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

Abstract

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.