Spin-polarized Josephson and quasiparticle currents in superconducting spin-filter tunnel junctions

TL;DR

This paper theoretically investigates how spin-filtering influences Josephson and quasiparticle currents in superconducting tunnel junctions, revealing new effects and explaining experimental observations related to spin-polarized supercurrents.

Contribution

It introduces a comprehensive theoretical model combining Green's functions and tunneling Hamiltonian to analyze spin-filter effects on superconducting junctions, highlighting the role of equal-spin Cooper pairs.

Findings

Equal-spin Cooper pairs contribute to Josephson current regardless of spin-filtering.

Differential conductance peaks depend on spin-filter polarization and magnetic field orientation.

The model explains experimental results and predicts new spin-polarized supercurrent phenomena.

Abstract

We present a theoretical study of the effect of spin-filtering on the Josephson and dissipative quasiparticle currents in a superconducting tunnel junction. By combining the quasiclassical Green's functions and the tunneling Hamiltonian method we describe the transport properties of a generic junction consisting of two superconducting leads with an effective exchange field h separated by a spin-filter insulating barrier. We show that besides the tunneling of Cooper pairs with total spin-projection Sz = 0 there is another contribution to the Josephson current due to equal-spin Cooper pairs. The latter is finite and not affected by the spin-filter effect provided that the fields h and the magnetization of the barrier are non-collinear . We also determine the quasiparticle current for a symmetric junction and show that the differential conductance may exhibit peaks at different values of the voltage depending on the polarization of the spin-filter, and the relative angle between the exchange fields and the magnetization of the barrier. Our findings provide a plausible explanation for existing experiments on Josephson junctions with magnetic barriers, predict new effects and show how spin-polarized supercurrents in hybrid structures can be created.