Second-harmonic current-phase relation in Josephson junctions with ferromagnetic barriers

TL;DR

This paper demonstrates the dominance of the second harmonic in the current-phase relation of superconductor-ferromagnet-superconductor junctions near a 0-$\

Contribution

It provides experimental evidence of a dominant second harmonic in Josephson junctions with ferromagnetic barriers, confirming theoretical predictions and revealing new physics in these systems.

Findings

Second harmonic dominates near the 0-$\

Half-integer Shapiro steps observed at the transition

Intrinsic second harmonic confirmed, ruling out alternative explanations

Abstract

We report the observation of a current-phase relation dominated by the second Josephson harmonic in superconductor-ferromagnet-superconductor junctions. The exotic current-phase relation is realized in the vicinity of a temperature-controlled 0-to-$\pi$ junction transition, at which the first Josephson harmonic vanishes. Direct current-phase relation measurements, as well as Josephson interferometry, non-vanishing supercurrent and half-integer Shapiro steps at the 0-$\pi$ transition self-consistently point to an intrinsic second harmonic term, making it possible to rule out common alternative origins of half-periodic behavior. While surprising for diffusive multimode junctions, the large second harmonic is in agreement with theory predictions for thin ferromagnetic interlayers.