We analyze the fractal dimension,
density, tortuosity, and size distribution of
microscopic vortex channels in a variety
of samples, as well as the voltage noise spectrum.
Weaker pinning produces relatively straight channels
while stronger pinning results in the
formation of one or more winding channels that carry all flow,
with each case producing a different voltage noise signature.
A remarkable drop in tortuosity
and noise power appears as a function of increasing disorder strength
when a transition in the channel behavior
occurs and the vortex-vortex interactions become less important.
These figures show the paths followed through the sample by the
vortices during a certain time interval.
Vortices enter on the left side of each figure and move towards
the right.
White areas of the sample contain vortices that were never depinned.
Pale blue areas of the sample indicate regions through which only
a few vortices moved. Dark blue areas of the sample were heavily
traveled by the vortices.
At very low pinning strengths, vortices flow throughout the
sample in channels that braid together. Tortuosity and noise
are low in these samples.
At moderately low pinning strengths, the channels become more well
defined, but still braid significantly. Tortuosity and noise in
these samples both increase.
At moderately high pinning strengths, the width of the channels
begins to collapse, and the amount of braiding in the channels
decreases. Tortuosity and noise both decrease.
At high pinning strengths, the main channels are very well defined
and isolated, with little braiding. Tortuosity and noise
are low.
C. J. Olson, C. Reichhardt, and F. Nori,
Phys. Rev. Lett. 80, 2197
(1998).
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Created by: Cynthia Olson Reichhardt
Last Modified: 1/1/02