994 research outputs found
Ultra-heavy cosmic rays: Theoretical implications of recent observations
Extreme ultraheavy cosmic ray observations (Z greater or equal 70) are compared with r-process models. A detailed cosmic ray propagation calculation is used to transform the calculated source distributions to those observed at the earth. The r-process production abundances are calculated using different mass formulae and beta-rate formulae; an empirical estimate based on the observed solar system abundances is used also. There is the continued strong indication of an r-process dominance in the extreme ultra-heavy cosmic rays. However it is shown that the observed high actinide/Pt ratio in the cosmic rays cannot be fit with the same r-process calculation which also fits the solar system material. This result suggests that the cosmic rays probably undergo some preferential acceleration in addition to the apparent general enrichment in heavy (r-process) material. As estimate also is made of the expected relative abundance of superheavy elements in the cosmic rays if the anomalous heavy xenon in carbonaceous chondrites is due to a fissioning superheavy element
Decoherence induced by Smith-Purcell radiation
The interaction between charged particles and the vacuum fluctuations of the
electromagnetic field induces decoherence, and therefore affects the contrast
of fringes in an interference experiment. In this article we show that if a
double slit experiment is performed near a conducting grating, the fringe
visibility is reduced. We find that the reduction of contrast is proportional
to the number of grooves in the conducting surface, and that for realistic
values of the parameters it could be large enough to be observed. The effect
can be understood in terms of the Smith-Purcell radiation produced by the
surface currents induced in the conductor.Comment: 10 pages, 3 figures. Improved discussion on experimental
perspectives. References added. Version to appear in Phys. Rev.
High-Contrast Interference in a Thermal Cloud of Atoms
The coherence properties of a gas of bosonic atoms above the BEC transition
temperature were studied. Bragg diffraction was used to create two spatially
separated wave packets, which interfere during expansion. Given sufficient
expansion time, high fringe contrast could be observed in a cloud of arbitrary
temperature. Fringe visibility greater than 90% was observed, which decreased
with increasing temperature, in agreement with a simple model. When the sample
was "filtered" in momentum space using long, velocity-selective Bragg pulses,
the contrast was significantly enhanced in contrast to predictions
Vortex mass in a superfluid at low frequencies
An inertial mass of a vortex can be calculated by driving it round in a
circle with a steadily revolving pinning potential. We show that in the low
frequency limit this gives precisely the same formula that was used by Baym and
Chandler, but find that the result is not unique and depends on the force field
used to cause the acceleration. We apply this method to the Gross-Pitaevskii
model, and derive a simple formula for the vortex mass. We study both the long
range and short range properties of the solution. We agree with earlier results
that the non-zero compressibility leads to a divergent mass. From the
short-range behavior of the solution we find that the mass is sensitive to the
form of the pinning potential, and diverges logarithmically when the radius of
this potential tends to zero.Comment: 4 page
Finite temperature coherence of the ideal Bose gas in an optical lattice
In current experiments with cold quantum gases in periodic potentials,
interference fringe contrast is typically the easiest signal in which to look
for effects of non-trivial many-body dynamics. In order better to calibrate
such measurements, we analyse the background effect of thermal decoherence as
it occurs in the absence of dynamical interparticle interactions. We study the
effect of optical lattice potentials, as experimentally applied, on the
condensed fraction of a non-interacting Bose gas in local thermal equilibrium
at finite temperatures. We show that the experimentally observed decrease of
the condensate fraction in the presence of the lattice can be attributed, up to
a threshold lattice height, purely to ideal gas thermodynamics; conversely we
confirm that sharper decreases in first-order coherence observed in stronger
lattices are indeed attributable to many-body physics. Our results also suggest
that the fringe visibility 'kinks' observed in F.Gerbier et al., Phys. Rev.
Lett. 95, 050404 (2005) may be explained in terms of the competition between
increasing lattice strength and increasing mean gas density, as the gaussian
profile of the red-detuned lattice lasers also increases the effective strength
of the harmonic trap
Confronting the trans-Planckian question of inflationary cosmology with dissipative effects
We provide a class of QFTs which exhibit dissipation above a threshold
energy, thereby breaking Lorentz invariance. Unitarity is preserved by coupling
the fields to additional degrees of freedom (heavy fields) which introduce the
rest frame. Using the Equivalence Principle, we define these theories in
arbitrary curved spacetime. We then confront the trans-Planckian question of
inflationary cosmology. When dissipation increases with the energy, the quantum
field describing adiabatic perturbations is completely damped at the onset of
inflation. However it still exists as a composite operator made with the
additional fields. And when these are in their ground state, the standard power
spectrum obtains if the threshold energy is much larger that the Hubble
parameter. In fact, as the energy redshifts below the threshold, the composite
operator behaves as if it were a free field endowed with standard vacuum
fluctuations. The relationship between our models and the Brane World scenarios
studied by Libanov and Rubakov displaying similar effects is discussed. The
signatures of dissipation will be studied in a forthcoming paper.Comment: 30 pages, 1 Figure, to appear in CQ
Out-of-equilibrium quantum fields with conserved charge
We study the out-of-equilibrium evolution of an O(2)-invariant scalar field
in which a conserved charge is stored. We apply a loop expansion of the
2-particle irreducible effective action to 3-loop order. Equations of motion
are derived which conserve both total charge and total energy yet allow for the
effects of scattering whereby charge and energy can transfer between modes.
Working in (1+1)-dimensions we solve the equations of motion numerically for a
system knocked out of equilibrium by a sudden temperature quench. We examine
the initial stages of the charge and energy redistribution. This provides a
basis from which we can understand the formation of Bose-Einstein condensates
from first principles.Comment: 11 pages, 5 figures, replacement with improved presentatio
Winding up by a quench: vortices in the wake of rapid Bose-Einstein condensation
A second order phase transition induced by a rapid quench can lock out
topological defects with densities far exceeding their equilibrium expectation
values. We use quantum kinetic theory to show that this mechanism, originally
postulated in the cosmological context, and analysed so far only on the mean
field classical level, should allow spontaneous generation of vortex lines in
trapped Bose-Einstein condensates of simple topology, or of winding number in
toroidal condensates.Comment: 4 pages, 2 figures; misprint correcte
Theory of 'which path' dephasing in single electron interference due to trace in conductive environment
A single-electron two-path interference (Young) experiment is considered
theoretically. The decoherence of an electron wave packet due to the 'which
path' trace left in the conducting (metallic) plate placed under the electron
trajectories is calculated using the many-body quantum description of the
electron gas reservoir.Comment: 11 pages, 5 figures, moderate changes, 1 new figure, updated
reference
Atom correlations and spin squeezing near the Heisenberg limit: finite system size effect and decoherence
We analyze a model for spin squeezing based on the so-called counter-twisting
Hamiltonian, including the effects of dissipation and finite system size. We
discuss the conditions under which the Heisenberg limit, i.e. phase sensitivity
, can be achieved. A specific implementation of this model based
on atom-atom interactions via quantized photon exchange is presented in detail.
The resulting excitation corresponds to the creation of spin-flipped atomic
pairs and can be used for fast generation of entangled atomic ensembles, spin
squeezing and apllications in quantum information processing. The conditions
for achieving strong spin squeezing with this mechanism are also analyzed.Comment: 15 pages, 8 figure
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