Coherent subgap transport in spin-split Josephson junctions

TL;DR

This paper presents the first experimental observation of subgap transport and multiple Andreev reflection in EuS/Al/AlOx/Al Josephson junctions, revealing spin-splitting effects and highly transparent conduction channels.

Contribution

It demonstrates that a single spin-split superconductor can exhibit even-odd MAR effects and characterizes the transmission channels using combined experiments and modeling.

Findings

Observation of split Andreev reflection peaks due to spin-splitting.

Identification of highly transparent conduction channels (~23%).

Confirmation of Josephson current through these channels.

Abstract

We report the first experimental observation of subgap transport in ferromagnetic insulator/superconductor/insulator/superconductor junctions realized in EuS/Al/AlOx/Al vertical stacks. Differential conductance measurements reveal multiple Andreev reflection peaks, with odd-order peaks split by the spin-splitting induced in the superconductor adjacent to EuS, while even-order peaks remain unaffected. Combining experiments with quasiclassical transport modeling, we extract the spin-splitting and the distribution of transmission channels, finding that a significant fraction ($\sim 23\%$) of highly transparent channels ($\tau \approx 0.9$) dominates transport. The observation of a Josephson current further confirms strong superconducting coupling through these channels. Our results demonstrate that a single spin-split superconductor is sufficient to observe the even-odd MAR effect. Our work establishes EuS/Al junctions as a versatile platform to study subgap transport, Josephson coupling, and spin-polarized superconducting phenomena.