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.