Triplet Cooper pairs induced in diffusive s-wave superconductors interfaced with strongly spin-polarized magnetic insulators or half-metallic ferromagnets

TL;DR

This paper investigates how spin-triplet Cooper pairs form in diffusive s-wave superconductors when interfaced with strongly spin-polarized magnetic materials, revealing their impact on the superconductor's properties and explaining recent experimental observations.

Contribution

It provides a comprehensive theoretical framework for understanding the influence of triplet pairs on superconductor properties in hybrid structures with magnetic insulators or half-metals.

Findings

Reproduces large zero-energy DOS peaks observed experimentally.

Shows triplet pairs significantly affect the superconductor's critical temperature.

Suggests some experimental effects are not solely due to long-range triplet proximity.

Abstract

Interfacing superconductors with strongly spin-polarized magnetic materials opens the possibility to discover new spintronic devices in which spin-triplet Cooper pairs play a key role. Motivated by the recent derivation of spin-polarized quasiclassical boundary conditions capable of describing such a scenario in the diffusive limit, we consider the emergent physics in hybrid structures comprised of a conventional s-wave superconductor (e.g. Nb, Al) and either strongly spin-polarized ferromagnetic insulators (e.g. EuO, GdN) or halfmetallic ferromagnets (e.g. CrO2, LCMO). In contrast to most previous works, we focus on how the superconductor itself is influenced by the proximity effect, and how the generated triplet Cooper pairs manifest themselves in the self-consistently computed density of states (DOS) and the superconducting critical temperature Tc. We provide a comprehensive treatment of how the superconductor and its properties are affected by the triplet pairs, demonstrating that our theory can reproduce the recent observation of an unusually large zero-energy peak in a superconductor interfaced with a half-metal, which even exceeds the normal-state DOS. We also discuss the recent observation of a large superconducting spin-valve effect with a Tc change ~1K in superconductor/half-metal structures, in which case our results indicate that the experiment cannot be explained fully by a long-ranged triplet proximity effect.