2,826 research outputs found
Focused-ion-beam-induced deposition of superconducting nanowires
Superconducting nanowires, with a critical temperature of 5.2 K, have been
synthesized using an ion-beam-induced deposition, with a Gallium focused ion
beam and Tungsten Carboxyl, W(CO)6, as precursor. The films are amorphous, with
atomic concentrations of about 40, 40, and 20 % for W, C, and Ga, respectively.
Zero Kelvin values of the upper critical field and coherence length of 9.5 T
and 5.9 nm, respectively, are deduced from the resistivity data at different
applied magnetic fields. The critical current density is Jc= 1.5 10^5 A/cm2 at
3 K. This technique can be used as a template-free fabrication method for
superconducting devices.Comment: Accepted for publication in Applied Physics Letter
Comparative Assessment of LES and URANS for Flow Over a Cylinder at a Reynolds Number of 3900
Numerical simulations utilising turbulence models based on the Reynolds Averaged Navier Stokes (RANS) equations generally exhibit poor performance in predicting separated flow around cylinders. This paper assesses potential improvements offered by the three-dimensional unsteady RANS and Large Eddy Simulation (LES) methodologies in replicating the flow around a cylinder at a Reynolds number, based on diameter, of 3900. The performance is assessed against corresponding experimental data and two-dimensional unsteady RANS turbulence simulations
Gravitational radiation from nonaxisymmetric spherical Couette flow in a neutron star
The gravitational wave signal generated by global, nonaxisymmetric shear
flows in a neutron star is calculated numerically by integrating the
incompressible Navier--Stokes equation in a spherical, differentially rotating
shell. At Reynolds numbers \Rey \gsim 3 \times 10^{3}, the laminar Stokes
flow is unstable and helical, oscillating Taylor--G\"ortler vortices develop.
The gravitational wave strain generated by the resulting kinetic-energy
fluctuations is computed in both and polarizations as a function
of time. It is found that the signal-to-noise ratio for a coherent,
-{\rm s} integration with LIGO II scales as for a star at 1 {\rm kpc} with angular velocity
. This should be regarded as a lower limit: it excludes pressure
fluctuations, herringbone flows, Stuart vortices, and fully developed
turbulence (for \Rey \gsim 10^{6}).Comment: (1) School of Physics, University of Melbourne, Parkville, VIC 3010,
Australia. (2) Departamento de Fisica, Escuela de Ciencias,Universidad de
Oriente, Cumana, Venezuela, (3) Department of Mechanical Engineering,
University of Melbourne, Parkville, VIC 3010, Australia. Accepted for
publication in The Astrophysical Journal Letter
Symmetric mode resonance of bubbles near a rigid boundary - the nonlinear case with time delay effects
A fundamental understanding of the effect of a surface on the resonance frequency of bubbles will be useful in the future development of diagnostic medical ultrasound equipment, and specifically in the area of targeted contrast agents for the screening and possible treatment of colon cancer. In this work we turn to the wall effects on the nonlinear resonance frequency response of air bubbles in water, following on from an earlier work which considered linear interactions (E. M. B. Payne, S. Illesinghe, A. Ooi, R. Manasseh, J. Acoust Soc. Am. 118, 2841-2849 (2005)). Numerical results for micron-sized bubbles near a rigid boundary are presented, showing the shift in frequency caused by the presence of the boundary and the presence of other bubbles. Time delay effects are also included, showing a damping of the frequency response. Simulations are limited to the special case where all bubbles are in phase (i.e., the symmetric mode), which refers to the case where all bubbles have the same initial conditions and are subjected to the same excitation pressure field. As a result they have identical time histories. An experimental method for measuring the frequency response of a single bubble attached to a surface is also briefly mentioned
Microcantilever Studies of Angular Field Dependence of Vortex Dynamics in BSCCO
Using a nanogram-sized single crystal of BSCCO attached to a microcantilever
we demonstrate in a direct way that in magnetic fields nearly parallel to the
{\it ab} plane the magnetic field penetrates the sample in the form of
Josephson vortices rather than in the form of a tilted vortex lattice. We
further investigate the relation between the Josephson vortices and the pancake
vortices generated by the perpendicular field component.Comment: 5 pages, 8 figure
Numerical Study of the Behaviour of Wall Shear Stress in Pulsatile Stenotic Flows
This paper presents a numerical study of pulsatile flow through an axisymmetric stenosed artery. Numerical calculations of the incompressible Navier-Stokes equations were carried out in an axisymmetric geometry to investigate how the wall shear stress (WSS) is affected by varying levels of stenosis contractions and pulse periods (reduced velocity). It is found that the distribution and strength of the WSS is closely correlated with the position of the vortex ring formed at the stenosis. Each vortex ring generates high WSS at the stenosis walls and this high WSS propagate downstream with the vortex ring. As the vortex ring convects downstream, it loses its strength due to viscous effects and WSS decreases in magnitude. In general, the strength of the vortex ring increases with increasing stenosis levels which leads to higher WSS values on the walls. The effect of smaller pulse period is to reduce the distance between the vortex rings, thus increasing the spatial variation of WSS along the stenosed artery
Vortex Matter Transition in BiSrCaCuO under Tilted Fields
Vortex phase diagram under tilted fields from the axis in
BiSrCaCuO is studied by local magnetization
hysteresis measurements using Hall probes. When the field is applied at large
angles from the axis, an anomaly () other than the well-known
peak effect () are found at fields below . The angular dependence of
the field is nonmonotonic and clearly different from that of
and depends on the oxygen content of the crystal. The results suggest existence
of a vortex matter transition under tilted fields. Possible mechanisms of the
transition are discussed.Comment: Revtex, 4 pages, some corrections are adde
Global three-dimensional flow of a neutron superfluid in a spherical shell in a neutron star
We integrate for the first time the hydrodynamic
Hall-Vinen-Bekarevich-Khalatnikov equations of motion of a -paired
neutron superfluid in a rotating spherical shell, using a pseudospectral
collocation algorithm coupled with a time-split fractional scheme. Numerical
instabilities are smoothed by spectral filtering. Three numerical experiments
are conducted, with the following results. (i) When the inner and outer spheres
are put into steady differential rotation, the viscous torque exerted on the
spheres oscillates quasiperiodically and persistently (after an initial
transient). The fractional oscillation amplitude () increases
with the angular shear and decreases with the gap width. (ii) When the outer
sphere is accelerated impulsively after an interval of steady differential
rotation, the torque increases suddenly, relaxes exponentially, then oscillates
persistently as in (i). The relaxation time-scale is determined principally by
the angular velocity jump, whereas the oscillation amplitude is determined
principally by the gap width. (iii) When the mutual friction force changes
suddenly from Hall-Vinen to Gorter-Mellink form, as happens when a rectilinear
array of quantized Feynman-Onsager vortices is destabilized by a counterflow to
form a reconnecting vortex tangle, the relaxation time-scale is reduced by a
factor of compared to (ii), and the system reaches a stationary state
where the torque oscillates with fractional amplitude about a
constant mean value. Preliminary scalings are computed for observable
quantities like angular velocity and acceleration as functions of Reynolds
number, angular shear, and gap width. The results are applied to the timing
irregularities (e.g., glitches and timing noise) observed in radio pulsars.Comment: 6 figures, 23 pages. Accepted for publication in Astrophysical
Journa
XAX: a multi-ton, multi-target detection system for dark matter, double beta decay and pp solar neutrinos
A multi-target detection system XAX, comprising concentric 10 ton targets of
136Xe and 129/131Xe, together with a geometrically similar or larger target of
liquid Ar, is described. Each is configured as a two-phase
scintillation/ionization TPC detector, enhanced by a full 4pi array of
ultra-low radioactivity Quartz Photon Intensifying Detectors (QUPIDs) replacing
the conventional photomultipliers for detection of scintillation light. It is
shown that background levels in XAX can be reduced to the level required for
dark matter particle (WIMP) mass measurement at a 10^-10 pb WIMP-nucleon cross
section, with single-event sensitivity below 10^-11 pb. The use of multiple
target elements allows for confirmation of the A^2 dependence of a coherent
cross section, and the different Xe isotopes provide information on the
spin-dependence of the dark matter interaction. The event rates observed by Xe
and Ar would modulate annually with opposite phases from each other for WIMP
mass >~100 GeV/c^2. The large target mass of 136Xe and high degree of
background reduction allow neutrinoless double beta decay to be observed with
lifetimes of 10^27-10^28 years, corresponding to the Majorana neutrino mass
range 0.01-0.1 eV, the most likely range from observed neutrino mass
differences. The use of a 136Xe-depleted 129/131Xe target will also allow
measurement of the pp solar neutrino spectrum to a precision of 1-2%.Comment: 16 pages with 17 figure
The London theory of the crossing-vortex lattice in highly anisotropic layered superconductors
A novel description of Josephson vortices (JVs) crossed by the pancake
vortices (PVs) is proposed on the basis of the anisotropic London theory. The
field distribution of a JV and its energy have been calculated for both dense
() PV lattices with distance
between PVs, and the nonlinear JV core size . It is shown that the
``shifted'' PV lattice (PVs displaced mainly along JVs in the crossing vortex
lattice structure), formed in high out-of-plane magnetic fields transforms into
the PV lattice ``trapped'' by the JV sublattice at a certain field, lower than
, where is the flux quantum, is the
anisotropy parameter and is the distance between CuO planes.
With further decreasing , the free energy of the crossing vortex lattice
structure (PV and JV sublattices coexist separately) can exceed the free energy
of the tilted lattice (common PV-JV vortex structure) in the case of with the in-plane penetration depth if the low
() or high ()
in-plane magnetic field is applied. It means that the crossing vortex structure
is realized in the intermediate field orientations, while the tilted vortex
lattice can exist if the magnetic field is aligned near the -axis and the
-plane as well. In the intermediate in-plane fields
, the
crossing vortex structure with the ``trapped'' PV sublattice seems to settle in
until the lock-in transition occurs since this structure has the lower energy
with respect to the tilted vortex structure in the magnetic field
oriented near the -plane.Comment: 15 pages, 6 figures, accepted for publication in PR
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