3,772 research outputs found
Radiation 'damping' in atomic photonic crystals
The force exerted on a material by an incident beam of light is dependent
upon the material's velocity in the laboratory frame of reference. This
velocity dependence is known to be diffcult to measure, as it is proportional
to the incident optical power multiplied by the ratio of the material velocity
to the speed of light. Here we show that this typically tiny effect is greatly
amplified in multilayer systems composed of resonantly absorbing atoms (e.g.
optically trapped 87Rb), which may exhibit ultra-narrow photonic band gaps. The
amplification of the effect is shown to be three orders of magnitude greater
than previous estimates for conventional photonic-band-gap materials, and
significant for material velocities of a few ms/s.Comment: 5 pages, 3 figure
Perinatal stem cells revisited: directions and indications at the crossroads between tissue regeneration and repair.
Perinatal stem cells research attracted great interest worldwide in recent years. Foetus-associated tissues contain
various populations of stem cells, most of which are comprised within the category of mesenchymal stem cells
(MSCs). This special issue collects both reviews and original
reports on all the perinatal stem cell types which are currently under investigation. These cells have multiple promising features: differentiative capacity towards mature cell
types of all the three germ layers, hypoimmunogenicity in
vitro and in vivo, ease of sourcing, ex vivo culture and stor-
age. In particular, immune modulation is viewed as a prom-
ising feature of many MSCs populations, since these cells,
once administered therapeutically, may be able to overcome,
or at least evade, the host immune response which may lead
to acute or chronic rejection of the transplant
Role of anisotropy in the F\"orster energy transfer from a semiconductor quantum well to an organic crystalline overlayer
We consider the non-radiative resonant energy transfer from a two-dimensional
Wannier exciton (donor) to a Frenkel exciton of a molecular crystal overlayer
(acceptor). We characterize the effect of the optical anisotropy of the organic
subsystem on this process. Using realistic values of material parameters, we
show that it is possible to change the transfer rate within typically a factor
of two depending on the orientation of the crystalline overlayer. The resonant
matching of donor and acceptor energies is also partly tunable via the organic
crystal orientation.Comment: 6 pages, 8 figure
A Gas-Kinetic Model for Shallow Water Flows in Presence of Wet/Dry Fronts
Experimental and Computational Hydraulic
Spatial Kramers–Kronig relations and the reflection of waves
When a planar dielectric medium has a permittivity profile that is an analytic function in the upper or lower half of the complex position plane x = x′ + ix″ then the real and imaginary parts of its permittivity are related by the spatial Kramers–Kronig relations. We find that such a medium will not reflect radiation incident from one side, whatever the angle of incidence. Using the spatial Kramers–Kronig relations, one can derive a real part of a permittivity profile from some given imaginary part (or vice versa) such that the reflection is guaranteed to be zero. This result is valid for both scalar and vector wave theories and may have relevance for designing materials that efficiently absorb radiation or for the creation of a new type of anti-reflection surface
Synchronization in Scale Free networks: The role of finite size effects
Synchronization problems in complex networks are very often studied by
researchers due to its many applications to various fields such as
neurobiology, e-commerce and completion of tasks. In particular, Scale Free
networks with degree distribution , are widely used in
research since they are ubiquitous in nature and other real systems. In this
paper we focus on the surface relaxation growth model in Scale Free networks
with , and study the scaling behavior of the fluctuations, in
the steady state, with the system size . We find a novel behavior of the
fluctuations characterized by a crossover between two regimes at a value of
that depends on : a logarithmic regime, found in previous
research, and a constant regime. We propose a function that describes this
crossover, which is in very good agreement with the simulations. We also find
that, for a system size above , the fluctuations decrease with
, which means that the synchronization of the system improves as
increases. We explain this crossover analyzing the role of the
network's heterogeneity produced by the system size and the exponent of the
degree distribution.Comment: 9 pages and 5 figures. Accepted in Europhysics Letter
Application of the spac method to ambient noise recorded in the vesuvius area (italy)
Noise measurements were recorded using a dense short-period seismic
array in Terzigno (Naples), a town that is located about 6 km from the Vesuvius crater. The
aim of this study was to calculate a surface velocity model of the area under investigation
through the application of the Spatial Autocorrelation (SPAC) method, with the hypotheses
that ambient noise is stationary both in time and space, and that it is composed of surface
dispersive waves. The correct knowledge of the surface structure is an important goal in
site-effects studies. Correlation coefficients were calculated as functions of the azimuth on
noise recorded at pairs of equally spaced stations in the frequency range of 1-8 Hz. Then,
the spatial average correlation coefficients were compared to estimates over long-term
recordings. The results appear to validate the hypothesis that ambient noise can be
considered as a stochastic process. The correlation-frequency curves have been fitted to
Bessel functions, from which the Rayleigh wave dispersion curve has been calculated. A
velocity model has been derived from the dispersion curve using both trial and error and a
standard inversion procedure. The results are consistent with those obtained from array
measurements in the area in other studies (Scarpa et al., 2003)
Site effects Estimation and Source_Scaling Dynamics for Local Earthquakes at Mt. Vesuvius, Italy
Local microearthquakes were used to estimate site effects and source
dynamic-scaling characteristics at Mt. Vesuvius, Italy. The selected data set is composed
of low magnitude events (1.1 £ Md £ 3.6) recorded in 1996 and 1999 by nine digital shortperiod
(1-Hz) seismic stations. Site response was evaluated by analysing data with three
different approaches: 1) spectral ratios method of S-waves with respect to the average
amplitude spectrum; 2) generalized inversion for site and source from the S-waves; and 3)
generalized inversion from the coda waves. The results obtained with all three methods
showed amplification of a factor of 1.5-2.5 in the 8-14 Hz frequency band for BKE and
SGV sites and an amplification of 3 in a narrow band around 8 Hz for the CDT site.
Method 2 allowed simultaneous determination of the source spectral shape for each
earthquake. By assuming an w2 source model, we estimated the seismic moment Mo and
corner frequency fc. The results show that most of the selected earthquakes are
characterized by stress drops of 10 bars. The present results are encouraging for further
investigation into the techniques for site-effect evaluation and for improving our knowledge
of the scaling law of the source spectrum at Mt. Vesuvius
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