129 research outputs found
Pinhole calculations of the Josephson effect in 3He-B
We study theoretically the dc Josephson effect between two volumes of
superfluid 3He-B. We first discuss how the calculation of the current-phase
relationships is divided into a mesoscopic and a macroscopic problem. We then
analyze mass and spin currents and the symmetry of weak links. In quantitative
calculations the weak link is assumed to be a pinhole, whose size is small in
comparison to the coherence length. We derive a quasiclassical expression for
the coupling energy of a pinhole, allowing also for scattering in the hole.
Using a selfconsistent order parameter near a wall, we calculate the
current-phase relationships in several cases. In the isotextural case, the
current-phase relations are plotted assuming a constant spin-orbit texture. In
the opposite anisotextural case the texture changes as a function of the phase
difference. For that we have to consider the stiffness of the macroscopic
texture, and we also calculate some surface interaction parameters. We analyze
the experiments by Marchenkov et al. We find that the observed pi states and
bistability hardly can be explained with the isotextural pinhole model, but a
good quantitative agreement is achieved with the anisotextural model.Comment: 20 pages, 21 figures, revtex
Contribution of toxicological analysis to the care of dimethyl fumarate dermatitis
Background:
Dimethyl fumarate (DMFu) is a fungicide which is used in Chinese manufactures of furniture and shoes to avoid mould spoiling of fabrics. In 2008, DMFu was found the responsible allergen for several cases of contact dermatitis from armchairs and shoes observed in Europe. In France a national toxicovigilance survey was set up and importation of products containing dimethyl fumarate is now forbidden. Case report: a 36 year-old woman, with no history of previous allergy, was hospitalized because of a severe acute eczema of her feet after wearing a new pair of boots inside which she had noticed desiccant sachets. She strongly reacted on patch testing to DMFu and to the content of a sachet which was identified as DMFu, both at 0.01%, 0.1%, 1% in petrolatum, and also to a piece of the fabric of her boots, patch tested as is.
Materials and method:
Boot fabrics and mould-proof sachets found in the boots were analysed by HPLC/UV/DAD and GC/MS after methanol extraction. Further samples of anti mould agent sachets or shoe fabrics from 5 other patients with suspicion of DMFu dermatitis were analysed with the same procedure. Some of them were transferred to the laboratory several months after healing of the dermatitis.
Results:
DMFu was found in all the samples from 1 to 100% in sachets or from 20 to 2000 ÎĽg/g in the fabrics of shoes, even after one year. These findings contributed to ensure the responsibility of DMFu in the dermatitis of the patients and demonstrate that DMFu may remain a long time in the contaminated fabrics after removal of the sachets. This study also points out the usefulness of the collaboration between dermatologists, biologists and poison centre practitioners
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
Geometric Laws of Vortex Quantum Tunneling
In the semiclassical domain the exponent of vortex quantum tunneling is
dominated by a volume which is associated with the path the vortex line traces
out during its escape from the metastable well. We explicitly show the
influence of geometrical quantities on this volume by describing point vortex
motion in the presence of an ellipse. It is argued that for the semiclassical
description to hold the introduction of an additional geometric constraint, the
distance of closest approach, is required. This constraint implies that the
semiclassical description of vortex nucleation by tunneling at a boundary is in
general not possible. Geometry dependence of the tunneling volume provides a
means to verify experimental observation of vortex quantum tunneling in the
superfluid Helium II.Comment: 4 pages, 2 figures, revised version to appear in Phys. Rev.
Analytical Estimate of the Critical Velocity for Vortex Pair Creation in Trapped Bose Condensates
We use a modified Thomas-Fermi approximation to estimate analytically the
critical velocity for the formation of vortices in harmonically trapped BEC. We
compare this analytical estimate to numerical calculations and to recent
experiments on trapped alkali condensates.Comment: 12 page
The Frequency Dependence of Critical-velocity Behavior in Oscillatory Flow of Superfluid Helium-4 Through a 2-micrometer by 2-micrometer Aperture in a Thin Foil
The critical-velocity behavior of oscillatory superfluid Helium-4 flow
through a 2-micrometer by 2-micrometer aperture in a 0.1-micrometer-thick foil
has been studied from 0.36 K to 2.10 K at frequencies from less than 50 Hz up
to above 1880 Hz. The pressure remained less than 0.5 bar. In early runs during
which the frequency remained below 400 Hz, the critical velocity was a
nearly-linearly decreasing function of increasing temperature throughout the
region of temperature studied. In runs at the lowest frequencies, isolated 2 Pi
phase slips could be observed at the onset of dissipation. In runs with
frequencies higher than 400 Hz, downward curvature was observed in the decrease
of critical velocity with increasing temperature. In addition, above 500 Hz an
alteration in supercritical behavior was seen at the lower temperatures,
involving the appearance of large energy-loss events. These irregular events
typically lasted a few tens of half-cycles of oscillation and could involve
hundreds of times more energy loss than would have occurred in a single
complete 2 Pi phase slip at maximum flow. The temperatures at which this
altered behavior was observed rose with frequency, from ~ 0.6 K and below, at
500 Hz, to ~ 1.0 K and below, at 1880 Hz.Comment: 35 pages, 13 figures, prequel to cond-mat/050203
Josephson effects in dilute Bose-Einstein condensates
We propose an experiment that would demonstrate the ``dc'' and ``ac''
Josephson effects in two weakly linked Bose-Einstein condensates. We consider a
time-dependent barrier, moving adiabatically across the trapping potential. The
phase dynamics are governed by a ``driven-pendulum'' equation, as in
current-driven superconducting Josephson junctions. At a critical velocity of
the barrier (proportional to the critical tunneling current), there is a sharp
transition between the ``dc'' and ``ac'' regimes. The signature is a sudden
jump of a large fraction of the relative condensate population. Analytical
predictions are compared with a full numerical solution of the time dependent
Gross-Pitaevskii equation, in an experimentally realistic situation.Comment: 4 pages, 1 figur
Josephson Effect between Condensates with Different Internal Structures
A general formula for Josephson current in a wide class of hybrid junctions
between different internal structures is derived on the basis of the Andreev
picture. The formula extends existing formulae and also enables us to analyze
novel B-phase/A-phase/B-phase (BAB) junctions in superfluid helium three
systems, which are accessible to experiments. It is predicted that BAB
junctions will exhibit two types of current-phase relations associated with
different internal symmetries. A ``pseudo-magnetic interface effect'' inherent
in the system is also revealed.Comment: 4 pages, 2 figure
Chemical-potential standard for atomic Bose-Einstein condensates
When subject to an external time periodic perturbation of frequency , a
Josephson-coupled two-state Bose-Einstein condensate responds with a constant
chemical potential difference , where is Planck's constant
and is an integer. We propose an experimental procedure to produce
ac-driven atomic Josephson devices that may be used to define a standard of
chemical potential. We investigate how to circumvent some of the specific
problems derived from the present lack of advanced atom circuit technology. We
include the effect of dissipation due to quasiparticles, which is essential to
help the system relax towards the exact Shapiro resonance, and set limits to
the range of values which the various physical quantities must have in order to
achieve a stable and accurate chemical potential difference between the
macroscopic condensates.Comment: 13 pages, 4 figure
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