Current-phase relationship of granular NbN weak links, inferred from Josephson interferometer characteristics

Small-area dc superconducting quantum interference devices (SQUID’s) were made using ultra-short variable-thickness microbridges of NbN. The bridges had an effective length of ∼500 Å and a width of ∼1.5 μm. Analysis of the response to magnetic flux permits interferences to be drawn about the current-phase relationship (CPR) of the bridges. Contrary to predictions of Ginzburg–Landau theory for microbridges of these dimensions, it is found that the CPR is single valued and probably close to ideal (sinusoidal) over a large temperature range (≳2.5 K). The discrepancy with theory may be due to the granular nature of the NbN films.