Coupling superconducting-ferromagnetic point contacts by Andreev reflections

Abstract

The properties of a device made of two point contacts between normal (N) or ferromagnetic (F) tips, and a superconductor (S), are discussed as a function of the spin polarization and the distance L between the contacts. When L is smaller than the superconductor coherence length ξ, nonlocal Andreev reflections occur: for opposite spin polarizations of the contacts, “mixed” Cooper pairs made of electrons coming one from each tip can be injected into the superconductor. This leads to unusual properties, for instance, the parallel resistance of two S/F contacts goes from infinity for full and equal polarizations, to a finite Andreev value for opposite ones.

Synchronized Andreev Transmission in Chains of SNS Junctions

“…Note that the direct and indirect transmissions here are different from the so-called "elastic co-tunneling" and "crossed Andreev tunneling" [23,30] processes where Bogoliubov quasiparticles tunnel below the gap through a thin (with the width of the order of the Cooper pair size) superconducting layer. The probability of these tunneling processes decreases exponentially if the width of the superconducting layer exceeds the Cooper pair size.…”

“…We discuss the common experimental situation of a diffusive regime where the most of the energy scales are smaller than /τ , where τ is the impurity scattering time. We assume the size, L C , of the central island to be much larger than the superconducting coherence length ξ, hence processes of subgap elastic cotunneling and/or direct Andreev tunneling [23] do not contribute much to the charge transfer. In general, this condition ensures that L C is large enough so that charges do not accumulate in the central island and Coulomb blockade effects are irrelevant for the quasiparticle transport.…”

Crossed conductance in ferromagnet/superconductor/ferromagnet double junctions: Role of out-of-equilibrium populations

“…The limit of equilibrium transport in FSF double junctions where the distance R between the contacts becomes smaller than ξ 9,10,11,12,13,14,15,16,17,18,19,20,21 , has been intensively investigated recently in connection with the determination of the so-called "crossed conductance". A voltage-biased crossed conductance experiment similar to the ones by Beckmann et al 20 in the geometry with lateral contacts on Fig.…”

Noise cross-correlation and Cooper pair splitting efficiency in multi-teminal superconductor junctions

“…A Cooper pair in the superconductor can be break up into two nonlocal entangled electrons that enter into different normal metal leads via the Cooper pair splitting (CPS) [9][10][11][12][13]. The CPS has been studied theoretically and experimentally [14][15][16][17][18], opening a door to test Bell inequalities in the solid state [19,20].…”