7,214 research outputs found
Weak disorder: anomalous transport and diffusion are normal yet again
Particles driven through a periodic potential by an external constant force
are known to exhibit a pronounced peak of the diffusion around a critical force
that defines the transition between locked and running states. It has recently
been shown both experimentally and numerically that this peak is greatly
enhanced if some amount of spatial disorder is superimposed on the periodic
potential. Here we show that beyond a simple enhancement lies a much more
interesting phenomenology. For some parameter regimes the system exhibits a
rich variety of behaviors from normal diffusion to superdiffusion, subdiffusion
and even subtransport.Comment: Substantial improvements in presentatio
Fraser of cryptophthalmosis syndrome: A case report
Fraser or cryptophthalmos syndrome is a rare autosomal recessive disorder characterized by major features such as cryptophthalmos, syndactyly, and abnormal genitalia. Consanguinity is reported in 15-24.8% of the cases. The diagnosis of this syndrome can be made on clinical examination; therefore we present the clinical findings of a rare case of Fraser syndrome in a female infant
Theoretical investigation of moir\'e patterns in quantum images
Moir\'e patterns are produced when two periodic structures with different
spatial frequencies are superposed. The transmission of the resulting structure
gives rise to spatial beatings which are called moir\'e fringes. In classical
optics, the interest in moir\'e fringes comes from the fact that the spatial
beating given by the frequency difference gives information about details(high
spatial frequency) of a given spatial structure. We show that moir\'e fringes
can also arise in the spatial distribution of the coincidence count rate of
twin photons from the parametric down-conversion, when spatial structures with
different frequencies are placed in the path of each one of the twin beams. In
other words,we demonstrate how moir\'e fringes can arise from quantum images
Density Perturbations in the Ekpyrotic Scenario
We study the generation of density perturbations in the ekpyrotic scenario
for the early universe, including gravitational backreaction. We expose
interesting subtleties that apply to both inflationary and ekpyrotic models.
Our analysis includes a detailed proposal of how the perturbations generated in
a contracting phase may be matched across a `bounce' to those in an expanding
hot big bang phase. For the physical conditions relevant to the ekpyrotic
scenario, we re-obtain our earlier result of a nearly scale-invariant spectrum
of energy density perturbations. We find that the perturbation amplitude is
typically small, as desired to match observation.Comment: 36 pages, compressed and RevTex file, one postscript figure file.
Minor typographical and numerical errors corrected, discussion added. This
version to appear in Physical Review
Hydrogen-Poor Disks in Compact X-Ray Binaries
We show that accretion disks in several compact X-ray binaries with
hydrogen-depleted donors are likely subject to a thermal ionization
instability, unless they are strongly irradiated. These disks are particularly
interesting in that their MHD-turbulent properties in the neutral phase may be
quite different from those of standard, hydrogen-rich disks.Comment: 10 pages, accepted for publication in ApJ
Quantum Fields in a Big Crunch/Big Bang Spacetime
We consider quantum field theory on a spacetime representing the Big
Crunch/Big Bang transition postulated in the ekpyrotic or cyclic cosmologies.
We show via several independent methods that an essentially unique matching
rule holds connecting the incoming state, in which a single extra dimension
shrinks to zero, to the outgoing state in which it re-expands at the same rate.
For free fields in our construction there is no particle production from the
incoming adiabatic vacuum. When interactions are included the total particle
production for fixed external momentum is finite at tree level. We discuss a
formal correspondence between our construction and quantum field theory on de
Sitter spacetime.Comment: 30 pages, RevTex file, five postscript figure file
Vector Bundle Moduli Superpotentials in Heterotic Superstrings and M-Theory
The non-perturbative superpotential generated by a heterotic superstring
wrapped once around a genus-zero holomorphic curve is proportional to the
Pfaffian involving the determinant of a Dirac operator on this curve. We show
that the space of zero modes of this Dirac operator is the kernel of a linear
mapping that is dependent on the associated vector bundle moduli. By explicitly
computing the determinant of this map, one can deduce whether or not the
dimension of the space of zero modes vanishes. It is shown that this
information is sufficient to completely determine the Pfaffian and, hence, the
non-perturbative superpotential as explicit holomorphic functions of the vector
bundle moduli. This method is illustrated by a number of non-trivial examples.Comment: 81 pages, LaTeX, corrected typo
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