78,852 research outputs found
Non-vanishing Heterotic Superpotentials on Elliptic Fibrations
We present models of heterotic compactification on Calabi-Yau threefolds and
compute the non-perturbative superpotential for vector bundle moduli. The key
feature of these models is that the threefolds, which are elliptically fibered
over del Pezzo surfaces, have homology classes with a unique holomorphic,
isolated genus-zero curve. Using the spectral cover construction, we present
vector bundles for which we can explicitly calculate the Pfaffians associated
with string instantons on these curves. These are shown to be non-zero, thus
leading to a non-vanishing superpotential in the 4D effective action. We
discuss, in detail, why such compactifications avoid the Beasley-Witten residue
theorem.Comment: 1 + 23 page
Dissociation Transition of a Composite Lattice of Magnetic Vortices in the Flux-Flow Regime of Two-Band Superconductors
In multiband superconductors, each superconducting condensate supports
vortices with fractional quantum flux. In the ground state, vortices in
different bands are spatially bounded together to form a composite vortex,
carrying one quantum flux \Phi_0. Here we predict dissociation of the composite
vortices lattice in the flux flow state due to the disparity of the vortex
viscosity and flux of the vortex in different bands. For a small driving
current, composite vortices start to deform, but the constituting vortices in
different bands move with the same velocity. For a large current, composite
vortices dissociate and vortices in different bands move with different
velocities. The dissociation transition shows up as an increase of flux flow
resistivity. In the dissociated phase, Shapiro steps are developed when an ac
current is superimposed with a dc current.Comment: 4.5 pages, 3 figure
A Thermal-Nonthermal Inverse Compton Model for Cyg X-1
Using Monte Carlo methods to simulate the inverse Compton scattering of soft
photons, we model the spectrum of the Galactic black hole candidate Cyg X-1,
which shows evidence of a nonthermal tail extending beyond a few hundred keV.
We assume an ad hoc sphere of leptons, whose energy distribution consists of a
Maxwellian plus a high energy power-law tail, and inject 0.5 keV blackbody
photons. The spectral data is used to constrain the nonthermal plasma fraction
and the power-law index assuming a reasonable Maxwellian temperature and
Thomson depth. A small but non-negligible fraction of nonthermal leptons is
needed to explain the power-law tail.Comment: 5 pages, 2 PostScript figure, uses aipproc.sty, to appear in
Proceedings of Fourth Compton Symposiu
Osculating and neighbour-avoiding polygons on the square lattice
We study two simple modifications of self-avoiding polygons. Osculating
polygons are a super-set in which we allow the perimeter of the polygon to
touch at a vertex. Neighbour-avoiding polygons are only allowed to have nearest
neighbour vertices provided these are joined by the associated edge and thus
form a sub-set of self-avoiding polygons. We use the finite lattice method to
count the number of osculating polygons and neighbour-avoiding polygons on the
square lattice. We also calculate their radius of gyration and the first
area-weighted moment. Analysis of the series confirms exact predictions for the
critical exponents and the universality of various amplitude combinations. For
both cases we have found exact solutions for the number of convex and
almost-convex polygons.Comment: 14 pages, 5 figure
Phase dynamics of inductively coupled intrinsic Josephson junctions and terahertz electromagnetic radiation
The Josephson effects associated with quantum tunneling of Cooper pairs
manifest as nonlinear relations between the superconductivity phase difference
and the bias current and voltage. Many novel phenomena appear, such as Shapiro
steps in dc cuurent-voltage (IV) characteristics of a Josephson junction under
microwave shining, which can be used as a voltage standard. Inversely, the
Josephson effects provide a unique way to generate high-frequency
electromagnetic (EM) radiation by dc bias voltage. The discovery of cuprate
high-Tc superconductors accelerated the effort to develop novel source of EM
waves based on a stack of atomically dense-packed intrinsic Josephson junctions
(IJJs), since the large superconductivity gap covers the whole terahertz
frequency band. Very recently, strong and coherent terahertz radiations have
been successfully generated from a mesa structure of
single crystal which works both as the source
of energy gain and as the cavity for resonance. It is then found theoretically
that, due to huge inductive coupling of IJJs produced by the nanometer junction
separation and the large London penetration depth of order of of
the material, a novel dynamic state is stabilized in the coupled sine-Gordon
system, in which kinks in phase differences are developed responding
to the standing wave of Josephson plasma and are stacked alternatively in the
c-axis. This novel solution of the inductively coupled sine-Gordon equations
captures the important features of experimental observations. The theory
predicts an optimal radiation power larger than the one available to date by
orders of magnitude, and thus suggests the technological relevance of the
phenomena.Comment: review article (69 pages, 30 figures
Driven classical diffusion with strong correlated disorder
We analyze one-dimensional motion of an overdamped classical particle in the
presence of external disorder potential and an arbitrary driving force F. In
thermodynamical limit the effective force-dependent mobility mu(F) is
self-averaging, although the required system size may be exponentially large
for strong disorder. We calculate the mobility mu(F) exactly, generalizing the
known results in linear response (weak driving force) and the perturbation
theory in powers of the disorder amplitude. For a strong disorder potential
with power-law correlations we identify a non-linear regime with a prominent
power-law dependence of the logarithm of mu(F) on the driving force.Comment: 4 pages, 2 figures include
The Role of Risk Attitude in Preference Rankings of Vaccine Use for Foot and Mouth Disease Eradication in the U.S.
Food Security and Poverty,
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