Necessary conditions for spin-resolved Josephson diode effect across strongly spin-polarized magnetic materials

TL;DR

This paper establishes the necessary conditions for observing charge and spin Josephson diode effects in junctions with strongly spin-polarized magnetic materials, emphasizing the roles of spin texture noncoplanarity, band contributions, and higher harmonics.

Contribution

It provides the first set of necessary conditions and a phenomenological model for the spin-resolved Josephson diode effect in magnetic superconductor junctions.

Findings

Noncoplanarity of spin texture is essential.

Both spin bands must contribute to transport.

Higher harmonics are required in the CPR.

Abstract

We present a set of necessary conditions for the appearance of charge and spin Josephson diode effects across strongly spin-polarized inhomogeneous magnetic materials (FM) placed between two spin-singlet superconductors. Noncoplanarity of the FM's spin texture gives rise to quantum geometric phases, $\Delta\varphi'$, that enter the Josephson current-phase relation (CPR) similarly to the superconducting phase difference, resulting in charge and spin Josephson diode effects. Our study shows that such effects appear if the CPR possesses no phase-inversion center, achieved under the following conditions. First, noncoplanarity of the spin texture is necessary to break the spatial inversion symmetry. Second, both spin bands have to contribute to the transport, i.e., the effect is absent in half-metallic junctions. Third, different band-specific densities of states are required, and this condition is ensured by the strong spin polarization of the FM. Finally, higher harmonics in the CPR are necessary, i.e., the effect is absent in the tunneling limit. However, even in this case, the CPR must not have a phase-inversion center, which is ensured by the restriction of the quantum geometric phase to values $\Delta\varphi'\neq k\pi/2, k\in\mathbb{Z}$. We formulate a minimal phenomenological model that incorporates all these points, qualitatively illustrating our theory.