Abstract
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 voltage1, typically with no phase shift between the superconductors in the lowest-energy state. Recently, Josephson junctions with ground-state phase shifts of π, proposed by theory three decades ago2, have been demonstrated3,4,5. In superconducting loops, π-junctions cause spontaneous circulation of persistent currents in zero magnetic field2, in analogy to spin-1/2 systems6. Here we use a scanning superconducting quantum interference device microscope7 to image the spontaneous zero-field currents in superconducting networks of temperature-controlled π-junctions with weakly ferromagnetic barriers3. We find an onset of spontaneous supercurrents at the 0–π transition temperature of the junctions, Tπ≈3 K. We image the currents in non-uniformly frustrated arrays consisting of cells with even and odd numbers of π-junctions. Such arrays are attractive model systems for studying the exotic phases of the two-dimensional XY-model8,9 and achieving scalable adiabatic quantum computers10.
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Acknowledgements
Research on π-Josephson junctions was supported by the National Science Foundation grant EIA-01-21568, by the US Civilian Research and Development Foundation (CRDF) grant RUP1-2691-CG-05 and by the Russian Foundation for Basic Research. The SSM imaging was supported by the Department of Energy Office of Basic Energy Sciences (DOE-BES) grant DEFG02-91-ER45439 through the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign.
Measurements, simulations and analysis were carried out by the Urbana group; arrays were fabricated by the Chernogolovka group; d.c. SQUID detectors were fabricated by the Naples group.
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Frolov, S., Stoutimore, M., Crane, T. et al. Imaging spontaneous currents in superconducting arrays of π-junctions. Nature Phys 4, 32–36 (2008). https://doi.org/10.1038/nphys780
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DOI: https://doi.org/10.1038/nphys780
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