Magnon influence on the superconducting density of states in superconductor$-$ferromagnetic-insulator bilayers

TL;DR

This paper explores how magnon interactions at superconductor-ferromagnetic-insulator interfaces alter the spin-split superconducting density of states, revealing complex mixing effects and odd-frequency pairing that extend beyond traditional models.

Contribution

It introduces a theoretical analysis of magnon-mediated electron spin-flip processes and their impact on spin-split superconductivity, including density of states modifications and odd-frequency pairing.

Findings

Magnon interactions cause partial mixing and reconstruction of spin-split quasiparticle branches.

The superconducting density of states shape is significantly altered by magnon effects.

An odd-frequency superconducting order parameter component is identified.

Abstract

Superconductor$-$ferromagnetic-insulator heterostructures are paradigmatic systems for studying the mutual influence of superconductivity and magnetism via proximity effects. In particular, spin-split superconductivity is realized in such structures. Recent experiments and theories demonstrate a rich variety of transport phenomena occurring in devices based on such heterostructures that suggest direct applications in thermoelectricity, low-dissipative spintronics, radiation detection, and sensing. In this work we investigate the influence of the electron-magnon interaction at the superconductor$-$ferromagnetic-insulator interface on the spin-split superconductivity. It is predicted that due to the magnon-mediated electron spin-flip processes the spin-split quasiparticle branches are partially mixed and reconstructed, and the BCS-like spin-split shape of the superconducting density of states, which is typical for superconductors in the effective exchange field, is strongly modified. An odd-frequency superconducting order parameter admixture to the leading singlet order parameter is also found. These findings expand the physical picture of spin-split superconductivity beyond the mean-field description of the ferromagnet exchange field.