10,878 research outputs found
Domain structure of epitaxial Co films with perpendicular anisotropy
Epitaxial hcp Cobalt films with pronounced c-axis texture have been prepared
by pulsed lased deposition (PLD) either directly onto Al2O3 (0001) single
crystal substrates or with an intermediate Ruthenium buffer layer. The crystal
structure and epitaxial growth relation was studied by XRD, pole figure
measurements and reciprocal space mapping. Detailed VSM analysis shows that the
perpendicular anisotropy of these highly textured Co films reaches the
magnetocrystalline anisotropy of hcp-Co single crystal material. Films were
prepared with thickness t of 20 nm < t < 100 nm to study the crossover from
in-plane magnetization to out-of-plane magnetization in detail. The analysis of
the periodic domain pattern observed by magnetic force microscopy allows to
determine the critical minimum thickness below which the domains adopt a pure
in-plane orientation. Above the critical thickness the width of the stripe
domains is evaluated as a function of the film thickness and compared with
domain theory. Especially the discrepancies at smallest film thicknesses show
that the system is in an intermediate state between in-plane and out-of-plane
domains, which is not described by existing analytical domain models
Laboratory assessment of antibacterial activity of zwitterionic 7-methoxyimino cephalosporins
Zwitterionic 7-methoxyimino cephalosporins (cefpirome, cefepime, cefclidin, DQ2556, FKO37 and SCE2787) possess a variable substitution at C3 which contains a quarernary nitrogen. These cephalosporins display low affinities for Class I /7-lactamase and rapid penetration through the outer membrane of Gram-negative bacilli, so that an increased number of periplasmic β-lactam molecules interact with PBP's per unit of time. As a consequence, the new zitterionic compounds remain active against some, but not all, ceftazidime-resistant Enterobacteriaceae producing high levels of Class Iβlactamase or Bush type 2bβlactamases. Antipseudomonas activities are generally similar to that of ceftazidime except that cefclidin is more active. The new zwitterionic compounds, especially cefpirome and FK037, express greater antistaphylococcal potency than does ceftazidime. A variety of animal models including meningitis and endocarditis have confirmed the potential of these compounds in-vivo. On the basis of structural and antibacterial characteristics, the expression ‘forth generation' is acceptable to describe the zwitterionic 7-methoxyimino cephalosporin
Exhaustion of Nucleation in a Closed System
We determine the distribution of cluster sizes that emerges from an initial
phase of homogeneous aggregation with conserved total particle density. The
physical ingredients behind the predictions are essentially classical:
Super-critical nuclei are created at the Zeldovich rate, and before the
depletion of monomers is significant, the characteristic cluster size is so
large that the clusters undergo diffusion limited growth. Mathematically, the
distribution of cluster sizes satisfies an advection PDE in "size-space".
During this creation phase, clusters are nucleated and then grow to a size much
larger than the critical size, so nucleation of super-critical clusters at the
Zeldovich rate is represented by an effective boundary condition at zero size.
The advection PDE subject to the effective boundary condition constitutes a
"creation signaling problem" for the evolving distribution of cluster sizes
during the creation era.
Dominant balance arguments applied to the advection signaling problem show
that the characteristic time and cluster size of the creation era are
exponentially large in the initial free-energy barrier against nucleation, G_*.
Specifically, the characteristic time is proportional to exp(2 G_*/ 5 k_B T)
and the characteristic number of monomers in a cluster is proportional to
exp(3G_*/5 k_B T). The exponentially large characteristic time and cluster size
give a-posteriori validation of the mathematical signaling problem. In a short
note, Marchenko obtained these exponentials and the numerical pre-factors, 2/5
and 3/5. Our work adds the actual solution of the kinetic model implied by
these scalings, and the basis for connection to subsequent stages of the
aggregation process after the creation era.Comment: Greatly shortened paper. Section on growth model removed. Added a
section analyzing the error in the solution of the integral equation. Added
reference
Ultra-bright and efficient single photon generation based on N-V centres in nanodiamonds on a solid immersion lens
Single photons are fundamental elements for quantum information technologies
such as quantum cryptography, quantum information storage and optical quantum
computing. Colour centres in diamond have proven to be stable single photon
sources and thus essential components for reliable and integrated quantum
information technology. A key requirement for such applications is a large
photon flux and a high efficiency. Paying tribute to various attempts to
maximise the single photon flux we show that collection efficiencies of photons
from colour centres can be increased with a rather simple experimental setup.
To do so we spin-coated nanodiamonds containing single nitrogen-vacancy colour
centres on the flat surface of a ZrO2 solid immersion lens. We found stable
single photon count rates of up to 853 kcts/s at saturation under continuous
wave excitation while having excess to more than 100 defect centres with count
rates from 400 kcts/s to 500 kcts/s. For a blinking defect centre we found
count rates up to 2.4 Mcts/s for time intervals of several ten seconds. It
seems to be a general feature that very high rates are accompanied by a
blinking behaviour. The overall collection efficiency of our setup of up to
4.2% is the highest yet reported for N-V defect centres in diamond. Under
pulsed excitation of a stable emitter of 10 MHz, 2.2% of all pulses caused a
click on the detector adding to 221 kcts/s thus opening the way towards diamond
based on-demand single photon sources for quantum applications
Three eras of micellization
Micellization is the precipitation of lipids from aqueous solution into aggregates with a broad distribution of aggregation number. Three eras of micellization are characterized in a simple kinetic model of Becker-Döring type. The model asigns the same constant energy to the (k-1) monomer-monomer bonds in a linear chain of k particles. The number of monomers decreases sharply and many clusters of small size are produced during the first era. During the second era, nuclei are increasing steadily in size until their distribution becomes a self-similar solution of the diffusion equation. Lastly, when the average size of the nuclei becomes comparable to its equilibrium value, a simple mean-field Fokker-Planck equation describes the final era until the equilibrium distribution is reached
Engineering chromium related single photon emitters in single crystal diamond
Color centers in diamond as single photon emitters, are leading candidates
for future quantum devices due to their room temperature operation and
photostability. The recently discovered chromium related centers are
particularly attractive since they possess narrow bandwidth emission and a very
short lifetime. In this paper we investigate the fabrication methodologies to
engineer these centers in monolithic diamond. We show that the emitters can be
successfully fabricated by ion implantation of chromium in conjunction with
oxygen or sulfur. Furthermore, our results indicate that the background
nitrogen concentration is an important parameter, which governs the probability
of success to generate these centers.Comment: 14 pages, 5 figure
Theory of surface deposition from boundary layers containing condensable vapour and particles
Heterogeneous condensation of vapours mixed with a carrier gas in the
stagnation point boundary layer flow near a cold wall is considered in the
presence of solid particles much larger than the mean free path of vapour
particles. The supersaturated vapour condenses on the particles by diffusion
and particles and droplets are thermophoretically attracted to the wall.
Assuming that the heat of vaporization is much larger than the Boltzmann
constant times the temperature far from the wall, vapour condensation occurs in
a {\em condensation layer} (CL). The CL width and characteristics depend on the
parameters of the problem, and a parameter yielding the rate of vapour
scavenging by solid particles is particularly important. Assuming that the CL
is so narrow that temperature, particle density and velocity do not change
appreciably inside it, an asymptotic theory is found, the -CL theory,
that approximates very well the vapour and droplet profiles, the dew point
shift and the deposition rates at the wall for wide ranges of the wall
temperature and the scavenging parameter . This theory
breaks down for very close to the maximum temperature yielding
non-zero droplet deposition rate, . For large , we can
either assume that the width of the CL is zero (0-CL theory, then the vapour
density reaches local equilibrium with the condensate immediately after it
enters the dew surface), or use a nonlinear multiple scales theory. The 0-CL
theory corrects the -CL theory for very close to
and any , whereas the multiple scales theory is
appropriate for large and moderate .Comment: 47 pages, 10 figures, to appear in J. Fluid Mec
Validation of northern latitude Tropospheric Emission Spectrometer stare ozone profiles with ARC-IONS sondes during ARCTAS: sensitivity, bias and error analysis
We compare Tropospheric Emission Spectrometer (TES) versions 3 and 4, V003 and V004, respectively, nadir-stare ozone profiles with ozonesonde profiles from the Arctic Intensive Ozonesonde Network Study (ARCIONS, http://croc.gsfc.nasa.gov/arcions/ during the Arctic Research on the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field mission. The ozonesonde data are from launches timed to match Aura's overpass, where 11 coincidences spanned 44° N to 71° N from April to July 2008. Using the TES "stare" observation mode, 32 observations are taken over each coincidental ozonesonde launch. By effectively sampling the same air mass 32 times, comparisons are made between the empirically-calculated random errors to the expected random errors from measurement noise, temperature and interfering species, such as water. This study represents the first validation of high latitude (>70°) TES ozone. We find that the calculated errors are consistent with the actual errors with a similar vertical distribution that varies between 5% and 20% for V003 and V004 TES data. In general, TES ozone profiles are positively biased (by less than 15%) from the surface to the upper-troposphere (~1000 to 100 hPa) and negatively biased (by less than 20%) from the upper-troposphere to the lower-stratosphere (100 to 30 hPa) when compared to the ozonesonde data. Lastly, for V003 and V004 TES data between 44° N and 71° N there is variability in the mean biases (from −14 to +15%), mean theoretical errors (from 6 to 13%), and mean random errors (from 9 to 19%)
Incorporating Inductances in Tissue-Scale Models of Cardiac Electrophysiology
In standard models of cardiac electrophysiology, including the bidomain and
monodomain models, local perturbations can propagate at infinite speed. We
address this unrealistic property by developing a hyperbolic bidomain model
that is based on a generalization of Ohm's law with a Cattaneo-type model for
the fluxes. Further, we obtain a hyperbolic monodomain model in the case that
the intracellular and extracellular conductivity tensors have the same
anisotropy ratio. In one spatial dimension, the hyperbolic monodomain model is
equivalent to a cable model that includes axial inductances, and the relaxation
times of the Cattaneo fluxes are strictly related to these inductances. A
purely linear analysis shows that the inductances are negligible, but models of
cardiac electrophysiology are highly nonlinear, and linear predictions may not
capture the fully nonlinear dynamics. In fact, contrary to the linear analysis,
we show that for simple nonlinear ionic models, an increase in conduction
velocity is obtained for small and moderate values of the relaxation time. A
similar behavior is also demonstrated with biophysically detailed ionic models.
Using the Fenton-Karma model along with a low-order finite element spatial
discretization, we numerically analyze differences between the standard
monodomain model and the hyperbolic monodomain model. In a simple benchmark
test, we show that the propagation of the action potential is strongly
influenced by the alignment of the fibers with respect to the mesh in both the
parabolic and hyperbolic models when using relatively coarse spatial
discretizations. Accurate predictions of the conduction velocity require
computational mesh spacings on the order of a single cardiac cell. We also
compare the two formulations in the case of spiral break up and atrial
fibrillation in an anatomically detailed model of the left atrium, and [...].Comment: 20 pages, 12 figure
Charging capacitors from thermal fluctuations using diodes
We theoretically consider a graphene ripple as a Brownian particle coupled to
an energy storage circuit. When circuit and particle are at the same
temperature, the second law forbids harvesting energy from the thermal motion
of the Brownian particle, even if the circuit contains a rectifying diode.
However, when the circuit contains a junction followed by two diodes wired in
opposition, the approach to equilibrium may become ultraslow. Detailed balance
is temporarily broken as current flows between the two diodes and charges
storage capacitors. The energy harvested by each capacitor comes from the
thermal bath of the diodes while the system obeys the first and second laws of
thermodynamics.Comment: 13 pages, 6 figure
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