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I have been confused over the wording used in this paper describing RSFQ logic circuits: http://www.mukhanov.com/uploads/LikharevSemenov-RSFQReview_IEEETAS91.pdf

From page 6: " If such a single flux quantum enters the array across an edge junction, it has no other choice than to leave it by crossing all the junctions in turn (flux crossing of the superconducting leads connecting the junctions is forbidden by the Meissner effect, and small inductances L of the leads make it impossible for the flux to be trapped in the loops of the array)."

The issue I have is the claim that flux cannot cross the superconducting leads. I realize the Meissner effect prohibits flux penetration through the bulk superconductor, but what about a ring of superconducting material with a non-superconducting core (air for example). The quote seems to imply that the flux should be conceptualized as maybe a very long string (with the ends eventually forming a loop) which is literally trapped by the superconducting ring.

In the literature there are lots of papers describing superconducting transformers (so I know they exist and function) … however the quote above seems to imply that a superconducting transformer would not allow a flux to thread into the secondary loop (the string would be "stuck" outside of the secondary). In this case, the operation of the transformer would be fundamentally different, i.e. the current in the secondary would not be caused by a brief EMF due to change in flux which causes the current to start moving and persist indefinitely due to zero resistance … rather the current in the secondary would be created to directly oppose, and even prevent, any flux from entering the loop.

In trying to answer this question I have attempted to find whether or not Abrikosov vortices can only form at the edge of the bulk superconductor and then migrate into the middle. Because if Abrikosov vortices can nucleate in the center of a bulk superconductor (as opposed to forming at the edge and migrating to the center), then it seems like flux should be able to "thread" a superconducting loop.

Apologies for any misterminology … I am a simple engineer

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