Evidence for non-trivial edge modes in binary multilayers formed by normal and magnetic nm-thick metallic films

Abstract

Abstract Unidirectional and backscatter-free propagation of charge carriers in three-dimensional materials is of fundamental interest in physics and high demand for practical implementations in quantum electronic circuits. Using phase-sensitive Josephson interferometry, we have identified edge-localized and chiral current-carrying modes in binary heterostructures formed by alternating nanometer-thick films of ferromagnetic (F = Ni) and normal (N = Al) metals with an insulating (I = Al/AlOx) interlayer which are coupling two superconducting (S = Nb) electrodes. Instead of conventional Fraunhofer-like patterns, the vertically stacked S(NF)10NI(NF)10NS Josephson junctions exhibit SQUID-like maximum supercurrent versus in-plane magnetic field characteristics with a normal (h/e) rather than superconducting (h/2e) flux quantum periodicity. The subsequent replacement of the I interlayer with a Josephson S'IS' junction (S/ is an ultrathin Nb film), which acts as a “bottleneck” controlling the supercurrent throughout the entire device, revealed the transformation of edge currents upon their contact with the S' plane into hybrid electron-hole modes propagating along it. Possible reasons for the emergence of chiral edge currents in the heterostructures based on conventional and well-studied materials are discussed.