4,510 research outputs found
Exploring barriers to 'Respondent driven sampling' in sex worker and drug-injecting sex worker populations in Eastern Europe
Respondent driven sampling (RDS) has been used in several counties to sample injecting drug users, sex workers (SWs) and men who have sex with men and as a means of collecting behavioural and biological health data. We report on the use of RDS in three separate studies conducted among SWs between 2004 and 2005 in the Russian Federation, Serbia, and Montenegro. Findings suggest that there are limitations associated with the use of RDS in SW populations in these regions. Findings highlight three main factors that merit further investigation as a means of assessing the feasibility and appropriateness of RDS in this high risk population: the network characteristics of SWs; the appropriate level of participant incentives; and lack of service contact. The highly controlled and hidden nature of SW organizations and weak SW social networks in the region can combine to undermine assumptions underpinning the feasibility of RDS approaches and potentially severely limit recruitment. We discuss the implications of these findings for recruitment and the use of monetary and non-monetary incentives in future RDS studies of SW populations in Eastern Europe
Fluctuations of the local density of states probe localized surface plasmons on disordered metal films
We measure the statistical distribution of the local density of optical
states (LDOS) on disordered semi-continuous metal films. We show that LDOS
fluctuations exhibit a maximum in a regime where fractal clusters dominate the
film surface. These large fluctuations are a signature of surface-plasmon
localization on the nanometer scale
Spontaneous decay of an emitter's excited state near a finite-length metallic carbon nanotube
The spontaneous decay of an excited state of an emitter placed in the
vicinity of a metallic single-wall carbon nanotube (SWNT) was examined
theoretically. The emitter-SWNT coupling strongly depends on the position of
the emitter relative to the SWNT, the length of the SWNT, the dipole transition
frequency and the orientation of the emitter. In the high-frequency regime,
dips in the spectrum of the spontaneous decay rate exist at the resonance
frequencies in the spectrum of the SWNT conductivity. In the
intermediate-frequency regime, the SWNT conductivity is very low, and the
spontaneous decay rate is practically unaffected by the SWNT. In the
low-frequency regime, the spectrum of the spontaneous decay rate contains
resonances at the antennas resonance frequencies for surface-wave propagation
in the SWNT. Enhancement of both the total and radiative spontaneous decay
rates by several orders in magnitude is predicted at these resonance
frequencies. The strong emitter-field coupling is achieved, in spite of the low
Q factor of the antenna resonances, due to the very high magnitude of the
electromagnetic field in the near-field zone. The vacuum Rabi oscillations of
the population of the excited emitter state are exhibited when the emitter is
coupled to an antenna resonance of the SWNT.Comment: 8 pages, 6 figure
Quality factor of thin-film Fabry-Perot resonators: dependence on interface roughness
Thin-film Fabry-Perot (F-P) optical resonators are studied for application as wavelength-selecting elements in on-chip spectrometers. The interface roughness between the different resonator layers (Al /PECVD SiO2 / Ag) is identified to be the primary source of light scattering and energy losses. It is
demonstrated that conventional IC fabrication yields layers with RMS interface roughness easily exceeding 10 nm. When applied to the visible spectral range, such a roughness causes significant degradation of the F-P filter quality factor. Moreover, the scattered light contributes to transmittance outside the narrow resonance band to which the F-P filter is tuned and overall device performance is decreased
Extrapolation-CAM Theory for Critical Exponents
By intentionally underestimating the rate of convergence of
exact-diagonalization values for the mass or energy gaps of finite systems, we
form families of sequences of gap estimates. The gap estimates cross zero with
generically nonzero linear terms in their Taylor expansions, so that
for each member of these sequences of estimates. Thus, the Coherent Anomaly
Method can be used to determine . Our freedom in deciding exactly how to
underestimate the convergence allows us to choose the sequence that displays
the clearest coherent anomaly. We demonstrate this approach on the
two-dimensional ferromagnetic Ising model, for which . We also use it
on the three-dimensional ferromagnetic Ising model, finding , in good agreement with other estimates.Comment: 21 pages, Submitted to Journal of Physics A; new section added
discussing rate of convergence and relation to Finite-Size Scalin
A Numerical Transfer-Matrix Study of Surface-Tension Anisotropy in Ising Models on Square and Cubic Lattices
We compute by numerical transfer-matrix methods the surface free energy
the surface stiffness coefficient and the single-step
free energy for Ising ferromagnets with
square-lattice and cubic-lattice geometries, into
which an interface is introduced by imposing antiperiodic or plus/minus
boundary conditions in one transverse direction. These quantities occur in
expansions of the angle-dependent surface tension, either for rough or for
smooth interfaces. The finite-size scaling behavior of the interfacial
correlation length provides the means of investigating and
The resulting transfer-matrix estimates are fully consistent with previous
series and Monte Carlo studies, although current computational technology does
not permit transfer-matrix studies of sufficiently large systems to show
quantitative improvement over the previous estimates.Comment: 40 pages, 17 figures available on request. RevTeX version 2.
Nanoplasmonic Renormalization and Enhancement of Coulomb Interactions
Nanostructured plasmonic metal systems are known to enhance greatly variety
of radiative and nonradiative optical processes, both linear and nonlinear,
which are due to the interaction of an electron in a molecule or semiconductor
with the enhanced local optical field of the surface plasmons. Principally
different are numerous many-body phenomena that are due to the Coulomb
interaction between charged particles: carriers (electrons and holes) and ions.
These include carrier-carrier or carrier-ion scattering, energy and momentum
transfer (including the drag effect), thermal equilibration, exciton formation,
impact ionization, Auger effects, etc. It is not widely recognized that these
and other many-body effects can also be modified and enhanced by the
surface-plasmon local fields. A special but extremely important class of such
many-body phenomena is constituted by chemical reactions at metal surfaces,
including catalytic reactions. Here, we propose a general and powerful theory
of the plasmonic enhancement of the many-body phenomena resulting in a closed
expression for the surface plasmon-dressed Coulomb interaction. We illustrate
this theory by computing this dressed interaction explicitly for an important
example of metal-dielectric nanoshells, which exhibits a reach resonant
behavior in both the magnitude and phase. This interaction is used to describe
the nanoplasmonic-enhanced Foerster energy transfer between nanocrystal quantum
dots in the proximity of a plasmonic nanoshell. Catalysis at nanostructured
metal surfaces, nonlocal carrier scattering and surface-enhanced Raman
scattering are discussed among other effects and applications where the
nanoplasmonic renormalization of the Coulomb interaction may be of principal
importance
Conjugate field and fluctuation-dissipation relation for the dynamic phase transition in the two-dimensional kinetic Ising model
The two-dimensional kinetic Ising model, when exposed to an oscillating
applied magnetic field, has been shown to exhibit a nonequilibrium,
second-order dynamic phase transition (DPT), whose order parameter Q is the
period-averaged magnetization. It has been established that this DPT falls in
the same universality class as the equilibrium phase transition in the
two-dimensional Ising model in zero applied field. Here we study for the first
time the scaling of the dynamic order parameter with respect to a nonzero,
period-averaged, magnetic `bias' field, H_b, for a DPT produced by a
square-wave applied field. We find evidence that the scaling exponent,
\delta_d, of H_b at the critical period of the DPT is equal to the exponent for
the critical isotherm, \delta_e, in the equilibrium Ising model. This implies
that H_b is a significant component of the field conjugate to Q. A finite-size
scaling analysis of the dynamic order parameter above the critical period
provides further support for this result. We also demonstrate numerically that,
for a range of periods and values of H_b in the critical region, a
fluctuation-dissipation relation (FDR), with an effective temperature
T_{eff}(T, P, H_0) depending on the period, and possibly the temperature and
field amplitude, holds for the variables Q and H_b. This FDR justifies the use
of the scaled variance of Q as a proxy for the nonequilibrium susceptibility,
\partial / \partial H_b, in the critical region.Comment: revised version; 31 pages, 12 figures; accepted by Phys. Rev.
Completeness of evanescent modes in layered dielectrics
In the presence of a dielectric slab, the modes of the free electromagnetic field comprise traveling modes, consisting of incoming, reflected, and transmitted parts, as well as trapped modes that are subject to repeated total internal reflection and emerge as evanescent field outside the slab. Traveling modes have a continuous range of frequencies, but trapped modes occur only at certain discrete frequencies. We solve the problem of which relative weight to use when summing over all modes, as commonly required in perturbative calculations. We demonstrate the correctness of our method by showing the completeness of electromagnetic field modes in the presence of a dielectric slab. We derive a convenient method of summing over all modes by means of a single contour integral, which is very useful in standard quantum electrodynamic calculations
Study of the effect of thermal processing on passion fruit juice by means of 1H NMR spectroscopy and chemometrics.
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