3,293 research outputs found
Non-invasive, near-field terahertz imaging of hidden objects using a single pixel detector
Terahertz (THz) imaging has the ability to see through otherwise opaque
materials. However, due to the long wavelengths of THz radiation
({\lambda}=300{\mu}m at 1THz), far-field THz imaging techniques are heavily
outperformed by optical imaging in regards to the obtained resolution. In this
work we demonstrate near-field THz imaging with a single-pixel detector. We
project a time-varying optical mask onto a silicon wafer which is used to
spatially modulate a pulse of THz radiation. The far-field transmission
corresponding to each mask is recorded by a single element detector and this
data is used to reconstruct the image of an object placed on the far side of
the silicon wafer. We demonstrate a proof of principal application where we
image a printed circuit board on the underside of a 115{\mu}m thick silicon
wafer with ~100{\mu}m ({\lambda}/4) resolution. With subwavelength resolution
and the inherent sensitivity to local conductivity provided by the THz probe
frequencies, we show that it is possible to detect fissures in the circuitry
wiring of a few microns in size. Imaging systems of this type could have other
uses where non-invasive measurement or imaging of concealed structures with
high resolution is necessary, such as in semiconductor manufacturing or in
bio-imaging
Density of kinks just after a quench in an overdamped system
A quench in an overdamped one dimensional model is studied by
analytical and numerical methods. For an infinite system or a finite system
with free boundary conditions, the density of kinks after the transition is
proportional to the eighth root of the rate of the quench. For a system with
periodic boundary conditions, it is proportional to the fourth root of the
rate. The critical exponent predicted in Zurek scenario is put in question.Comment: 4 pages in RevTex + 1 .ps fil
Formation of Topological Defects with Explicit Symmetry Breaking
We demonstrate a novel mechanism for the formation of topological defects in
a first order phase transition for theories in the presence of small explicit
symmetry breaking terms. We carry out numerical simulations of collisions of
two bubbles in 2+1 dimensions for a field theory where U(1) global symmetry is
spontaneously as well as explicitly broken. In the coalesced region of bubble
walls, field oscillations result in the decay of the coalesced portion in a
large number of defects (e.g. ten vortices and anti-vortices). We discuss the
implications of our results for axionic strings in the early Universe, for
baryon formation in quark-gluon plasma, and for electric or magnetic field.Comment: Latex file, 8 pages, 6 uuencoded postscript figure
The Hubble Diagram of Type Ia Supernovae in Non-Uniform Pressure Universes
We use the redshift-magnitude relation, as derived by D\c{a}browski (1995),
for the two exact non-uniform pressure spherically symmetric Stephani universes
with the observer positioned at the center of symmetry, to test the agreement
of these models with recent observations of high redshift type Ia supernovae
(SNIa), as reported in Perlmutter et al. (1997). By a particular choice of
model parameters, we show that these models give an excellent fit to the
observed redshifts and (corrected) B band apparent magnitudes of the SNIa data,
but for an age of the Universe which is typically about two Gyr greater than in
the corresponding Friedmann model.
Based on a value of and assuming , the P97 data
implies a Friedmann age of at most 13 Gyr and in fact a best-fit (for ) age of only 10 Gyr. Our Stephani models, on the other hand, can give a
good fit to the P97 data with an age of up to 15 Gyr and could, therefore,
significantly alleviate the conflict between recent cosmological and
astrophysical age predictions. The choice of model parameters is quite robust:
one requires only that the non-uniform pressure parameter, , in one of the
models is negative and satisfies |a| \lte 3 km s Mpc. By
allowing slightly larger, negative, values of one may `fine tune' the model
to give an even better fit to the P97 data.Comment: 36 pages, 2 tables, 6 figures, AAS Latex 4.0, vastly revised version,
new title and abstract, to appear in Ap
A test for within-lake niche differentiation in the nine-spined sticklebacks (Pungitius pungitius)
Specialization for the use of different resources can lead to ecological speciation. Accordingly, there are numerous examples of ecologically specialized pairs of fish species in postglacial lakes. Using a polymorphic panel of single nucleotide variants, we tested for genetic footprints of within-lake population stratification in nine-spined sticklebacks (Pungitius pungitius) collected from three habitats (viz. littoral, benthic, and pelagic) within a northern Swedish lake. Analyses of admixture, population structure, and relatedness all supported the conclusion that the fish from this lake form a single interbreeding unit.Peer reviewe
Evolution of Fields in a Second Order Phase Transition
We analyse the evolution of scalar and gauge fields during a second order
phase transition using a Langevin equation approach. We show that topological
defects formed during the phase transition are stable to thermal fluctuations.
Our method allows the field evolution to be followed throughout the phase
transition, for both expanding and non-expanding Universes. The results verify
the Kibble mechanism for defect formation during phase transitions.Comment: 12 pages of text plus 17 diagrams available on request, DAMTP 94-8
Cosmic String Formation from Correlated Fields
We simulate the formation of cosmic strings at the zeros of a complex
Gaussian field with a power spectrum , specifically
addressing the issue of the fraction of length in infinite strings. We make two
improvements over previous simulations: we include a non-zero random background
field in our box to simulate the effect of long-wavelength modes, and we
examine the effects of smoothing the field on small scales. The inclusion of
the background field significantly reduces the fraction of length in infinite
strings for . Our results are consistent with the possibility that
infinite strings disappear at some in the range ,
although we cannot rule out , in which case infinite strings would
disappear only at the point where the mean string density goes to zero. We
present an analytic argument which suggests the latter case. Smoothing on small
scales eliminates closed loops on the order of the lattice cell size and leads
to a ``lattice-free" estimate of the infinite string fraction. As expected,
this fraction depends on the type of window function used for smoothing.Comment: 24 pages, latex, 10 figures, submitted to Phys Rev
Avoiding selection bias in gravitational wave astronomy
When searching for gravitational waves in the data from ground-based
gravitational wave detectors it is common to use a detection threshold to
reduce the number of background events which are unlikely to be the signals of
interest. However, imposing such a threshold will also discard some real
signals with low amplitude, which can potentially bias any inferences drawn
from the population of detected signals. We show how this selection bias is
naturally avoided by using the full information from the search, considering
both the selected data and our ignorance of the data that are thrown away, and
considering all relevant signal and noise models. This approach produces
unbiased estimates of parameters even in the presence of false alarms and
incomplete data. This can be seen as an extension of previous methods into the
high false rate regime where we are able to show that the quality of parameter
inference can be optimised by lowering thresholds and increasing the false
alarm rate.Comment: 13 pages, 2 figure
Big bang simulation in superfluid 3He-B -- Vortex nucleation in neutron-irradiated superflow
We report the observation of vortex formation upon the absorption of a
thermal neutron in a rotating container of superfluid He-B. The nuclear
reaction n + He = p + H + 0.76MeV heats a cigar shaped region of the
superfluid into the normal phase. The subsequent cooling of this region back
through the superfluid transition results in the nucleation of quantized
vortices. Depending on the superflow velocity, sufficiently large vortex rings
grow under the influence of the Magnus force and escape into the container
volume where they are detected individually with nuclear magnetic resonance.
The larger the superflow velocity the smaller the rings which can expand. Thus
it is possible to obtain information about the morphology of the initial defect
network. We suggest that the nucleation of vortices during the rapid cool-down
into the superfluid phase is similar to the formation of defects during
cosmological phase transitions in the early universe.Comment: 4 pages, LaTeX file, 4 figures are available at
ftp://boojum.hut.fi/pub/publications/lowtemp/LTL-95009.p
Hadronic Regge Trajectories: Problems and Approaches
We scrutinized hadronic Regge trajectories in a framework of two different
models --- string and potential. Our results are compared with broad spectrum
of existing theoretical quark models and all experimental data from PDG98. It
was recognized that Regge trajectories for mesons and baryons are not straight
and parallel lines in general in the current resonance region both
experimentally and theoretically, but very often have appreciable curvature,
which is flavor-dependent. For a set of baryon Regge trajectories this fact is
well described in the considered potential model. The standard string models
predict linear trajectories at high angular momenta J with some form of
nonlinearity at low J.Comment: 15 pages, 9 figures, LaTe
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