561 research outputs found
Nonrandom Mixing Models of HIV Transmission
Models of HIV transmission and the AIDS epidemic generally assume random mixing among those infected with HIV and those who are not. For sexually transmitted HIV, this implies that individuals select sex partners without regard to attributes such as familiarity, attractiveness, or risk of infection. This paper formulates a model for examining the impact of nonrandom mixing on HIV transmission. We present threshold conditions that determine when HIV epidemics can occur within the framework of this model. Nonrandom mixing is introduced by assuming that sexually active individuals select sex partners to minimize the risk of infection. In addition to variability in risky sex rates, some versions of our model allow for error (or noise) in information exchanged between prospective partners. We investigate several models including random partner selection (or proportionate mixing), segregation of the population by risky sex rates, a probabilistic combination of segregation and random selection induced by imperfect information (or preferred mixing), and a model of costly search with perfect information. We develop examples which show that nonrandom mixing can lead to epidemics that are more severe or less severe than random mixing. For reasonable parameter choices describing the AIDS epidemic, however, the results suggest that random mixing models overstate the number of HIV infections that will occur.AIDs; Random Mixing Models; Search Costs
Collective effects in charge transfer within a hybrid organic-inorganic system
A collective electron transfer (ET) process was discovered by studying the
current noise in a field effect transistor with light-sensitive gate formed by
nanocrystals linked by organic molecules to its surface. Fluctuations in the ET
through the organic linker are reflected in the fluctuations of the transistor
conductivity. The current noise has an avalanche character. Critical exponents
obtained from the noise power spectra, avalanche distributions, and the
dependence of the average avalanche size on avalanche duration are consistent
with each other. A plausible model is proposed for this phenomenonComment: 15 pages 4 figures. Accepted for publication in Physical Review
Letter
Longitudinal and transverse noise in a moving Vortex Lattice
We have studied the longitudinal and the transverse velocity fluctuations of
a moving vortex lattice (VL) driven by a transport current. They exhibit both
the same broad spectrum and the same order of magnitude. These two components
are insensitive to the velocity and to a small bulk perturbation. This means
that no bulk averaging over the disorder and no VL crystallization are
observed. This is consistently explained referring to a previously proposed
noisy flow of surface current whose elementary fluctuator is measured
isotropic.Comment: accepted for publication in Phys Rev
Economic Costs of Influenza-Related Work Absenteeism
AbstractBackgroundInfluenza vaccinations are currently advocated only for individuals over age 50. However, vaccination of all working-age people may be warranted based on reduced absenteeism from work.ObjectiveThis study aims to quantify the association between lost workdays and influenza, controlling for other factors. A secondary aim of the study is to assess the net benefit of expanded vaccination in a workplace setting.Research DesignMultivariate regression analyses of the 1996 Medical Expenditure Panel Survey Household Component are used to estimate the number of workdays missed because of influenza-like illness (ILI) when controlling for other health, demographic, and employment factors. Mean productivity costs are measured in terms of absences from work and valued in dollar terms. The net benefit of influenza vaccination is estimated using a simple decision analysis.Subjects and MeasuresHealth, demographic, and employment data for employed individuals between the ages of 22 and 64 years are analyzed.ResultsThe average number of workdays missed due to ILI was 1.30 days, and the average work loss was valued at $137 per person. The vaccine strategy was not preferred in the baseline analysis; however, this result was sensitive to assumptions regarding the incidence of influenza, the cost of delivering the vaccine, and the productivity impact of worker absenteeism. Moreover, nonproductivity benefits of vaccination were omitted.ConclusionsThe economic attractiveness of expanded investment in influenza vaccination hinges on employer- and population-specific assumptions. Our analysis provides a simple framework within which competing considerations of disease epidemiology, worker productivity, and economic cost may be weighed
Experimental study of the correlation length of critical-current fluctuations in the presence of surface disorder: Probing vortex long-range interactions
We report on critical currents and voltage noise measurements in Niobium
strips in the superconducting state, in the presence of a bulk vortex lattice
() and in the surface superconducting state ().
For homogeneous surfaces, the correlation length of the current fluctuations
can be associated with the electromagnetic skin depth of vortex superficial
instabilities. The modification of the surface state by means of low energy
irradiation induces a strong modification of the critical current and of the
noise. The appearance of a corner frequency in the spectral domain can be
linked with the low wave-vectors of the artificial corrugation. Since this
latter occurs only for , we propose that the long-range
interactions allow the correlation length to extend up to values imposed by the
surface topography.Comment: accepted for publication in PR
Evolution of the fishtail-effect in pure and Ag-doped MG-YBCO
We report on magnetic measurements carried out in a textured
YBaCuO and YBa(CuAg)O (at
0.02) crystals. The so-called fishtail-effect (FE) or second
magnetization peak has been observed in a wide temperature range
0.4~~0.8 for . The origin of the FE arises for
the competition between surface barrier and bulk pinning. This is confirmed in
a non-monotonically behavior of the relaxation rate . The value
for Ag-doped crystals is larger than for the pure one due to the presence of
additional pinning centers, above all on silver atoms.Comment: 6 pages, 6 figure
Equilibration and Dynamic Phase Transitions of a Driven Vortex Lattice
We report on the observation of two types of current driven transitions in
metastable vortex lattices. The metastable states, which are missed in usual
slow transport measurements, are detected with a fast transport technique in
the vortex lattice of undoped
2H-NbSe. The transitions are seen by following the evolution of these
states when driven by a current. At low currents we observe an equilibration
transition from a metastable to a stable state, followed by a dynamic
crystallization transition at high currents.Comment: 5 pages, 4 figure
V-I characteristics in the vicinity of order-disorder transition in vortex matter
The shape of the V-I characteristics leading to a peak in the differential
resistance r_d=dV/dI in the vicinity of the order-disorder transition in NbSe2
is investigated. r_d is large when measured by dc current. However, for a small
Iac on a dc bias r_d decreases rapidly with frequency, even at a few Hz, and
displays a large out-of-phase signal. In contrast, the ac response increases
with frequency in the absence of dc bias. These surprisingly opposite phenomena
and the peak in r_d are shown to result from a dynamic coexistence of two
vortex matter phases rather than from the commonly assumed plastic depinning.Comment: 12 pages 4 figures. Accepted for publication in PRB rapi
Abrupt Transition between Thermally-Activated Relaxation and Quantum Tunneling in a Molecular Magnet
We report Hall sensor measurements of the magnetic relaxation of Mn
acetate as a function of magnetic field applied along the easy axis of
magnetization. Data taken at a series of closely-spaced temperatures between
0.24 K and 1.4 K provide strong new evidence for an abrupt ``first-order''
transition between thermally-assisted relaxation and magnetic decay via quantum
tunneling.Comment: 4 pages, including 7 figure
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