22,599 research outputs found
Stabilizing nuclear spins around semiconductor electrons via the interplay of optical coherent population trapping and dynamic nuclear polarization
We experimentally demonstrate how coherent population trapping (CPT) for
donor-bound electron spins in GaAs results in autonomous feedback that prepares
stabilized states for the spin polarization of nuclei around the electrons. CPT
was realized by excitation with two lasers to a bound-exciton state.
Transmission studies of the spectral CPT feature on an ensemble of electrons
directly reveal the statistical distribution of prepared nuclear spin states.
Tuning the laser driving from blue to red detuned drives a transition from one
to two stable states. Our results have importance for ongoing research on
schemes for dynamic nuclear spin polarization, the central spin problem and
control of spin coherence.Comment: 5 pages, 4 figure
Split-gate quantum point contacts with tunable channel length
We report on developing split-gate quantum point contacts (QPCs) that have a
tunable length for the transport channel. The QPCs were realized in a
GaAs/AlGaAs heterostructure with a two- dimensional electron gas (2DEG) below
its surface. The conventional design uses 2 gate fingers on the wafer surface
which deplete the 2DEG underneath when a negative gate voltage is applied, and
this allows for tuning the width of the QPC channel. Our design has 6 gate
fingers and this provides additional control over the form of the electrostatic
potential that defines the channel. Our study is based on electrostatic
simulations and experiments and the results show that we developed QPCs where
the effective channel length can be tuned from about 200 nm to 600 nm.
Length-tunable QPCs are important for studies of electron many-body effects
because these phenomena show a nanoscale dependence on the dimensions of the
QPC channel
A Look At Three Different Scenarios for Bulge Formation
In this paper, we present three qualitatively different scenarios for bulge
formation: a secular evolution model in which bulges form after disks and
undergo several central starbursts, a primordial collapse model in which bulges
and disks form simultaneously, and an early bulge formation model in which
bulges form prior to disks. We normalize our models to the local z=0
observations of de Jong & van der Kruit (1994) and Peletier & Balcells (1996)
and make comparisons with high redshift observations. We consider model
predictions relating directly to bulge-to-disk properties. As expected, smaller
bulge-to-disk ratios and bluer bulge colors are predicted by the secular
evolution model at all redshifts, although uncertainties in the data are
currently too large to differentiate strongly between the models.Comment: 19 pages, 6 figures, accepted for publication in the Astrophysical
Journa
De (biologische) boer als landschapsondernemer
Doel van het project was om inzicht te verkrijgen in beelden en verwachtingen van consumenten ten aanzien van hoe biologische ondernemers omgaan met natuur, landschap en biodiversiteit op hun bedrijf. Deze inzichten kunnen door ondernemers én door beleidsmakers gebruikt worden om activiteiten voor onderhoud en behoud van natuur en landschap te stimuleren. Uit het onderzoek blijkt dat er intuïtief gezien volgens de respondenten een groot verschil is tussen hoe biologische en reguliere ondernemers omgaan met hun boerderij en directe omgeving, hoewel dat rationeel gesproken niet zo zou moeten zijn. Als je gekozen hebt voor het boerenbestaan krijg je daar namelijk de verantwoordelijkheid voor natuur en dieren automatisch bij. Anders “had je maar op kantoor moeten gaan werken”. De kern van het verschil zit in focus op productie versus focus op duurzame balans. Daar waar bij de reguliere ondernemer alles in het teken staat van economische productie laat de biologische ondernemer meerdere belangen meewegen. Dat betekent dat hij socialer en respectvoller is in de omgang met mensen, dieren, natuur en omgeving. Hij is meer begaan met milieu en klimaat en heeft meer aandacht voor de flora en fauna in zijn omgeving. Dat vertaalt zich in meer begroeiing en meer afwisseling in het landschap. De reguliere ondernemer is daarentegen gericht op efficiency en minimaliseren van onderhoud; daarom is alles in zijn omgeving kaal en monotoon en niet altijd goed onderhoude
Structure of Disk Dominated Galaxies I. Bulge/Disk Parameters, Simulations, and Secular Evolution
(Abridged) A robust analysis of galaxy structural parameters, based on the
modeling of bulge and disk brightnesses in the BVRH bandpasses, is presented
for 121 face-on and moderately inclined late-type spirals. Each surface
brightness (SB) profile is decomposed into a sum of a generalized Sersic bulge
and an exponential disk. The reliability and limitations of our bulge-to-disk
(B/D) decompositions are tested with extensive simulations of galaxy brightness
profiles (1D) and images (2D). Galaxy types are divided into 3 classes
according to their SB profile shapes; Freeman Type-I and Type-II, and a third
``Transition'' class for galaxies whose profiles change from Type-II in the
optical to Type-I in the infrared. We discuss possible interpretations of
Freeman Type-II profiles. The Sersic bulge shape parameter for nearby Type-I
late-type spirals shows a range between n=0.1-2 but, on average, the underlying
surface density profile for the bulge and disk of these galaxies is adequately
described by a double-exponential distribution. We confirm a coupling between
the bulge and disk with a scale length ratio r_e/h=0.22+/-0.09, or
h_bulge/h_disk=0.13+/-0.06 for late-type spirals, in agreement with recent
N-body simulations of disk formation and models of secular evolution. This
ratio increases from ~0.20 for late-type spirals to ~0.24 for earlier types.
The similar scaling relations for early and late-type spirals suggest
comparable formation and/or evolution scenarios for disk galaxies of all Hubble
types.Comment: 78 pages with 23 embedded color figures + tables of galaxy structural
parameters. Accepted for publication in the Astrophysical Journal. The
interested reader is strongly encouraged to ignore some of the low res
figures within; instead, download the high resolution version from
http://www.astro.ubc.ca/people/courteau/public/macarthur02_disks.ps.g
Noncompact gaugings, chiral reduction and dual sigma models in supergravity
We show that the half-maximal SU(2) gauged supergravity with topological mass
term admits coupling of an arbitrary number of n vector multiplets. The chiral
circle reduction of the ungauged theory in the dual 2-form formulation gives
N=(1,0) supergravity in 6D coupled to 3p scalars that parametrize the coset
SO(p,3)/SO(p)x SO(3), a dilaton and (p+3) axions with p < n+1. Demanding that
R-symmetry gauging survives in 6D is shown to put severe restrictions on the 7D
model, in particular requiring noncompact gaugings. We find that the SO(2,2)
and SO(3,1) gauged 7D supergravities give a U(1)_R, and the SO(2,1) gauged 7D
supergravity gives an Sp(1)_R gauged chiral 6D supergravities coupled to
certain matter multiplets. In the 6D models obtained, with or without gauging,
we show that the scalar fields of the matter sector parametrize the coset
SO(p+1,4)/SO(p+1)x SO(4), with the (p+3) axions corresponding to its abelian
isometries. In the ungauged 6D models, upon dualizing the axions to 4-form
potentials, we obtain coupling of p linear multiplets and one special linear
multiplet to chiral 6D supergravity.Comment: 41 pages, late
Dust Attenuation in Late-Type Galaxies. I. Effects on Bulge and Disk Components
We present results of new Monte Carlo calculations made with the DIRTY code
of radiative transfer of stellar and scattered radiation for a dusty giant
late-type galaxy like the Milky Way, which illustrate the effect of the
attenuation of stellar light by internal dust on the integrated photometry of
the individual bulge and disk components. Here we focus on the behavior of the
attenuation function, the color excess, and the fraction of light scattered or
directly transmitted towards the outside observer as a function of the total
amount of dust and the inclination of the galaxy, and the structure of the
dusty interstellar medium (ISM) of the disk. We confirm that dust attenuation
produces qualitatively and quantitatively different effects on the integrated
photometry of bulge and disk, whatever the wavelength. In addition, we find
that the structure of the dusty ISM affects more sensitively the observed
magnitudes than the observed colors of both bulge and disk. Finally, we show
that the contribution of the scattered radiation to the total monochromatic
light received by the outside observer is significant, particularly at UV
wavelengths, even for a two-phase, clumpy, dusty ISM. Thus understanding dust
scattering properties is fundamental for the interpretation of extragalactic
observations in the rest-frame UV.Comment: 62 pages, 28 eps-figures, 1 table, accepted for publication in ApJ
Main Journa
Simulations of Dust in Interacting Galaxies I: Dust Attenuation
A new Monte-Carlo radiative-transfer code, Sunrise, is used in conjunction
with hydrodynamic simulations of major galaxy mergers to calculate the effects
of dust in such systems. The simulations are in good agreement with
observations of dust absorption in starburst galaxies, and the dust has a
profound effect on their appearance. The dust attenuation increases with
luminosity such that at peak luminosities ~90% of the bolometric luminosity is
absorbed by dust. In general, the detailed appearance of the merging event
depends on the stage of the merger and the geometry of the encounter. The
fraction of bolometric energy absorbed by the dust, however, is a robust
quantity that can be predicted from the intrinsic properties bolometric
luminosity, baryonic mass, star-formation rate, and metallicity of the system.
This paper presents fitting formulae, valid over a wide range of masses and
metallicities, from which the absorbed fraction of luminosity (and consequently
also the infrared dust luminosity) can be predicted. The attenuation of the
luminosity at specific wavelengths can also be predicted, albeit with a larger
scatter due to the variation with viewing angle. These formulae for dust
attenuation appear to be valid for both isolated and interacting galaxies, are
consistent with earlier studies, and would be suitable for inclusion in
theoretical models, e.g. semi-analytic models of galaxy formation.Comment: 12 pages, 10 figures, submitted to Ap
On Star Formation and the Non-Existence of Dark Galaxies
We investigate whether a baryonic dark galaxy or `galaxy without stars' could
persist indefinitely in the local universe, while remaining stable against star
formation. To this end, a simple model has been constructed to determine the
equilibrium distribution and composition of a gaseous protogalactic disk.
Specifically, we determine the amount of gas that will transit to a Toomre
unstable cold phase via the H2 cooling channel in the presence of a UV--X-ray
cosmic background radiation field.
All but one of the models are predicted to become unstable to star formation.
Moreover, we find that all our model objects would be detectable via HI line
emission, even in the case that star formation is potentially avoided. These
results are consistent with the non-detection of isolated extragalactic HI
clouds with no optical counterpart (galaxies without stars) by HIPASS.
Additionally, where star formation is predicted to occur, we determine the
minimum interstellar radiation field required to restore gravothermal
stability, which we then relate to a minimum global star formation rate. This
leads to the prediction of a previously undocumented relation between HI mass
and star formation rate that is observed for a wide variety of dwarf galaxies
in the HI mass range 10^8--10^10 M_sun. The existence of such a relation
strongly supports the notion that the well observed population of dwarf
galaxies represent the minimum rates of self-regulating star formation in the
universe. (Barely abridged)Comment: 19 pages, 8 figures, TeX using emulateapj.cls, v2 accepted for
publication in ApJ (16/8/5) with one figure deleted and a number of minor
clarifying revision
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