5,711 research outputs found
On the Relative Sensitivity of Mass-sensitive Chemical Microsensors
In this work, the chemical sensitivity of mass-sensitive chemical microsensors with a uniform layer sandwich structure vibrating in their lateral or in-plane flexural modes is investigated. It is experimentally verified that the relative chemical sensitivity of such resonant microsensors is -to a first order- independent of the microstructure\u27s in-plane dimensions and the flexural eigenmode used, and only depends on the layer thicknesses and densities as well as the sorption properties of the sensing film. Important implications for the design of mass-sensitive chemical microsensors are discussed, whereby the designer can focus on the layer stack to optimize the chemical sensitivity and on the in-plane dimensions and mode shape to optimize the resonator\u27s frequency stability
The decay and collisions of dark solitons in superfluid Fermi gases
We study soliton collisions and the decay of solitons into sound in
superfluid Fermi gases across the Bose-Einstein condensate to
Bardeen-Cooper-Schrieffer (BEC-BCS) crossover by performing numerical
simulations of the time-dependent Bogoliubov-de Gennes equations. This decay
process occurs when the solitons are accelerated to the bulk pair-breaking
speed by an external potential. A similar decay process may occur when solitons
are accelerated by an inelastic collision with another soliton. We find that
soliton collisions become increasingly inelastic as we move from the BEC to BCS
regimes, and the excess energy is converted into sound. We interpret this
effect as being due to evolution of Andreev bound states localized within the
soliton.Comment: 9 pages, 5 figure
On the photodissociation of H2 by the first stars
The first star formation in the universe is expected to take place within
small protogalaxies, in which the gas is cooled by molecular hydrogen. However,
if massive stars form within these protogalaxies, they may suppress further
star formation by photodissociating the H2. We examine the importance of this
effect by estimating the timescale on which significant H2 is destroyed. We
show that photodissociation is significant in the least massive protogalaxies,
but becomes less so as the protogalactic mass increases. We also examine the
effects of photodissociation on dense clumps of gas within the protogalaxy. We
find that while collapse will be inhibited in low density clumps, denser ones
may survive to form stars.Comment: 13 pages, 10 figures. Minor revisions to match version accepted by
MNRA
Discriminating cool-water from warm-water carbonates and their diagenetic environments using element geochemistry: the Oligocene Tikorangi Formation (Taranaki Basin) and the dolomite effect
Fields portrayed within bivariate element plots have been used to distinguish between carbonates formed in warm- (tropical) water and cool- (temperate) water depositional settings. Here, element concentrations (Ca, Mg, Sr, Na, Fe, and Mn) have been determined for the carbonate fraction of bulk samples from the late Oligocene Tikorangi Formation, a subsurface, mixed dolomite-calcite, cool-water limestone sequence in Taranaki Basin, New Zealand. While the occurrence of dolomite is rare in New Zealand Cenozoic carbonates, and in cool-water carbonates more generally, the dolomite in the Tikorangi carbonates is shown to have a dramatic effect on the "traditional" positioning of cool-water limestone fields within bivariate element plots. Rare undolomitised, wholly calcitic carbonate samples in the Tikorangi Formation have the following average composition: Mg 2800 ppm; Ca 319 100 ppm; Na 800 ppm; Fe 6300 ppm; Sr 2400 ppm; and Mn 300 ppm. Tikorangi Formation dolomite-rich samples (>15% dolomite) have average values of: Mg 53 400 ppm; Ca 290 400 ppm; Na 4700 ppm; Fe 28 100 ppm; Sr 5400 ppm; and Mn 500 ppm. Element-element plots for dolomite-bearing samples show elevated Mg, Na, and Sr values compared with most other low-Mg calcite New Zealand Cenozoic limestones. The increased trace element contents are directly attributable to the trace element-enriched nature of the burial-derived dolomites, termed here the "dolomite effect". Fe levels in the Tikorangi Formation carbonates far exceed both modern and ancient cool-water and warm-water analogues, while Sr values are also higher than those in modern Tasmanian cool-water carbonates, and approach modern Bahaman warm-water carbonate values. Trace element data used in conjunction with more traditional petrographic data have aided in the diagenetic interpretation of the carbonate-dominated Tikorangi sequence. The geochemical results have been particularly useful for providing more definitive evidence for deep burial dolomitisation of the deposits under the influence of marine-modified pore fluids
Soliton Magnetization Dynamics in Spin-Orbit Coupled Bose-Einstein Condensates
Ring-trapped Bose-Einstein condensates subject to spin-orbit coupling support
localized dark soliton excitations that show periodic density dynamics in real
space. In addition to the density feature, solitons also carry a localized
pseudo-spin magnetization that exhibits a rich and tunable dynamics. Analytic
results for Rashba-type spin-orbit coupling and spin-invariant interactions
predict a conserved magnitude and precessional motion for the soliton
magnetization that allows for the simulation of spin-related geometric phases
recently seen in electronic transport measurements.Comment: 3 figures, 5 page
Radiative feedback from an early X-ray background
The first generation of stars (commonly known as population III) are expected
to form in low-mass protogalaxies in which molecular hydrogen is the dominant
coolant. Radiation from these stars will rapidly build up an extragalactic
ultraviolet background capable of photodissociating H2, and it is widely
believed that this background will suppress further star formation in low-mass
systems.
However, star formation will also produce an extragalactic X-ray background.
This X-ray background, by increasing the fractional ionization of protogalactic
gas, promotes H2 formation and reduces the effectiveness of ultraviolet
feedback.
In this paper, we examine which of these backgrounds has the dominant effect.
Using a simple model for the growth of the UV and X-ray backgrounds, together
with a detailed one-dimensional model of protogalactic chemical evolution, we
examine the effects of the X-ray backgrounds produced by a number of likely
source models. We show that in several cases, the resulting X-ray background is
strong enough to offset UV photodissociation in large H2-cooled protogalaxies.
On the other hand, small protogalaxies (those with virial temperatures T_vir <
2000K) remain dominated by the UV background in all of the models we examine.
We also briefly investigate the effects of the X-ray background upon the
thermal and chemical evolution of the diffuse IGM.Comment: 19 pages, 10 figures. Presentation improved, thanks to helpful
comments by the referee. Accepted by MNRA
Errors in kinematic distances and our image of the Milky Way Galaxy
Errors in the kinematic distances, under the assumption of circular gas
orbits, were estimated by performing synthetic observations of a model disk
galaxy. It was found that the error is < 0.5 kpc for most of the disk when the
measured rotation curve was used, but larger if the real rotation curve is
applied. In both cases, the error is significantly larger at the positions of
the spiral arms. The error structure is such that, when kinematic distances are
used to develope a picture of the large scale density distribution, the most
significant features of the numerical model are significantly distorted or
absent, while spurious structure appears. By considering the full velocity
field in the calculation of the kinematic distances, most of the original
density structures can be recovered.Comment: Accepted for publication in A
The continuity of the inversion and the structure of maximal subgroups in countably compact topological semigroups
In this paper we search for conditions on a countably compact
(pseudo-compact) topological semigroup under which: (i) each maximal subgroup
in is a (closed) topological subgroup in ; (ii) the Clifford part
(i.e. the union of all maximal subgroups) of the semigroup is a
closed subset in ; (iii) the inversion is continuous; and (iv) the projection ,
, onto the subset of idempotents of ,
is continuous
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