Josephson junctions and arrays made by focused helium ion beam*

Planar Josephson junctions and series arrays were fabricated in magnesium diboride (MgB₂) thin films grown by hybrid physical-chemical vapor deposition. The junction barrier is created by locally damaging the crystal structure of the MgB₂ with a 30 keV He⁺ ion beam focused to a diameter < 1 nm. Prior irradiation experiments over large areas showed a critical dose of 8x10¹⁵/cm² for complete T_c suppression. Single-track irradiation results in Josephson coupling across the damaged region for a narrow dose window between 0.8–4x10¹⁶/cm². All junctions in this window show resistively-shunted I-V behavior, Shapiro steps under microwave radiation and Fraunhofer-pattern modulation of the critical current in magnetic field at temperatures as high as 26 K, indicative of highly uniform barrier properties. A 10-junction series array shows giant Shapiro steps and modulation of critical current in magnetic field. Analysis on a 30-junction series array shows a spread in critical current of ~12%, drastically lower than spreads reported in MgB2 junctions fabricated by other techniques. This work demonstrates the potential of the focused He⁺ ion beam damage technique in MgB₂ Josephson multi-junction circuit applications.