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Persistent current in small superconducting rings

Yuval Oreg - Weizmann Institute, Israel

We study theoretically the contribution of fluctuating Cooper pairs to the persistent current in superconducting rings threaded by a magnetic flux. For sufficiently small rings, in whichthe coherence length $\xi$ exceeds the radius $R$, mean field theory predicts a full reduction of the transition temperature to zero near half-integer flux. We find that nevertheless a very large current is expected to persist in the ring as a consequence of Cooper pair fluctuations that do not condense. For larger rings with $R\gg \xi$ we calculate analytically the susceptibility in the critical region of
strong fluctuations and show that it reflects competition of two
interacting complex order parameters.


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