sharp, high-contrast resonances in the optical
response of a broad class of systems, wherein interference effects are
generated by coherent perturbation or interaction of dark states. The
properties of these resonances can be manipulated to design a desired atomic
response.Comment: 4 pages, 3 figures, RevTeX, submitted to PRL; changed three numbers
in Fig. 3 (caption
Diluted Networks of Nonlinear Resistors and Fractal Dimensions of Percolation Clusters
We study random networks of nonlinear resistors, which obey a generalized
Ohm's law, . Our renormalized field theory, which thrives on an
interpretation of the involved Feynman Diagrams as being resistor networks
themselves, is presented in detail. By considering distinct values of the
nonlinearity r, we calculate several fractal dimensions characterizing
percolation clusters. For the dimension associated with the red bonds we show
that at least to order {\sl O} (\epsilon^4),
with being the correlation length exponent, and , where d
denotes the spatial dimension. This result agrees with a rigorous one by
Coniglio. Our result for the chemical distance, d_{\scriptsize min} = 2 -
\epsilon /6 - [ 937/588 + 45/49 (\ln 2 -9/10 \ln 3)] (\epsilon /6)^2 + {\sl O}
(\epsilon^3) verifies a previous calculation by one of us. For the backbone
dimension we find D_B = 2 + \epsilon /21 - 172 \epsilon^2 /9261 + 2 (- 74639 +
22680 \zeta (3))\epsilon^3 /4084101 + {\sl O} (\epsilon^4), where , in agreement to second order in with a two-loop
calculation by Harris and Lubensky.Comment: 29 pages, 7 figure
High efficiency photon counting using stopped light
Single-photon detection and photon counting play a central role in a large
number of quantum communication and computation protocols. While the efficiency
of state-of-the-art photo-detectors is well below the desired limits, quantum
state measurements in trapped ions can be carried out with efficiencies
approaching 100%. Here, we propose a method that can in principle achieve ideal
photon counting, by combining the techniques of photonic quantum memory and
ion-trap fluorescence detection: after mapping the quantum state of a
propagating light pulse onto metastable collective excitations of a trapped
cold atomic gas, it is possible to monitor the resonance fluorescence induced
by an additional laser field that only couples to the metastable excited state.
Even with a photon collection/detection efficiency as low as 10%, it is
possible to achieve photon counting with efficiency approaching 100%.Comment: 4 page
Exactly solvable model with two conductor-insulator transitions driven by impurities
We present an exact analysis of two conductor-insulator transitions in the
random graph model. The average connectivity is related to the concentration of
impurities. The adjacency matrix of a large random graph is used as a hopping
Hamiltonian. Its spectrum has a delta peak at zero energy. Our analysis is
based on an explicit expression for the height of this peak, and a detailed
description of the localized eigenvectors and of their contribution to the
peak. Starting from the low connectivity (high impurity density) regime, one
encounters an insulator-conductor transition for average connectivity
1.421529... and a conductor-insulator transition for average connectivity
3.154985.... We explain the spectral singularity at average connectivity
e=2.718281... and relate it to another enumerative problem in random graph
theory, the minimal vertex cover problem.Comment: 4 pages revtex, 2 fig.eps [v2: new title, changed intro, reorganized
text
Towards a Microscopic Model of Magnetoelectric Interactions in Ni3V2O8
We develop a microscopic magnetoelectric coupling in NiVO (NVO)
which gives rise to the trilinear phenomenological coupling used previously to
explain the phase transition in which magnetic and ferroelectric order
parameters appear simultaneously. Using combined neutron scattering
measurements and first-principles calculations of the phonons in NVO, we
determine eleven phonons which can induce the observed spontaneous
polarization. Among these eleven phonons, we find that a few of them can
actually induce a significant dipole moment. Using the calculated atomic
charges, we find that the required distortion to induce the observed dipole
moment is very small (~0.001 \AA) and therefore it would be very difficult to
observe the distortion by neutron-powder diffraction. Finally, we identify the
derivatives of the exchange tensor with respect to atomic displacements which
are needed for a microscopic model of a spin-phonon coupling in NVO and which
we hope will be obtained from a fundamental quantum calculation such as LDA+U.
We also analyze two toy models to illustrate that the Dzyaloskinskii-Moriya
interaction is very important for coexisting of magnetic and ferroelectric
order but it is not the only mechanism when the local site symmetry of the
system is low enough.Comment: 20 pages, 10 figure
Comparison of regional blood flow values measured by radioactive and fluorescent microspheres
Fluorescent microspheres (FM) have become an attractive alternative to radioactive microspheres (RM) for the measurement of regional blood flow (RBF). The aim of the present study was to investigate the comparability of both methods by measuring RBF with FM and RM. Eight anaesthetised pigs received simultaneous, left atrial injections of FM and RM with a diameter of 15 mum at six different time points. Blood reference samples were collected from the descending aorta. RBF was determined in tissue samples of the myocardium, spleen and kidneys of all 8 animals. After radioactivity of the tissue samples was determined, the samples were processed automatically for measuring fluorescence using a recently developed filter device (SPU). RBF was calculated with both the isotope and spectrometric data of both methods for each sample resulting in a total of 10,512 blood flow values. The comparison of the RBF values yielded high linear correlation (mean r(2) = 0.95 +/- 0.03 to 0.97 +/- 0.02) and excellent agreement (bias 5.4-6.7%, precision 9.9-16.5%) of both methods. Our results indicate the validity of MS and of the automated tissue processing technique by means of the SPU. Copyright (C) 2002 S. Karger AG, Basel
Comparing Online with Brick and Mortar Course Learning Outcomes: An Analysis of Quantitative Methods Curriculum in Public Administration
Teaching graduate students in an intensive adult-learning format presents a special challenge for quantitative analytical competencies. Students often lack necessary background, skills and motivation to deal with quantitative-skill-based course work. This study compares learning outcomes for graduate students enrolled in three course sections (cohorts) taking a quantitative methods course in a public administration program. One cohort of students was taught online, while two student cohorts were taught face-to-face in a traditional classroom setting. Most of the online students resided in the same geographic location as the “brick-and-mortar’ students. While student backgrounds and demographics were comparable, there were notable differences in their levels of self-directed learning readiness and persistence.
These differences illustrate both course design and modality features for a comparison between online and traditional brick-and-mortar learning environments. We find that predictors of student performance in an online environment are rather well described by the Self-Directed Learning Theory (SDL) and Self-Regulated Learning Theory (SRL). A statistically significant difference was found in the pretest-posttest mean scores, which indicates that students learned the course content for quantitative methods in the online section differently from those in the brick-and-mortar section. Overall, students enrolled in the online section (cohort) performed better on the posttest than did students enrolled in traditional “brick-and-mortar” classes. An age variable shows that older students performed much better than younger students on the posttest. Other differences in learning outcomes between the online and brick-and-mortar sections are analyzed in the study. Stakeholders in online education should be interested in these outcomes
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