Planar Josephson junctions fabricated with magnesium diboride films

Josephson junctions were fabricated in three planar configurations using a focused ion beam (FIB) to cut ≥70 nm gaps in MgB₂ bridges. In two of the approaches - narrowed microbridges where a 0.1 μm bridge was left in place after the FIB cut or thinned microbridges where a 20-40 nm-thick film layer remained uncut - we reproduced junction results obtained by other researchers but with I_c(T)>0 at substantially higher temperatures, >32 K. Measurements were made of critical current modulation in an applied magnetic field and I-V curves were measured with the chips exposed to 1-10 GHz radiation. In the third configuration, S-N-S structures were made by filling the gap made by the FIB with a noble metal. This configuration is preferred to S-S´-S since the normal-conductor coherence length of 40-100 nm for a clean metal roughly matches the width of the FIB cut. Junction R_n measurements showed that ex-situ Au deposited after a low-energy argon ion cleaning was not as effective as in-situ Pt deposition for obtaining low-resistance S-N interfaces.