Long range statistical fluctuations of the crossed Josephson current

TL;DR

This paper studies the statistical fluctuations of the crossed Josephson current in a ferromagnetic-superconductor junction, revealing long-range coherence effects mediated by fluctuations that depend on the ferromagnet's phase coherence length.

Contribution

It demonstrates that the root mean square of the crossed Josephson current scales with the square root of the junction area and can be long-range due to fluctuations, extending beyond the exchange length.

Findings

Root mean square of current proportional to sqrt of area

Long-range coherence mediated by fluctuations over phase coherence length

Predicts fluctuation-induced Josephson current under specific length conditions

Abstract

We investigate the crossed Josephson effect in a geometry consisting of a double ferromagnetic bridge between two superconductors, with tunnel interfaces. The crossed Josephson current vanishes on average because the Andreev reflected hole does not follow the same sequence of impurities as the incoming electron. We show that i) the root mean square of the crossed Josephson current distribution is proportional to the square root of the junction area; and ii) the coherent coupling mediated by fluctuations is ``long range'' since it decays over the ferromagnet phase coherence length $l_\phi$, larger than the exchange length. We predict a crossed Josephson current due to fluctuations if the length of the ferromagnets is smaller than $l_\phi$ and larger than the exchange length $\xi_h$.