Signature of odd-frequency pairing correlations induced by a magnetic interface

TL;DR

This paper demonstrates how magnetic interfaces induce a transition from even- to odd-frequency pairing correlations in superconductor-normal metal systems, revealing a robust method to generate odd-frequency superconductivity.

Contribution

It provides analytical and numerical evidence of odd-frequency pairing induced by magnetic interfaces, including the effects of various scattering mechanisms and the inverse proximity effect.

Findings

Conversion from even- to odd-frequency pairing at a critical spin-polarization

Enhancement of density of states above the normal state near the Fermi level

Robustness of odd-frequency correlations against scattering and interface parameters

Abstract

We investigate the mutual proximity effect in a normal metal contacted to a superconductor through a magnetic interface. Analytical and self-consistent numerical results are presented, and we consider both the diffusive and ballistic regimes. We focus on the density of states in both the normal and superconducting region, and find that the presence of spin-dependent phase-shifts occurring at the interface qualitatively modifies the density of states. In particular, we find that the proximity-induced pairing amplitudes in the normal metal region undergo a conversion at the Fermi level from pure even-frequency to odd-frequency. Above a critical value of the interface spin-polarization (or, equivalently, for fixed interface spin-polarization, above a critical interface resistance), only odd frequency correlations remain. This is accompanied by the replacement of the familiar proximity minigap or pseudogap in the normal layer by an enhancement of the density of states above its normal state value for energies near the chemical potential. The robustness of this effect towards inelastic scattering, impurity scattering, and the depletion of the superconducting order parameter close to the interface is investigated. We also study the inverse proximity effect in the diffusive limit. We find that the above-mentioned conversion persists also for thin superconducting layers comparable in size to the superconducting coherence length $\xi_\text{S}$, as long as the inverse proximity effect is relatively weak. Concomitantly, we find a shift in the critical interface resistance where the pairing conversion occurs. Our findings suggest a robust and simple method for producing purely odd-frequency superconducting correlations, that can be tested experimentally.