Ferromagnetic 0-pi Josephson junctions

TL;DR

This paper reports on the fabrication and analysis of 0-$$ Josephson junctions with ferromagnetic layers, demonstrating controllable ground states and vortex phenomena relevant for quantum computing applications.

Contribution

It introduces optimized fabrication of high-quality 0-$$ junctions with variable ferromagnetic layer thickness, enabling the study of degenerate ground states and vortex behavior.

Findings

Successful fabrication of homogeneous SIFS junctions

Observation of 0 and  ground states depending on parameters

Detection of vortex states carrying fractional magnetic flux

Abstract

We present a study on low-$T_c$ superconductor-insulator-ferromagnet-superconductor (SIFS) Josephson junctions. SIFS junctions have gained considerable interest in recent years because they show a number of interesting properties for future classical and quantum computing devices. We optimized the fabrication process of these junctions to achieve a homogeneous current transport, ending up with high-quality samples. Depending on the thickness of the ferromagnetic layer and on temperature, the SIFS junctions are in the ground state with a phase drop either 0 or $\pi$. By using a ferromagnetic layer with variable step-like thickness along the junction, we obtained a so-called 0-$\pi$ Josephson junction, in which 0 and $\pi$ ground states compete with each other. At a certain temperature the 0 and $\pi$ parts of the junction are perfectly symmetric, i.e. the absolute critical current densities are equal. In this case the degenerate ground state corresponds to a vortex of supercurrent circulating clock- or counterclockwise and creating a magnetic flux which carries a fraction of the magnetic flux quantum $\Phi_0$.