Purely even harmonic Josephson current due to crossed pair transmission across strongly spin-polarized materials

TL;DR

This paper demonstrates that in strongly spin-polarized ferromagnetic Josephson junctions, the second harmonic of the current-phase relation arises from a process involving four Cooper pairs, leading to a net charge transfer of 4e.

Contribution

It introduces a detailed diagrammatic analysis showing that the dominant process involves four Cooper pairs, resulting in purely even harmonic Josephson current in strongly spin-polarized systems.

Findings

The second harmonic is generated by a process involving four Cooper pairs.

The net charge transfer across the interface is 4e.

Numerical results confirm the analytical picture in diffusive junctions.

Abstract

We revisit the problem of the second harmonic generation in the current-phase relation across ferromagnetic bilayers placed between BCS superconductors. In particular, we consider a strongly spin-polarized metallic ferromagnet coupled to two superconducting leads via thin spin-active (left) and non-spin-active (right) insulating layers. The system is examined in the framework of the quasiclassical Green$^\prime$s function formalism, both in the ballistic (Eilenberger) and the diffusive (Usadel) limit. Strong spin polarization allows for neglecting short-range mixed-spin correlations, and the Josephson supercurrent across the ferromagnet is fully mediated by long-range equal-spin triplet correlations. Using a diagrammatic technique for ballistic propagators introduced in Refs. [1-3], we describe the relevant Andreev processes responsible for the effective conversion of two spin-singlet Cooper pairs in the superconductor into two $\uparrow\uparrow$ and $\downarrow\downarrow$ pairs in the ferromagnet. Contrary to the naive picture of direct conversion, we show that the lowest order process involves four Cooper pairs in the superconductor, among which three are incoming, and one is outgoing, giving rise to net charge transport of 4e across the non-spin-active interface. The self-consistent numerical treatment of the diffusive junction, typically more relevant in experiments, confirms this picture quantitatively.