Imaging spontaneous currents in superconducting arrays of π - 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, 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, have been demonstrated. In superconducting loops,pi-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 to image the spontaneous zero-field currents in superconducting networks of temperature-controlled pi-junctions with weakly ferromagnetic barriers. We find an onset of spontaneous supercurrents at the 0–pi 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 pi-junctions. Such arrays are attractive model systems for studying the exotic phases of the two-dimensional XY-model and achieving scalable adiabatic quantum computers.