Coherent effects in double-barrier ferromagnet/superconductor/ferromagnet junctions

TL;DR

This paper investigates quantum transport in ferromagnet/superconductor/ferromagnet junctions, deriving analytical conductance spectra and revealing effects like Andreev reflection oscillations and spin polarization without accumulation.

Contribution

It provides the first analytical expressions for charge and spin conductance in coherent FSF junctions considering various interface and alignment conditions.

Findings

Periodic vanishing of Andreev reflection at geometrical resonances.

Non-trivial spin polarization in antiparallel alignment without excess spin accumulation.

Oscillations of differential conductance due to quasiparticle interference.

Abstract

Coherent quantum transport in ferromagnet/superconductor/ferromagnet (FSF) double-barrier junctions is studied. Analytic expressions for charge and spin conductance spectra are derived for the general case of insulating interfaces (from metallic to tunnel limit), the Fermi velocity mismatch, and for parallel (P) and antiparallel (AP) alignment of the electrode magnetizations. We focus on two characteristic features of finite size and coherency: subgap electronic transport, 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. In contrast with the case of incoherent transport, a non-trivial spin-polarization without the excess spin accumulation is found for the AP alignment.