4,578 research outputs found
Two-Level Systems in Evaporated Amorphous Silicon
In -beam evaporated amorphous silicon (-Si), the densities of two-level
systems (TLS), and , determined from specific heat
and internal friction measurements, respectively, have been shown to
vary by over three orders of magnitude. Here we show that and
are proportional to each other with a constant of
proportionality that is consistent with the measurement time dependence
proposed by Black and Halperin and does not require the introduction of
additional anomalous TLS. However, and depend strongly
on the atomic density of the film () which depends on both film
thickness and growth temperature suggesting that the -Si structure is
heterogeneous with nanovoids or other lower density regions forming in a dense
amorphous network. A review of literature data shows that this atomic density
dependence is not unique to -Si. These findings suggest that TLS are not
intrinsic to an amorphous network but require a heterogeneous structure to
form
Flux-ratio anomalies from discs and other baryonic structures in the Illustris simulation
The flux ratios in the multiple images of gravitationally lensed quasars can
provide evidence for dark matter substructure in the halo of the lensing galaxy
if the flux ratios differ from those predicted by a smooth model of the lensing
galaxy mass distribution. However, it is also possible that baryonic structures
in the lensing galaxy, such as edge-on discs, can produce flux-ratio anomalies.
In this work, we present the first statistical analysis of flux-ratio anomalies
due to baryons from a numerical simulation perspective. We select galaxies with
various morphological types in the Illustris simulation and ray-trace through
the simulated halos, which include baryons in the main lensing galaxies but
exclude any substructures, in order to explore the pure baryonic effects. Our
ray-tracing results show that the baryonic components can be a major
contribution to the flux-ratio anomalies in lensed quasars and that edge-on
disc lenses induce the strongest anomalies. We find that the baryonic
components increase the probability of finding high flux-ratio anomalies in the
early-type lenses by about 8% and by about 10 - 20% in the disc lenses. The
baryonic effects also induce astrometric anomalies in 13% of the mock lenses.
Our results indicate that the morphology of the lens galaxy becomes important
in the analysis of flux-ratio anomalies when considering the effect of baryons,
and that the presence of baryons may also partially explain the discrepancy
between the observed (high) anomaly frequency and what is expected due to the
presence of subhalos as predicted by the CDM simulations.Comment: 16 pages, 11 figures, accepted by MNRA
Imaging the Cosmic Matter Distribution using Gravitational Lensing of Pregalactic HI
21-cm emission from neutral hydrogen during and before the epoch of cosmic
reionisation is gravitationally lensed by material at all lower redshifts.
Low-frequency radio observations of this emission can be used to reconstruct
the projected mass distribution of foreground material, both light and dark. We
compare the potential imaging capabilities of such 21-cm lensing with those of
future galaxy lensing surveys. We use the Millennium Simulation to simulate
large-area maps of the lensing convergence with the noise, resolution and
redshift-weighting achievable with a variety of idealised observation
programmes. We find that the signal-to-noise of 21-cm lens maps can far exceed
that of any map made using galaxy lensing. If the irreducible noise limit can
be reached with a sufficiently large radio telescope, the projected convergence
map provides a high-fidelity image of the true matter distribution, allowing
the dark matter halos of individual galaxies to be viewed directly, and giving
a wealth of statistical and morphological information about the relative
distributions of mass and light. For instrumental designs like that planned for
the Square Kilometer Array (SKA), high-fidelity mass imaging may be possible
near the resolution limit of the core array of the telescope.Comment: version accepted for publication in MNRAS (reduced-resolution
figures
Cosmological Information in the Gravitational Lensing of Pregalactic HI
We study the constraints which the next generation of radio telescopes could
place on the nature of dark energy, dark matter and inflation by studying the
gravitational lensing of high redshift 21 cm emission, and we compare with the
constraints obtainable from wide-angle surveys of galaxy lensing. If the
reionization epoch is effectively at z ~ 8 or later, very large amounts of
cosmological information will be accessible to telescopes like SKA and LOFAR.
We use simple characterizations of reionization history and of proposed
telescope designs to investigate how well the two-dimensional convergence power
spectrum, the three-dimensional matter power spectrum, the evolution of the
linear growth function, and the standard cosmological parameters can be
measured from radio data. The power spectra can be measured accurately over a
wide range of wavenumbers at z ~ 2, and the evolution in the cosmic energy
density can be probed from z ~ 0.5 to z ~ 7. This results in a characterization
of the shape of the power spectra (i.e. of the nature of dark matter and of
inflationary structure generation) which is potentially more precise than that
obtained from galaxy lensing surveys. On the other hand, the dark energy
parameters in their conventional parametrization (Omega_Lambda, w_o, w_a) are
somewhat less well constrained by feasible 21 cm lensing surveys than by an
all-sky galaxy lensing survey although a 21 cm surveys might be more powerful
than galaxy surveys for constraining models with "early" dark energy. Overall,
the best constraints come from combining surveys of the two types. This results
in extremely tight constraints on dark matter and inflation, and improves
constraints on dark energy, as judged by the standard figure of merit, by more
than an order of magnitude over either survey alone.Comment: submitted to MNRAS, 12 pages, error in computer code corrected which
changed constraints on some cosmological parameters, change to lensing
estimator to improve performanc
The PCA Lens-Finder: application to CFHTLS
We present the results of a new search for galaxy-scale strong lensing
systems in CFHTLS Wide. Our lens-finding technique involves a preselection of
potential lens galaxies, applying simple cuts in size and magnitude. We then
perform a Principal Component Analysis of the galaxy images, ensuring a clean
removal of the light profile. Lensed features are searched for in the residual
images using the clustering topometric algorithm DBSCAN. We find 1098 lens
candidates that we inspect visually, leading to a cleaned sample of 109 new
lens candidates. Using realistic image simulations we estimate the completeness
of our sample and show that it is independent of source surface brightness,
Einstein ring size (image separation) or lens redshift. We compare the
properties of our sample to previous lens searches in CFHTLS. Including the
present search, the total number of lenses found in CFHTLS amounts to 678,
which corresponds to ~4 lenses per square degree down to i=24.8. This is
equivalent to ~ 60.000 lenses in total in a survey as wide as Euclid, but at
the CFHTLS resolution and depth.Comment: 21 pages, 12 figures, accepted for publication on A&
Laser Phase and Frequency Stabilization Using Atomic Coherence
We present a novel and simple method of stabilizing the laser phase and
frequency by polarization spectroscopy of an atomic vapor. In analogy to the
Pound-Drever-Hall method, which uses a cavity as a memory of the laser phase,
this method uses atomic coherence (dipole oscillations) as a phase memory of
the transmitting laser field. A preliminary experiment using a distributed
feedback laser diode and a rubidium vapor cell demonstrates a
shot-noise-limited laser linewidth reduction (from 2 MHz to 20 kHz). This
method would improve the performance of gas-cell-based optical atomic clocks
and magnetometers and facilitate laser-cooling experiments using narrow
transitions.Comment: 7 pages, 6 figures, appendix on the derivation of Eq.(3) (transfer
function for a polarization-spectroscopy-based frequency discriminator) has
been adde
Resonant enhancement of ultracold photoassociation rate by electric field induced anisotropic interaction
We study the effects of a static electric field on the photoassociation of a
heteronuclear atom-pair into a polar molecule. The interaction of permanent
dipole moment with a static electric field largely affects the ground state
continuum wave function of the atom-pair at short separations where
photoassociation transitions occur according to Franck-Condon principle.
Electric field induced anisotropic interaction between two heteronuclear ground
state atoms leads to scattering resonances at some specific electric fields.
Near such resonances the amplitude of scattering wave function at short
separation increases by several orders of magnitude. As a result,
photoaasociation rate is enhanced by several orders of magnitude near the
resonances. We discuss in detail electric field modified atom-atom scattering
properties and resonances. We calculate photoassociation rate that shows giant
enhancement due to electric field tunable anisotropic resonances. We present
selected results among which particularly important are the excitations of
higher rotational levels in ultracold photoassociation due to electric field
tunable resonances.Comment: 14 pages,9 figure
Cavity Assisted Nondestructive Laser Cooling of Atomic Qubits
We analyze two configurations for laser cooling of neutral atoms whose
internal states store qubits. The atoms are trapped in an optical lattice which
is placed inside a cavity. We show that the coupling of the atoms to the damped
cavity mode can provide a mechanism which leads to cooling of the motion
without destroying the quantum information.Comment: 12 page
Collisional Control of Ground State Polar Molecules and Universal Dipolar Scattering
We explore the impact of the short range interaction on the scattering of
ground state polar molecules, and study the transition from a weak to strong
dipolar scattering over an experimentally reasonable range of energies and
electric field values. In the strong dipolar limit, the scattering scales with
respect to a dimensionless quantity defined by mass, induced dipole moment, and
collision energy. The scaling has implications for all quantum mechanical
dipolar scattering, and therefore this universal dipolar scaling provides
estimates of scattering cross sections for any dipolar system.Comment: 4 pages, 2 figure
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