376 research outputs found
Reduced leakage current in Josephson tunnel junctions with codeposited barriers
Josephson junctions were fabricated using two different methods of barrier
formation. The trilayers employed were Nb/Al-AlOx/Nb on sapphire, where the
first two layers were epitaxial. The oxide barrier was formed either by
exposing the Al surface to O2 or by codepositing Al in an O2 background. The
codeposition process yielded junctions that showed the theoretically predicted
subgap current and no measurable shunt conductance. In contrast, devices with
barriers formed by thermal oxidation showed a small shunt conductance in
addition to the predicted subgap current.Comment: 3 pages, 4 figure
Uniform shear flow in dissipative gases. Computer simulations of inelastic hard spheres and (frictional) elastic hard spheres
In the preceding paper (cond-mat/0405252), we have conjectured that the main
transport properties of a dilute gas of inelastic hard spheres (IHS) can be
satisfactorily captured by an equivalent gas of elastic hard spheres (EHS),
provided that the latter are under the action of an effective drag force and
their collision rate is reduced by a factor (where is
the constant coefficient of normal restitution). In this paper we test the
above expectation in a paradigmatic nonequilibrium state, namely the simple or
uniform shear flow, by performing Monte Carlo computer simulations of the
Boltzmann equation for both classes of dissipative gases with a dissipation
range and two values of the imposed shear rate .
The distortion of the steady-state velocity distribution from the local
equilibrium state is measured by the shear stress, the normal stress
differences, the cooling rate, the fourth and sixth cumulants, and the shape of
the distribution itself. In particular, the simulation results seem to be
consistent with an exponential overpopulation of the high-velocity tail. The
EHS results are in general hardly distinguishable from the IHS ones if
, so that the distinct signature of the IHS gas (higher
anisotropy and overpopulation) only manifests itself at relatively high
dissipationsComment: 23 pages; 18 figures; Figs. 2 and 9 include new simulations; two new
figures added; few minor changes; accepted for publication in PR
System of elastic hard spheres which mimics the transport properties of a granular gas
The prototype model of a fluidized granular system is a gas of inelastic hard
spheres (IHS) with a constant coefficient of normal restitution . Using
a kinetic theory description we investigate the two basic ingredients that a
model of elastic hard spheres (EHS) must have in order to mimic the most
relevant transport properties of the underlying IHS gas. First, the EHS gas is
assumed to be subject to the action of an effective drag force with a friction
constant equal to half the cooling rate of the IHS gas, the latter being
evaluated in the local equilibrium approximation for simplicity. Second, the
collision rate of the EHS gas is reduced by a factor , relative
to that of the IHS gas. Comparison between the respective Navier-Stokes
transport coefficients shows that the EHS model reproduces almost perfectly the
self-diffusion coefficient and reasonably well the two transport coefficients
defining the heat flux, the shear viscosity being reproduced within a deviation
less than 14% (for ). Moreover, the EHS model is seen to agree
with the fundamental collision integrals of inelastic mixtures and dense gases.
The approximate equivalence between IHS and EHS is used to propose kinetic
models for inelastic collisions as simple extensions of known kinetic models
for elastic collisionsComment: 20 pages; 6 figures; change of title; few minor changes; accepted for
publication in PR
Radiographic viewing conditions at Johannesburg Hospital
Purpose: To measure the luminance level of X-ray viewing boxes and ambient lighting levels in reporting rooms as a quality assurance procedure, and to compare the results with those recommended by the Directorate of Radiatio
Transition from phase slips to the Josephson effect in a superfluid 4He weak link
The rich dynamics of flow between two weakly coupled macroscopic quantum
reservoirs has led to a range of important technologies. Practical development
has so far been limited to superconducting systems, for which the basic
building block is the so-called superconducting Josephson weak link. With the
recent observation of quantum oscillations in superfluid 4He near 2K, we can
now envision analogous practical superfluid helium devices. The characteristic
function which determines the dynamics of such systems is the current-phase
relation Is(phi), which gives the relationship between the superfluid current
Is flowing through a weak link and the quantum phase difference phi across it.
Here we report the measurement of the current-phase relation of a superfluid
4He weak link formed by an array of nano-apertures separating two reservoirs of
superfluid 4He. As we vary the coupling strength between the two reservoirs, we
observe a transition from a strongly coupled regime in which Is(phi) is linear
and flow is limited by 2pi phase slips, to a weak coupling regime where Is(phi)
becomes the sinusoidal signature of a Josephson weak link.Comment: 12 pages, 4 figure
The Politics of Exhaustion and the Externalization of British Border Control. An Articulation of a Strategy Designed to Deter, Control and Exclude
In response to contemporary forms of human mobility, there has been a continued hardening of borders seeking to deter, control and exclude certain groups of people from entering nation states in Europe, North America and Australasia. Within this context, a disconcerting evolution of new and increasingly sophisticated forms of border control measures have emerged, which often play out within bilateral arrangements of âexternalisedâ or âoffshoreâ border controls. Drawing on extensive firstâhand field research among displaced people in Calais, Paris and Brussels in 2016â2019, this paper argues that the externalization of the British border to France is contingent upon a harmful strategy, which can be understood as the âpolitics of exhaustion.â This is a raft of (micro) practices and methods strategically aimed to deter, control and exclude certain groups of people on the move who have been profiled as âundesirable,â with a detrimental (un)intended impact on human lives
Non-Newtonian Couette-Poiseuille flow of a dilute gas
The steady state of a dilute gas enclosed between two infinite parallel
plates in relative motion and under the action of a uniform body force parallel
to the plates is considered. The Bhatnagar-Gross-Krook model kinetic equation
is analytically solved for this Couette-Poiseuille flow to first order in the
force and for arbitrary values of the Knudsen number associated with the shear
rate. This allows us to investigate the influence of the external force on the
non-Newtonian properties of the Couette flow. Moreover, the Couette-Poiseuille
flow is analyzed when the shear-rate Knudsen number and the scaled force are of
the same order and terms up to second order are retained. In this way, the
transition from the bimodal temperature profile characteristic of the pure
force-driven Poiseuille flow to the parabolic profile characteristic of the
pure Couette flow through several intermediate stages in the Couette-Poiseuille
flow are described. A critical comparison with the Navier-Stokes solution of
the problem is carried out.Comment: 24 pages, 5 figures; v2: discussion on boundary conditions added; 10
additional references. Published in a special issue of the journal "Kinetic
and Related Models" dedicated to the memory of Carlo Cercignan
Gibberellin Biosynthetic Inhibitors Make Human Malaria Parasite Plasmodium falciparum Cells Swell and Rupture to Death
Malaria remains as one of the most devastating infectious disease, and continues to exact an enormous toll in medical cost and days of labor lost especially in the tropics. Effective malaria control and eventual eradication remain a huge challenge, with efficacious antimalarials as important intervention/management tool. Clearly new alternative drugs that are more affordable and with fewer side effects are desirable. After preliminary in vitro assays with plant growth regulators and inhibitors, here, we focus on biosynthetic inhibitors of gibberellin, a plant hormone with many important roles in plant growth, and show their inhibitory effect on the growth of both apicomplexa, Plasmodium falciparum and Toxoplasma gondii. Treatment of P. falciparum cultures with the gibberellin biosynthetic inhibitors resulted in marked morphological changes that can be reversed to a certain degree under hyperosmotic environment. These unique observations suggest that changes in the parasite membrane permeability may explain the pleiotropic effects observed within the intracellular parasites
- âŠ