Spontaneous Currents in Josephson Devices

TL;DR

This paper theoretically investigates spontaneous supercurrents in Josephson devices with ferromagnetic weak links between d-wave superconductors, revealing conditions for unconventional coupling, spontaneous currents, and anomalous flux quantization.

Contribution

It introduces a theoretical model for unconventional Josephson coupling in ferromagnetic weak links between d-wave superconductors, highlighting new conditions for spontaneous currents and flux quantization.

Findings

Unconventional coupling occurs at small misorientations with strong ferromagnetic barriers.

Spontaneous supercurrents depend on reduced inductance and appear above a critical value.

Anomalous flux quantization is observed for large inductance values.

Abstract

The unconventional Josephson coupling in a ferromagnetic weak link between d-wave superconductors is studied theoretically. For strong ferromagnetic barrier influence, the unconventional coupling, with ground state phase difference across the link $0<\phi_{\rm gs}\leq \pi$, is obtained at small crystal misorientation of the superconducting electrodes, in contrast to the case of normal metal barrier, where it appears at large misorientations. In both cases, with decreasing temperature there is an increasing range of misorientations, where $\phi_{\rm gs}$ varies continuously between 0 and $\pi$. When the weak link is a part of a superconducting ring, this is accompanied by the flow of spontaneous supercurrent, of intensity which depends (for a given misorientation) on the reduced inductance $l=2\pi LI_c(T)/\Phi_0$, and is non-zero only for $l$ greater than a critical value. For $l\gg 1$, another consequence of the unconventional coupling is the anomalous quantization of the magnetic flux.