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Coherent Subgap Transport in Spin-Split Josephson Junctions
David Caldevilla-Asenjo1, Gorm Ole Steffensen2, Sara Catalano1,3
1Centro de Fisica de Materiales, CSIC-UPV/EHU, 20018 Donostia-San Sebastian, Spain.
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 an approximate distribution of transmission channels, finding that roughly a hundred highly transmitting (τ>0.3) channels will dominate 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 multiple Andreev reflection effect. Our work establishes EuS/Al junctions as a versatile platform to study subgap transport, Josephson coupling, and spin-polarized superconducting phenomena.
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