6,278 research outputs found
Microlensing of gamma ray bursts by stars and MACHOs
The microlensing interpretation of the optical afterglow of GRB 000301C seems
naively surprising, since a simple estimate of the stellar microlensing rate
gives less than one in four hundred for a flat Omega_Lambda=0.7 cosmology,
whereas one event was seen in about thirty afterglows. Considering baryonic
MACHOs making up half of the baryons in the universe, the microlensing
probability per burst can be roughly 5% for a GRB at redshift z=2. We explore
two effects that may enhance the probability of observing microlensed gamma-ray
burst afterglows: binary lenses and double magnification bias. We find that the
consideration of binary lenses can increase the rate only at the ~15% level. On
the other hand, because gamma-ray bursts for which afterglow observations exist
are typically selected based on fluxes at widely separated wavebands which are
not necessarily well correlated (e.g. localization in X-ray, afterglow in
optical/infrared), magnification bias can operate at an enhanced level compared
to the usual single-bias case. We find that existing estimates of the slope of
the luminosity function of gamma-ray bursts, while as yet quite uncertain,
point to enhancement factors of more than three above the simple estimates of
the microlensing rate. We find that the probability to observe at least one
microlensing event in the sample of 27 measured afterglows can be 3-4% for
stellar lenses, or as much as 25 Omega_lens for baryonic MACHOs. We note that
the probability to observe at least one event over the available sample of
afterglows is significant only if a large fraction of the baryons in the
universe are condensed in stellar-mass objects. (ABRIDGED)Comment: 22 pages, 4 figures, 2 table
Gauge Consistent Wilson Renormalization Group I: Abelian Case
A version of the Wilson Renormalization Group Equation consistent with gauge
symmetry is presented. A perturbative renormalizability proof is established. A
wilsonian derivation of the Callan-Symanzik equation is given.Comment: Latex2e, 39 pages, 3 eps figures. Revised version to appear in Int.
J. Mod. Phy
Photon number discrimination without a photon counter and its application to reconstructing non-Gaussian states
The non-linearity of a conditional photon-counting measurement can be used to
`de-Gaussify' a Gaussian state of light. Here we present and experimentally
demonstrate a technique for photon number resolution using only homodyne
detection. We then apply this technique to inform a conditional measurement;
unambiguously reconstructing the statistics of the non-Gaussian one and two
photon subtracted squeezed vacuum states. Although our photon number
measurement relies on ensemble averages and cannot be used to prepare
non-Gaussian states of light, its high efficiency, photon number resolving
capabilities, and compatibility with the telecommunications band make it
suitable for quantum information tasks relying on the outcomes of mean values.Comment: 4 pages, 3 figures. Theory section expanded in response to referee
comment
Simulations of lattice animals and trees
The scaling behaviour of randomly branched polymers in a good solvent is
studied in two to nine dimensions, using as microscopic models lattice animals
and lattice trees on simple hypercubic lattices. As a stochastic sampling
method we use a biased sequential sampling algorithm with re-sampling, similar
to the pruned-enriched Rosenbluth method (PERM) used extensively for linear
polymers. Essentially we start simulating percolation clusters (either site or
bond), re-weigh them according to the animal (tree) ensemble, and prune or
branch the further growth according to a heuristic fitness function. In
contrast to previous applications of PERM, this fitness function is {\it not}
the weight with which the actual configuration would contribute to the
partition sum, but is closely related to it. We obtain high statistics of
animals with up to several thousand sites in all dimension 2 <= d <= 9. In
addition to the partition sum (number of different animals) we estimate
gyration radii and numbers of perimeter sites. In all dimensions we verify the
Parisi-Sourlas prediction, and we verify all exactly known critical exponents
in dimensions 2, 3, 4, and >= 8. In addition, we present the hitherto most
precise estimates for growth constants in d >= 3. For clusters with one site
attached to an attractive surface, we verify the superuniversality of the
cross-over exponent at the adsorption transition predicted by Janssen and
Lyssy. Finally, we discuss the collapse of animals and trees, arguing that our
present version of the algorithm is also efficient for some of the models
studied in this context, but showing that it is {\it not} very efficient for
the `classical' model for collapsing animals.Comment: 17 pages RevTeX, 29 figures include
Photon-Phonon-assisted tunneling through a single-molecular quantum dot
Based on exactly mapping of a many-body electron-phonon interaction problem
onto a one-body problem, we apply the well-established nonequilibrium Green
function technique to solve the time-dependent phonon-assisted tunneling at low
temperature through a single-molecular quantum dot connected to two leads,
which is subject to a microwave irradiation field. It is found that in the
presence of the electron-phonon interaction and the microwave irradiation
field, the time-average transmission and the nonlinear differential conductance
display additional peaks due to pure photon absorption or emission processes
and photon-absorption-assisted phonon emission processes. The variation of the
time-average current with frequency of the microwave irradiation field is also
studied.Comment: 9 pages, 6 figures, submitted to Phys. Rev. B. accepted by Phys. Rev.
Phase diagram of the metal-insulator transition in 2D electronic systems
We investigated the interdependence of the effects of disorder and carrier
correlations on the metal-insulator transition in two-dimensional electronic
systems. We present a quantitative metal-insulator phase diagram. Depending on
the carrier density we find two different types of metal-insulator transition -
a continuous localization for rs=<8 and a discontinuous transition at higher
rs. The critical level of disorder at the transition decreases with decreasing
carrier density. At very low carrier densities we find that the system is
always insulating. The value of the conductivity at the transition is
consistent with recent experimental measurements. The self-consistent method
which we have developed includes the effects of both disorder and correlations
on the transition, using a density relaxation theory with the Coulomb
correlations determined from numerical simulation data.Comment: 4 pages, RevTeX + epsf, 5 figures. New comments on conducting phase
and on the conductivity. References updated and correcte
Eikonal Evolution and Gluon Radiation
We give a simple quantum mechanical formulation of the eikonal propagation
approximation, which has been heavily used in recent years in problems
involving hadronic interactions at high energy. This provides a unified
framework for several approaches existing in the literature. We illustrate this
scheme by calculating the total, elastic, inelastic and diffractive DIS cross
sections, as well as gluon production in high energy hadronic collisions. From
the q-qbar-g-component of the DIS cross sections, we straightforwardly derive
low x evolution equations for inelastic and diffractive DIS distribution
functions. In all calculations, we provide all order 1/N corrections to the
results existing in the literature.Comment: 40 pages, LaTeX, 3 eps-figures, typos corrected, to be published in
PR
The Einstein-Podolsky-Rosen paradox: from concepts to applications
This Colloquium examines the field of the EPR Gedankenexperiment, from the
original paper of Einstein, Podolsky and Rosen, through to modern theoretical
proposals of how to realize both the continuous-variable and discrete versions
of the EPR paradox. We analyze the relationship with entanglement and Bell's
theorem, and summarize the progress to date towards experimental confirmation
of the EPR paradox, with a detailed treatment of the continuous-variable
paradox in laser-based experiments. Practical techniques covered include
continuous-wave parametric amplifier and optical fibre quantum soliton
experiments. We discuss current proposals for extending EPR experiments to
massive-particle systems, including spin-squeezing, atomic position entangle-
ment, and quadrature entanglement in ultra-cold atoms. Finally, we examine
applications of this technology to quantum key distribution, quantum
teleportation and entanglement-swapping.Comment: Colloquium in press in Reviews of Modern Physics, accepted Dec 200
Dynamic image potential at an Al(111) surface
We evaluate the electronic self-energy Sigma(E) at an Al(111) surface using the GW space-time method. This self-energy automatically includes the image potential V-im not present in any local-density approximation for exchange and correlation. We solve the energy-dependent quasiparticle equations and calculate the effective local potential experienced by electrons in the near-surface region. The relative contribution of exchange proves to be very different for states above the Fermi level. The image-plane position for interacting electrons is closer to the surface than for the purely electrostatic effects felt by test charges, and, like its classical counterpart, is drawn inwards by the effects of atomic structure
Solution of the relativistic Dirac-Hulthen problem
The one-particle three-dimensional Dirac equation with spherical symmetry is
solved for the Hulthen potential. The s-wave relativistic energy spectrum and
two-component spinor wavefunctions are obtained analytically. Conforming to the
standard feature of the relativistic problem, the solution space splits into
two distinct subspaces depending on the sign of a fundamental parameter in the
problem. Unique and interesting properties of the energy spectrum are pointed
out and illustrated graphically for several values of the physical parameters.
The square integrable two-component wavefunctions are written in terms of the
Jacobi polynomials. The nonrelativistic limit reproduces the well-known
nonrelativistic energy spectrum and results in Schrodinger equation with a
"generalized" three-parameter Hulthen potential, which is the sum of the
original Hulthen potential and its square.Comment: 13 pages, 3 color figure
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