573 research outputs found
Meissner response of superconductors with inhomogeneous penetration depths
We discuss the Meissner response to a known field source of superconductors
having inhomogeneities in their penetration depth. We simplify the general
problem by assuming that the perturbations of the fields by the penetration
depth inhomogeneities are small. We present expressions for inhomogeneities in
several geometries, but concentrate for comparison with experiment on planar
defects, perpendicular to the sample surfaces, with superfluid densities
different from the rest of the samples. These calculations are relevant for
magnetic microscopies, such as Scanning Superconducting Quantum Interference
Device (SQUID) and Magnetic Force Microscope, which image the local diamagnetic
susceptibility of a sample.Comment: 9 pages, 4 figure
Determining the vortex tilt relative to a superconductor surface
It is of interest to determine the exit angle of a vortex from a
superconducting surface, since this affects the intervortex interactions and
their consequences. Two ways to determine this angle are to image the vortex
magnetic fields above the surface, or the vortex core shape at the surface. In
this work we evaluate the field h(x, y, z) above a flat superconducting surface
x, y and the currents J(x,y) at that surface for a straight vortex tilted
relative to the normal to the surface, for both the isotropic and anisotropic
cases. In principle, these results can be used to determine the vortex exit
tilt angle from analyses of magnetic field imaging or density of states data.Comment: 5 pages, 4 figure
Three-dimensional analysis of the Pratt and Whitney alternate design SSME fuel turbine
The three dimensional viscous time-mean flow in the Pratt and Whitney alternate design space shuttle main engine fuel turbine is simulated using the average passage Navier-Stokes equations. The migration of secondary flows generated by upstream blade rows and their effect on the performance of downstream blade rows is studied. The present simulation confirms that the flow in this two stage turbine is highly three dimensional and dominated by the tip leakage flow. The tip leakage vortex generated by the first blade persists through the second blade and adversely affects its performance. The greatest mixing of the inlet total temperature distortion occurs in the second vane and is due to the large leakage vortex generated by the upstream rotor. It is assumed that the predominant spanwise mixing mechanism in this low aspect ratio turbine is the radial transport due to the deterministically unsteady vortical flow generated by upstream blade rows. A by-product of the analysis is accurate pressure and heat loads for all blade rows under the influence of neighboring blade rows. These aero loads are useful for advanced structural analysis of the vanes and blades
Direct Investigation of Superparamagnetism in Co Nanoparticle Films
A direct probe of superparamagnetism was used to determine the complete
anisotropy energy distribution of Co nanoparticle films. The films were
composed of self-assembled lattices of uniform Co nanoparticles 3 nm or 5 nm in
diameter, and a variable temperature scanning-SQUID microscope was used to
measure temperature-induced spontaneous magnetic noise in the samples. Accurate
measurements of anisotropy energy distributions of small volume samples will be
critical to magnetic optimization of nanoparticle devices and media.Comment: 4 pages, 4 figures. Submitted to Physical Review Letter
Ground state and bias current induced rearrangement of semifluxons in 0-pi long Josephson junctions
We investigate numerically a long Josephson junction with several phase
pi-discontinuity points. Such junctions are usually fabricated as a ramp
between an anisotropic cuprate superconductor like YBCO and an isotropic metal
superconductor like Nb. From the top, they look like zigzags with pi-jumps of
the Josephson phase at the corners. These pi-jumps, at certain conditions, lead
to the formation of half-integer flux quanta, which we call semifluxons (SF),
pinned at the corners. We show (a) that the spontaneous formation of SFs
depends on the junction length, (b) that the ground state without SFs can be
converted to a state with SFs by applying a bias current, (c) that the SF
configuration can be rearranged by the bias current. All these effects can be
observed using a SQUID microscope.Comment: ~8 pages, 6 figures, submitted to PR
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