Spin-polarized currents in superconducting films

TL;DR

This paper develops a microscopic theory for spin-polarized quantum transport in superconducting films with ferromagnetic electrodes, revealing interference effects, conductance oscillations, and spin polarization phenomena.

Contribution

It introduces a comprehensive model for FSF junctions considering various interface transparencies and coherency effects, highlighting the impact on conductance and spin polarization.

Findings

Periodic vanishing of Andreev reflection at geometrical resonances

Oscillations of differential conductance due to quasiparticle interference

Non-trivial spin polarization in antiparallel magnetic alignment

Abstract

We present a microscopic theory of coherent quantum transport through a superconducting film between two ferromagnetic electrodes. The scattering problem is solved for the general case of ferromagnet/superconductor/ferromagnet (FSF) double-barrier junction, including the interface transparency from metallic to tunnel limit, and the Fermi velocity mismatch. Charge and spin conductance spectra of FSF junctions are calculated for parallel (P) and antiparallel (AP) alignment of the electrode magnetization. Limiting cases of nonmagnetic normal-metal electrodes (NSN) and of incoherent transport are also presented. We focus on two characteristic features of finite size and coherency: subgap tunneling of electrons, and oscillations of the differential conductance. Periodic vanishing of the Andreev reflection at the energies of geometrical resonances above the superconducting gap is a striking consequence of the quasiparticle interference. Also, the non-trivial spin-polarization of the current is found for FSF junctions in AP alignment. This is in contrast with the incoherent transport, where the unpolarized current is accompanied by excess spin accumulation and destruction of superconductivity. Application to spectroscopic measurements of the superconducting gap and the Fermi velocity is also discussed.