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Calculated current stream lines for the cases of (a) a bend and (b) a suddenly widening “stub” region. The increased density of stream lines near the inner corners shows the current crowding. The dashed curve in (a) represents the median contour: half the integrated current flows on one side of the contour and half flows on the other side. An inner corner shaped according to this contour will exhibit no current crowding and is said to be optimally rounded.
SEM images of test structures measured in these experiments: (a) a straight line segment; (b) an optimally rounded bend; (c) a bend; and (d) a suddenly widening “stub” region. The minimum inner radius of curvature achievable in our fabrication process was approximately 60 nm. The light spot visible on (d) is due to charging after zooming in on the wire-contact transition region.
Histograms of 1000 measured critical currents each, for 9 distinct devices: (a) straight lines (3 devices) 1-μm wide; (b) corners with optimally designed inner corner curves (2 devices, straight segments were also 1-μm wide); (c) corners as in (b), but in which the inner corner was fabricated to be as sharp as possible (3 devices); and (d) lines with a wider region (stub) extending 1 μm to one side (1 device). The bin width is 0.5 μA.
Temperature dependence of (a) the critical current and (b) the width of the critical current distribution for straight wires (black squares), optimally rounded corners (blue circles), sharp corners (red triangles), and a wire with a stub (green diamonds). The dashed curve in (a) is a fit following Kupriyanov and Lukichev.7
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