Localized Edge States in Stacked Al/Ni Multilayers: Possible Evidence of Chiral Hinge Modes

Abstract: Here, we report experimental evidence suggesting the emergence of robust, possibly chiral, edge states in artificially engineered multilayers composed of alternating nanometer-thick layers of nonmagnetic aluminum (Al) and ferromagnetic nickel (Ni). Using phase-sensitive Josephson interferometry, we observed distinct SQUID-like oscillations (instead of the conventional Fraunhofer patterns) in the maximum supercurrent versus in-plane probing magnetic field patterns, which can be associated with one-dimensional current-carrying modes localized at the sample boundaries. These results were obtained for multilayers consisting of up to ten Al/Ni bilayers sandwiched between superconducting Nb electrodes to form Josephson junctions. The spatially confined flow of supercurrent suggests the possible presence of chiral Andreev edge states reminiscent of those found in higher-order topological insulators, despite the absence of strong spin-orbit coupling or intrinsic topological band structure. The discovery of edge-localized charge transport in structures made of materials without intrinsic topological order challenges the prevailing understanding of topological phenomena and highlights the possibility of developing topological metamaterials as a tunable platform for exploring nontrivial edge physics.