High frequency properties of all-NbN nanobridges with gap structure in I-V curves

All-NbN nanobridges with gap structure in I-V curves have been reproducibly constructed using RIE and lift-off techniques. The nanobridges had a width of 2 μm, and a thickness of < 30 nm. The length or nanobridge was about of the order of 3 to 5 coherence length of epitaxial NbN films. The nanobridges had nearly ideal characteristics: sharply defined critical current, high resistance, well-defined gap structure at about 4 mV, large IcRNproducts of ∼3 mV, and low excess current. Small-area dc SQUIDs were made using the nanobridges, and analyses of the response to magnetic flux were performed. The current-phase relationship of the nano-bridges was found to be close to sinusoidal. The maximum LC resonant voltage was about 1.2 mV, corresponding to a frequency of 580 GHz. The IF peak was obtained up to the bias voltage of about 4 mV in 101 GHz Josephson mixing.