19,882 research outputs found
Supersymmetric features of the Maxwell fish-eye lens
We provide a supersymmetric analysis of the Maxwell fisheye (MF) wave problem
at zero energy. Working in the so-called sector, we obtain the
corresponding superpartner (fermionic) MF effective potential within Witten's
one-dimensional (radial) supersymmetric procedure.Comment: 5 pages in Latex, one figure not included has been published as Fig.
2 in Phys. Lett. A 208 (1995) 33-3
Effects of Bulk Viscosity on Cosmological Evolution
The effect of bulk viscisity on the evolution of the homogeneous and
isotropic cosmological models is considered. Solutions are found, with a
barotropic equation of state, and a viscosity coefficient that is proportional
to a power of the energy density of the universe. For flat space, power law
expansions, related to extended inflation are found as well as exponential
solutions, related to old inflation; also a solution with expansion that is an
exponential of an exponential of the time is found.Comment: 8 pages, latex, no figure
Detection of motional ground state population of a trapped ion using delayed pulses
Efficient preparation and detection of the motional state of trapped ions is
important in many experiments ranging from quantum computation to precision
spectroscopy. We investigate the stimulated Raman adiabatic passage (STIRAP)
technique for the manipulation of motional states in a trapped ion system. The
presented technique uses a Raman coupling between two hyperfine ground states
in Mg, implemented with delayed pulses, which removes a single
phonon independent of the initial motional state. We show that for a thermal
state the STIRAP population transfer is more efficient than a stimulated Raman
Rabi pulse on a motional sideband. In contrast to previous implementations, a
large detuning of more than 200 times the natural linewidth of the transition
is used. This approach renders STIRAP suitable for atoms in which resonant
laser fields would populate fluorescing excited states and thus impede the
STIRAP process. We use the technique to measure the wavefunction overlap of
excited motional states with the motional ground state. This is an important
application for photon recoil spectroscopy and other force sensing applications
that utilize the high sensitivity of the motional state of trapped ions to
external fields. Furthermore, a determination of the ground state population
enables a simple measurement of the ion's temperature.Comment: 17 pages, 7 figure
Environmental dependence of AGN activity in the supercluster A901/2
We present XMM data for the supercluster A901/2, at z ~ 0.17, which is
combined with deep imaging and 17-band photometric redshifts (from the COMBO-17
survey), 2dF spectra and Spitzer 24um data, to identify AGN in the
supercluster. The 90ksec XMM image contains 139 point sources, of which 11 are
identified as supercluster AGN with L_X(0.5-7.5keV) > 1.7x10^41 erg/cm2/s. The
host galaxies have M_R < -20 and only 2 of 8 sources with spectra could have
been identified as AGN by the detected optical emission lines. Using a large
sample of 795 supercluster galaxies we define control samples of massive
galaxies with no detected AGN. The local environments of the AGN and control
samples differ at >98 per cent significance. The AGN host galaxies lie
predominantly in areas of moderate projected galaxy density and with more local
blue galaxies than the control sample, with the exception of one very bright
Type I AGN very near the centre of a cluster. These environments are similar
to, but not limited to, cluster outskirts and blue groups. Despite the large
number of potential host galaxies, no AGN are found in regions with the highest
galaxy density (excluding some cluster cores where emission from the ICM
obscures moderate luminosity AGN). AGN are also absent from the areas with
lowest galaxy density. We conclude that the prevalence of cluster AGN is linked
to their environment.Comment: 20 pages, 15 figures. MNRAS accepted. Version with full resolution
figures, including Figure 14, is available at
http://www.sc.eso.org/~rgilmour
Precision isotope shift measurements in Ca using highly sensitive detection schemes
We demonstrate an efficient high-precision optical spectroscopy technique for
single trapped ions with non-closed transitions. In a double-shelving
technique, the absorption of a single photon is first amplified to several
phonons of a normal motional mode shared with a co-trapped cooling ion of a
different species, before being further amplified to thousands of fluorescence
photons emitted by the cooling ion using the standard electron shelving
technique. We employ this extension of the photon recoil spectroscopy technique
to perform the first high precision absolute frequency measurement of the
D P transition in Ca,
resulting in a transition frequency of kHz.
Furthermore, we determine the isotope shift of this transition and the
S P transition for Ca,
Ca and Ca ions relative to Ca with an
accuracy below 100 kHz. Improved field and mass shift constants of these
transitions as well as changes in mean square nuclear charge radii are
extracted from this high resolution data
Cryogenic micro-calorimeters for mass spectrometric identification of neutral molecules and molecular fragments
We have systematically investigated the energy resolution of a magnetic
micro-calorimeter (MMC) for atomic and molecular projectiles at impact energies
ranging from to 150 keV. For atoms we obtained absolute energy
resolutions down to eV and relative energy resolutions
down to . We also studied in detail the MMC
energy-response function to molecular projectiles of up to mass 56 u. We have
demonstrated the capability of identifying neutral fragmentation products of
these molecules by calorimetric mass spectrometry. We have modeled the MMC
energy-response function for molecular projectiles and conclude that
backscattering is the dominant source of the energy spread at the impact
energies investigated. We have successfully demonstrated the use of a detector
absorber coating to suppress such spreads. We briefly outline the use of MMC
detectors in experiments on gas-phase collision reactions with neutral
products. Our findings are of general interest for mass spectrometric
techniques, particularly for those desiring to make neutral-particle mass
measurements
Modelling of epitaxial film growth with a Ehrlich-Schwoebel barrier dependent on the step height
The formation of mounded surfaces in epitaxial growth is attributed to the
presence of barriers against interlayer diffusion in the terrace edges, known
as Ehrlich-Schwoebel (ES) barriers. We investigate a model for epitaxial growth
using a ES barrier explicitly dependent on the step height. Our model has an
intrinsic topological step barrier even in the absence of an explicit ES
barrier. We show that mounded morphologies can be obtained even for a small
barrier while a self-affine growth, consistent with the Villain-Lai-Das Sarma
equation, is observed in absence of an explicit step barrier. The mounded
surfaces are described by a super-roughness dynamical scaling characterized by
locally smooth (faceted) surfaces and a global roughness exponent .
The thin film limit is featured by surfaces with self-assembled
three-dimensional structures having an aspect ratio (height/width) that may
increase or decrease with temperature depending on the strength of step
barrier.Comment: To appear in J. Phys. Cond. Matter; 3 movies as supplementary
materia
Cosmological Implications of the Uncertainty in H– Destruction Rate Coefficients
In primordial gas, molecular hydrogen forms primarily through associative detachment of H- and H, thereby destroying the H-. The H- anion can also be destroyed by a number of other reactions, most notably by mutual neutralization with protons. However, neither the associative detachment nor the mutual neutralization rate coefficients are well determined: both may be uncertain by as much as an order of magnitude. This introduces a corresponding uncertainty into the H2 formation rate, which may have cosmological implications. Here we examine the effect that these uncertainties have on the formation of H2 and the cooling of protogalactic gas in a variety of situations. We show that the effect is particularly large for protogalaxies forming in previously ionized regions, affecting our predictions of whether or not a given protogalaxy can cool and condense within a Hubble time, and altering the strength of the ultraviolet background that is required to prevent collapse
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