Spontaneous currents and half-integer Shapiro steps in Superconductor-Ferromagnet-Superconductor 0-pi Josephson junctions

TL;DR

This paper investigates Nb-CuNi-Nb Josephson junctions with spatially varying barriers, revealing spontaneous currents and half-integer Shapiro steps due to 0-pi interface effects and phase-locking phenomena.

Contribution

It demonstrates the existence of spontaneous circulating currents and half-integer Shapiro steps caused by spatial variations in the junction barrier, a novel insight into 0-pi junction behavior.

Findings

Critical current density changes sign along the junction width.

Spontaneous circulating currents occur at 0-pi interfaces.

Half-integer Shapiro steps are explained by phase-locking of these currents.

Abstract

We study Superconductor-Ferromagnet-Superconductor (Nb-Cu0.47 Ni0.53-Nb) Josephson junctions with spatial variations in the barrier thickness. Critical current vs. magnetic flux diffraction patterns indicate that the critical current density changes sign along the width of the junctions, creating interfaces between 0 and pi junction regions around which spontaneous currents can circulate. Shapiro steps observed at half-integer Josephson voltages can be explained by the phase-locking of the spontaneous circulating currents to the applied rf modulation.