Reduced Critical Current Spread in Planar MgB2 Josephson Junction Array Made by Focused Helium Ion Beam

We have fabricated series arrays of closely spaced planar Josephson junctions on MgB2 films using a 30 keV focused helium ion beam. Uniformity of junction parameters within the arrays is sufficient for achieving phase-lock into an applied microwave signal, and flat giant Shapiro steps are observed. The spread in critical current of a 60-Josephson junction array is estimated to be less than 3.5%, significantly better than reported in MgB2 junctions fabricated by other techniques. These results demonstrate the potential of the focused He
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ion beam irradiation technique in MgB2 Josephson multi-junction circuit applications such as quantum voltage standards.