1,145 research outputs found
Non-integer flux quanta for a spherical superconductor
A thin film superconductor shaped into a spherical shell at whose center lies
the end of long thin solenoid in which there is an integer flux has
been previously extensively studied numerically as a model of a two-dimensional
superconductor. The emergent flux from the solenoid produces a radial B-field
at the superconducting shell and vortices in the superconducting film. We
study here the effects of including a second solenoid (carrying a flux )
which is inserted inside the first solenoid but passing right across the
sphere. This Aharonov-Bohm (AB) flux does not have to be quantized to make the
order parameter single valued. The Ginzburg-Landau (GL) free energy is
minimized at fixed as a function of and it is found that the minimum is
usually achieved when the AB flux is half a flux quantum, but depending on
the minimum may be at or values which are not obvious rational
fractions.Comment: 6 pages, RevTeX, 5 figures include
A novel filtration system for point of care washing of cellular therapy products
The cell therapy industry would greatly benefit from a simple point of care solution to remove Dimethyl Sulfoxide (DMSO) from small volume thawed cell suspensions prior to injection. We have designed and validated a novel dead-end filtration device, which takes advantage of the higher density of thawed cell suspensions to remove the DMSO and protein impurities from the cell suspension without fouling the filter membrane. The filter was designed to avoid fluid circuits and minimize the surface area that is contacted by the cell suspension, thus reducing cell losses by design. The filtration process was established through optimization of the fluid flow configuration, backflush cycles and filter geometry. Overall, this novel filtration device allows for a 1 mL of thawed cryopreserved cell suspensions, containing 107 cells of a foetal lung fibroblast cell line (MRC-5), to be washed in less than 30 minutes. More than 95% of the DMSO and up to 94% of the Albumin- Fluorescein-Isothiocyanate content can be removed while the viable cell recovery is higher than 80%. We have also demonstrated that this system can be used for bone marrow-derived human mesenchymal stem cells with more than 73% cell recovery and 85% DMSO reduction. This is the first time that a dead end (normal) filtration process has been used to successfully wash high density human cell suspensions. In practice, this novel solid-liquid separation technology fills the need for small volume washing in closed processing systems for cellular therapies
Vortices in a Thin Film Superconductor with a Spherical Geometry
We report results from Monte Carlo simulations of a thin film superconductor
in a spherical geometry within the lowest Landau level approximation. We
observe the absence of a phase transition to a low temperature vortex solid
phase with these boundary conditions; the system remains in the vortex liquid
phase for all accessible temperatures. The correlation lengths are measured for
phase coherence and density modulation. Both lengths display identical
temperature dependences, with an asymptotic scaling form consistent with a
continuous zero temperature transition. This contrasts with the first order
freezing transition which is seen in the alternative quasi-periodic boundary
conditions. The high temperature perturbation theory and the ground states of
the spherical system suggest that the thermodynamic limit of the spherical
geometry is the same as that on the flat plane. We discuss the advantages and
drawbacks of simulations with different geometries, and compare with current
experimental conclusions. The effect of having a large scale inhomogeneity in
the applied field is also considered.Comment: This replacment contains substantial revisions: the new article is
twice as long with new and different results on the thermodynamic limit on
the sphere plus a full discussion on the alternative boundary conditions used
in simulations in the LLL approximation. 19 pages, 12 encapsulated PostScript
figures, 1 JPEG figure, uses RevTeX (with epsf
Vortex Collisions: Crossing or Recombination?
We investigate the collision of two vortex lines moving with viscous dynamics
and driven towards each other by an applied current. Using London theory in the
approach phase we observe a non-trivial vortex conformation producing
anti-parallel segments; their attractive interaction triggers a violent
collision. The collision region is analyzed using the time-dependent
Ginzburg-Landau equation. While we find vortices will always recombine through
exchange of segments, a crossing channel appears naturally through a double
collision process.Comment: 4 pages, 3 figure
Characteristics of First-Order Vortex Lattice Melting: Jumps in Entropy and Magnetization
We derive expressions for the jumps in entropy and magnetization
characterizing the first-order melting transition of a flux line lattice. In
our analysis we account for the temperature dependence of the Landau parameters
and make use of the proper shape of the melting line as determined by the
relative importance of electromagnetic and Josephson interactions. The results
agree well with experiments on anisotropic YBaCuO and
layered BiSrCaCuO materials and reaffirm the validity of
the London model.Comment: 4 pages. We have restructured the paper to emphasize that in the
London scaling regime (appropriate for YBCO) our results are essentially
exact. We have also emphasized that a major controversy over the relevance of
the London model to describe VL melting has been settled by this wor
Interstitial Fractionalization and Spherical Crystallography
Finding the ground states of identical particles packed on spheres has
relevance for stabilizing emulsions and a venerable history in the literature
of theoretical physics and mathematics. Theory and experiment have confirmed
that defects such as disclinations and dislocations are an intrinsic part of
the ground state. Here we discuss the remarkable behavior of vacancies and
interstitials in spherical crystals. The strain fields of isolated
disclinations forced in by the spherical topology literally rip interstitials
and vacancies apart, typically into dislocation fragments that combine with the
disclinations to create small grain boundary scars. The fractionation is often
into three charge-neutral dislocations, although dislocation pairs can be
created as well. We use a powerful, freely available computer program to
explore interstitial fractionalization in some detail, for a variety of power
law pair potentials. We investigate the dependence on initial conditions and
the final state energies, and compare the position dependence of interstitial
energies with the predictions of continuum elastic theory on the sphere. The
theory predicts that, before fragmentation, interstitials are repelled from
5-fold disclinations and vacancies are attracted. We also use vacancies and
interstitials to study low energy states in the vicinity of "magic numbers"
that accommodate regular icosadeltahedral tessellations.Comment: 21 pages, 9 figure
Evaporation of the pancake-vortex lattice in weakly-coupled layered superconductors
We calculate the melting line of the pancake-vortex system in a layered
superconductor, interpolating between two-dimensional (2D) melting at high
fields and the zero-field limit of single-stack evaporation. Long-range
interactions between pancake vortices in different layers permit a mean-field
approach, the ``substrate model'', where each 2D crystal fluctuates in a
substrate potential due to the vortices in other layers. We find the thermal
stability limit of the 3D solid, and compare the free energy to a 2D liquid to
determine the first-order melting transition and its jump in entropy.Comment: 4 pages, RevTeX, two postscript figures incorporated using eps
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Motivations for innovation in the built environment: new directions for research
Innovation in the built environment involves multiple actors with diverse motivations. Policy-makers find it difficult to promote changes that require cooperation from these numerous and dispersed actors and to align their sometimes divergent interests. Established research traditions on the economics and management of innovation pay only limited attention to stakeholder choices, engagement and motivation. This paper reviews the insights that emerge as research in these traditions comes into contact with work on innovation from sociological and political perspectives. It contributes by highlighting growing areas of research on user involvement in complex innovation, collective action, distributed innovation and transition management. To differing extents, these provide approaches to incorporate the motivations of different actors into theoretical understanding. These indicate new directions for research that promise to enrich understanding of innovation
Disorder Induced Transitions in Layered Coulomb Gases and Superconductors
A 3D layered system of charges with logarithmic interaction parallel to the
layers and random dipoles is studied via a novel variational method and an
energy rationale which reproduce the known phase diagram for a single layer.
Increasing interlayer coupling leads to successive transitions in which charge
rods correlated in N>1 neighboring layers are nucleated by weaker disorder. For
layered superconductors in the limit of only magnetic interlayer coupling, the
method predicts and locates a disorder-induced defect-unbinding transition in
the flux lattice. While N=1 charges dominate there, N>1 disorder induced defect
rods are predicted for multi-layer superconductors.Comment: 4 pages, 2 figures, RevTe
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