Imaging spontaneous currents in superconducting arrays of p-junctions

A charge current can flow between two superconductors separated by a thin barrier. This phenomenon is the Josephson e ff ect, which enables a current to tunnel at zero voltage , typically with no phase shift between the superconductors in the lowest-energy state. Recently, Josephson junctions with ground-state phase shifts of ?, proposed by theory three decades ago 2 , have been demonstrated . In superconducting loops, ?-junctions cause spontaneous circulation of persistent currents in zero magnetic field, in analogy to spin-1/2 systems . Here we use a scanning superconducting quantum interference device microscope 7 to image the spontaneous zero-field currents in superconducting networks of temperature-controlled ?-junctions with weakly ferromagnetic barriers. We find an onset of spontaneous supercurrents at the 0-? transition temperature of the junctions, T ? ? 3 K. We image the currents in non-uniformly frustrated arrays consisting of cells with even and odd numbers of ?-junctions. Such arrays are attractive model systems for studying the exotic phases of the two-dimensional XY-model and achieving scalable adiabatic quantum computers.