146 research outputs found
The roles of latent heating and dust in the structure and variability of the northern Martian polar vortex
The winter polar vortices on Mars are annular in terms of their potential
vorticity (PV) structure, a phenomenon identified in observations, reanalysis
and some numerical simulations. Some recent modeling studies have proposed that
condensation of atmospheric carbon dioxide at the winter pole is a contributing
factor to maintaining the annulus through the release of latent heat. Dust and
topographic forcing are also known to be causes of internal and interannual
variability in the polar vortices. However, coupling between these factors
remains uncertain, and previous studies of their impact on vortex structure and
variability have been largely limited to a single Martian global climate model
(MGCM). Here, by further developing a novel MGCM, we decompose the relative
roles of latent heat and dust as drivers for the variability and structure of
the northern Martian polar vortex. We also consider how Martian topography
modifies the driving response. By also analyzing a reanalysis dataset we show
that there is significant dependence in the polar vortex structure and
variability on the observations assimilated. In both model and reanalysis, high
atmospheric dust loading (such as that seen during a global dust storm) can
disrupt the vortex, cause the destruction of PV in the low-mid altitudes (> 0.1
hPa), and significantly reduce spatial and temporal vortex variability. Through
our simulations, we find that the combination of dust and topography primarily
drives the eddy activity throughout the Martian year, and that although latent
heat release can produce an annular vortex, it has a relatively minor effect on
vortex variability.Comment: 16 pages, 14 figures, The Planetary Science Journa
Wave Energy Amplification in a Metamaterial based Traveling Wave Structure
We consider the interaction between a particle beam and a propagating
electromagnetic wave in the presence of a metamaterial. We show that the
introduction of a metamaterial gives rise to a novel dispersion curve which
determines a unique wave particle relationship, via the frequency dependence of
the metamaterial and the novel ability of metamaterials to exhibit simultaneous
negative permittivity and permeability. Using a modified form of Madey's
theorem we find that the novel dispersion of the metamaterial leads to a
amplification of the EM wave power
Mesoscopic Ferromagnet/Superconductor Junctions and the Proximity Effect
We have measured the electrical transport of submicron ferromagnets (Ni) in
contact with a mesoscopic superconductor (Al) for a range of interface
resistances. In the geometry measured, the interface and the ferromagnet are
measured separately. The ferromagnet itself shows no appreciable
superconducting proximity effect, but the ferromagnet/superconductor interface
exhibits strong temperature, field and current bias dependences. These effects
are dependent on the local magnetic field distribution near the interface
arising from the ferromagnet. We find that the temperature dependences may be
fit to a modified version of the Blonder-Tinkham-Klapwijk theory for
normal-superconductor transport.Comment: 4 eps fig
Resonant transmission of normal electrons through Andreev states in ferromagnets
Giant oscillations of the conductance of a superconductor - ferromagnet -
superconductor Andreev interferometer are predicted. The effect is due to the
resonant transmission of normal electrons through Andreev levels when the
voltage applied to the ferromagnet is close to ( is the
spin-dependant part of the electron energy). The effect of bias voltage and
phase difference between the superconductors on the current and the
differential conductance is presented. These efects allow a direct spectroscopy
of Andreev levels in the ferromagnet.Comment: 4 pages, 4 figure
Spin-accumulation and Andreev-reflection in a mesoscopic ferromagnetic wire
The electron transport though ferromagnetic metal-superconducting hybrid
devices is considered in the non-equilibrium Green's function formalism in the
quasiclassical approximation. Attention if focused on the limit in which the
exchange splitting in the ferromagnet is much larger than the superconducting
energy gap. Transport properties are then governed by an interplay between
spin-accumulation close to the interface and Andreev reflection at the
interface. We find that the resistance can either be enhanced or lowered in
comparison to the normal case and can have a non-monotonic temperature and
voltage dependence. In the non-linear voltage regime electron heating effects
may govern the transport properties, leading to qualitative different behaviour
than in the absence of heating effects. Recent experimental results on the
effect of the superconductor on the conductance of the ferromagnet can be
understood by our results for the energy-dependent interface resistance
together with effects of spin- accumulation without invoking long range pairing
correlations in the ferromagnet.Comment: 15 pages, 12 figures included, submitted to PR
Proximity effects at ferromagnet-superconductor interfaces
We study proximity effects at ferromagnet superconductor interfaces by
self-consistent numerical solution of the Bogoliubov-de Gennes equations for
the continuum, without any approximations. Our procedures allow us to study
systems with long superconducting coherence lengths. We obtain results for the
pair potential, the pair amplitude, and the local density of states. We use
these results to extract the relevant proximity lengths. We find that the
superconducting correlations in the ferromagnet exhibit a damped oscillatory
behavior that is reflected in both the pair amplitude and the local density of
states. The characteristic length scale of these oscillations is approximately
inversely proportional to the exchange field, and is independent of the
superconducting coherence length in the range studied. We find the
superconducting coherence length to be nearly independent of the ferromagnetic
polarization.Comment: 13 Pages total. Compressed .eps figs might display poorly, but will
print fin
Conceptual design of a nonscaling fixed field alternating gradient accelerator for protons and carbon ions for charged particle therapy
Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.The conceptual design for a nonscaling fixed field alternating gradient accelerator suitable for charged particle therapy (the use of protons and other light ions to treat some forms of cancer) is described.EPSR
Study of 16 Portuguese activated sludge systems based on filamentous bacteria populations and their relationships with environmental parameters
A survey in 16 activated sludge waste water treatment plants (WWTP) was conducted to contribute to the knowledge of the environmental parameters that determine the composition of the filamentous community. A total of 128 samples of mixed liquor from municipal WWTP were collected during 2 years, and 22 filamentous morphotypes were identified. The most frequent and abundant filamentous bacteria were, in both cases and by this order, type 0041/0675, type 0092, Microthrix parvicella and 1851, nocardioforms and Haliscomenobacter hydrossis. Concerning dominance, type 1851 was the most frequently dominant morphotype, followed by M. parvicella and types 0092 and 0041/0675. These were also, and by this order, the dominant morphotypes during bulking occurrences. Significant correlations were obtained between the abundance of filamentous bacteria and environmental parameters, but multivariate statistical analysis only confirmed the correlation between type 0092 and Sludge Volume Index (SVI), emphasizing the association of this filament with bulking. The discussion of the results in light of published works was complicated by the random use of terms such as frequency, abundance, and dominance with different and often unclear meanings. This reinforces the need of clarifying these terms when discussing the causes of filamentous overgrowth in WWTP.Portuguese Foundation for Science and Technology (FCT) and the European Community fund FEDER, through Program COMPETE, in the ambit of the Projects
FCOMP-01-0124-FEDER-007025 (PTDC/AMB/68393/2006), PEst-OE/EQB/LA0023/2013, RECI/BBB-EBI/0179/2012 (FCOMP-01-0124-FEDER-027462), and the Project BBioEnv - Biotechnology and
Bioengineering for a sustainable world,REF. NORTE-07-0124-
FEDER-000048, co-funded by the Programa Operacional Regional do
Norte (ON.2 – O Novo Norte), QREN, FEDER. PhD grant SFRH/BD/64848/200
Opportunities for mesoscopics in thermometry and refrigeration: Physics and applications
This review presents an overview of the thermal properties of mesoscopic
structures. The discussion is based on the concept of electron energy
distribution, and, in particular, on controlling and probing it. The
temperature of an electron gas is determined by this distribution:
refrigeration is equivalent to narrowing it, and thermometry is probing its
convolution with a function characterizing the measuring device. Temperature
exists, strictly speaking, only in quasiequilibrium in which the distribution
follows the Fermi-Dirac form. Interesting nonequilibrium deviations can occur
due to slow relaxation rates of the electrons, e.g., among themselves or with
lattice phonons. Observation and applications of nonequilibrium phenomena are
also discussed. The focus in this paper is at low temperatures, primarily below
4 K, where physical phenomena on mesoscopic scales and hybrid combinations of
various types of materials, e.g., superconductors, normal metals, insulators,
and doped semiconductors, open up a rich variety of device concepts. This
review starts with an introduction to theoretical concepts and experimental
results on thermal properties of mesoscopic structures. Then thermometry and
refrigeration are examined with an emphasis on experiments. An immediate
application of solid-state refrigeration and thermometry is in ultrasensitive
radiation detection, which is discussed in depth. This review concludes with a
summary of pertinent fabrication methods of presented devices.Comment: Close to the version published in RMP; 59 pages, 35 figure
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Interannual SAM modulation of Antarctic sea ice extent does not account for its long-term trends: implications for the role of ozone depletion
The expansion of Antarctic sea ice since 1979 in the presence of increasing greenhouse gases remains one of the most puzzling features of current climate change. Some studies have proposed that the formation of the ozone hole, via the Southern Annular Mode, might explain that expansion, and a recent study highlighted a robust causal link between summertime Southern Annular Mode (SAM) anomalies and sea ice anomalies in the subsequent autumn. Here we show that many models are able to capture this relationship between the SAM and sea ice, but also emphasize that the SAM only explains a small fraction of the year-to-year variability. Finally, examining multidecadal trends, in models and observations, we confirm the findings of several previous studies and conclude that the SAM – and thus the ozone hole – are not the primary drivers of the sea ice expansion around Antarctica in recent decades
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