All-NbN nanobridges as Josephson mixers

Using RIE (reactive in etching) and lift-off techniques, all-NbN nanobridges with gap structure in the I-V (current-voltage) curves have been reproducibly constructed for testing as millimeter-wave mixers. The nanobridges were constructed on NbN/MgO/NbN edge junctions. They were characterized by measurements of their I-V curve and dI/dV versus V and by mixing experiments at 105.9 GHz. These devices had high normal-state resistance, and showed almost ideal Josephson response to external magnetic flux and to millimeter-wave irradiation. The I -V curve remained nonhysteretic from T_c to 4.2 K, and millimeter-wave-induced steps in the I-V curve were observed up to ~3 mV. The cooling of the NbN nanobridges was quite good due to their small size and VTB geometry and the high thermal conductivity of the MgO films. Hence the Josephson response of these devices to millimeter waves was comparatively less limited by self-heating