Superconductivity-enhanced spin pumping: Role of Andreev resonances

TL;DR

This paper investigates how spin-resolved Andreev bound states in a superconductor|ferromagnetic-insulator structure enhance spin pumping, revealing resonant subgap channels that act as a Fabry-Pérot resonator for spin transport.

Contribution

It demonstrates the role of Andreev resonances in boosting spin transfer in superconductor|ferromagnetic-insulator hybrids, highlighting their impact on spin flow physics.

Findings

Spin-resolved Andreev bound states affect spin angular momentum transfer.

Resonant Andreev states create highly transmitting subgap spin channels.

Resonance acts as a Fabry-Pérot resonator for spin pumping.

Abstract

We describe a simple hybrid superconductor$|$ferromagnetic-insulator structure manifesting spin-resolved Andreev bound states in which dynamic magnetization is employed to probe spin related physics. We show that, at low bias and below $T_c$, the transfer of spin angular momentum pumped by an externally driven ferromagnetic insulator is greatly affected by the formation of spin-resolved Andreev bound states. Our results indicate that these bound states capture the essential physics of condensate-facilitated spin flow. For finite thicknesses of the superconducting layer, comparable to the coherence length, resonant Andreev bound states render highly transmitting subgap spin transport channels. We point out that the resonant enhancement of the subgap transport channels establishes a prototype Fabry-P\'erot resonator for spin pumping.