Josephson effect in S/F/S junctions: spin bandwidth asymmetry vs. Stoner exchange

TL;DR

This paper investigates the dc Josephson effect in S/F/S junctions, comparing spin bandwidth asymmetry and Stoner exchange mechanisms, revealing differences in $0- ext{pi}$ transitions and critical currents through theoretical analysis.

Contribution

It provides a general formula for Andreev levels considering arbitrary parameters and compares the phase diagrams for different ferromagnetic mechanisms.

Findings

More $0- ext{pi}$ transitions occur with spin bandwidth asymmetry.

Larger critical currents are associated with bandwidth asymmetry.

Ground state phase difference can differ between mechanisms even with equal polarization.

Abstract

We analyze the dc Josephson effect in a ballistic superconductor/ferromagnet/superconductor junction by means of the Bogoliubov-de Gennes equations in the quasiclassical Andreev approximation. We consider the possibility of ferromagnetism originating from a mass renormalization of carriers of opposite spin, i.e. a spin bandwidth asymmetry. We provide a general formula for Andreev levels which is valid for arbitrary interface transparency, exchange interaction, and bandwidth asymmetry. We analyze the current-phase relation, the critical current, and the free energy in the short junction regime. We demonstrate that a larger number of $0-\pi$ transitions caused by a change in junction width or polarization magnitude is expected when ferromagnetism is driven by spin bandwidth asymmetry compared to Stoner magnetism, and that the same mechanism is likely responsible for the flowing of a larger critical current. We compare the phase diagrams of two identical junctions differing only in the mechanism by which the mid layer becomes magnetic, i.e. Stoner magnetism or bandwidth asymmetry. In particular, we point out that for these two cases, the phase difference across the junction in the ground state need not be the same, even if the polarizations are equal.