Imaging spontaneous currents in superconducting arrays of pi-junctions

A charge current can flow between two superconductors separated by a thin barrier. This phenomenon is the Josephson effect, which enables a current to tunnel at zero voltage(1), typically with no phase shift between the superconductors in the lowest-energy state. Recently, Josephson junctions with ground-state phase shifts of pi, proposed by theory three decades ago(2), have been demonstrated(3-5). In superconducting loops, pi-junctions cause spontaneous circulation of persistent currents in zero magnetic field(2), in analogy to spin-1/2 systems(6). Here we use a scanning superconducting quantum interference device microscope(7) to image the spontaneous zero-field currents in superconducting networks of temperature-controlled pi-junctions with weakly ferromagnetic barriers(3). We find an onset of spontaneous supercurrents at the 0-pi transition temperature of the junctions, T pi approximate to 3 K. We image the currents in non-uniformly frustrated arrays consisting of cells with even and odd numbers of pi-junctions. Such arrays are attractive model systems for studying the exotic phases of the two-dimensional XY-model(8,9) and achieving scalable adiabatic quantum computers(10).